trunk/doc/pcre.txt

-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. Neither has
the pcredemo program. There are separate text files for the pcregrep and
pcretest commands.
-----------------------------------------------------------------------------


PCRE(3)                                                                PCRE(3)


NAME
       PCRE - Perl-compatible regular expressions


INTRODUCTION

       The  PCRE  library is a set of functions that implement regular expres-
       sion pattern matching using the same syntax and semantics as Perl, with
       just  a few differences. Some features that appeared in Python and PCRE
       before they appeared in Perl are also available using the  Python  syn-
       tax,  there  is  some  support for one or two .NET and Oniguruma syntax
       items, and there is an option for requesting some  minor  changes  that
       give better JavaScript compatibility.

       The  current implementation of PCRE corresponds approximately with Perl
       5.12, including support for UTF-8 encoded strings and  Unicode  general
       category  properties.  However,  UTF-8  and  Unicode  support has to be
       explicitly enabled; it is not the default. The  Unicode  tables  corre-
       spond to Unicode release 5.2.0.

       In  addition to the Perl-compatible matching function, PCRE contains an
       alternative function that matches the same compiled patterns in a  dif-
       ferent way. In certain circumstances, the alternative function has some
       advantages.  For a discussion of the two matching algorithms,  see  the
       pcrematching page.

       PCRE  is  written  in C and released as a C library. A number of people
       have written wrappers and interfaces of various kinds.  In  particular,
       Google  Inc.   have  provided  a comprehensive C++ wrapper. This is now
       included as part of the PCRE distribution. The pcrecpp page has details
       of  this  interface.  Other  people's contributions can be found in the
       Contrib directory at the primary FTP site, which is:

       ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre

       Details of exactly which Perl regular expression features are  and  are
       not supported by PCRE are given in separate documents. See the pcrepat-
       tern and pcrecompat pages. There is a syntax summary in the  pcresyntax
       page.

       Some  features  of  PCRE can be included, excluded, or changed when the
       library is built. The pcre_config() function makes it  possible  for  a
       client  to  discover  which  features are available. The features them-
       selves are described in the pcrebuild page. Documentation about  build-
       ing  PCRE  for various operating systems can be found in the README and
       NON-UNIX-USE files in the source distribution.

       The library contains a number of undocumented  internal  functions  and
       data  tables  that  are  used by more than one of the exported external
       functions, but which are not intended  for  use  by  external  callers.
       Their  names  all begin with "_pcre_", which hopefully will not provoke
       any name clashes. In some environments, it is possible to control which
       external  symbols  are  exported when a shared library is built, and in
       these cases the undocumented symbols are not exported.


USER DOCUMENTATION

       The user documentation for PCRE comprises a number  of  different  sec-
       tions.  In the "man" format, each of these is a separate "man page". In
       the HTML format, each is a separate page, linked from the  index  page.
       In  the  plain  text format, all the sections, except the pcredemo sec-
       tion, are concatenated, for ease of searching. The sections are as fol-
       lows:

         pcre              this document
         pcre-config       show PCRE installation configuration information
         pcreapi           details of PCRE's native C API
         pcrebuild         options for building PCRE
         pcrecallout       details of the callout feature
         pcrecompat        discussion of Perl compatibility
         pcrecpp           details of the C++ wrapper
         pcredemo          a demonstration C program that uses PCRE
         pcregrep          description of the pcregrep command
         pcrematching      discussion of the two matching algorithms
         pcrepartial       details of the partial matching facility
         pcrepattern       syntax and semantics of supported
                             regular expressions
         pcreperform       discussion of performance issues
         pcreposix         the POSIX-compatible C API
         pcreprecompile    details of saving and re-using precompiled patterns
         pcresample        discussion of the pcredemo program
         pcrestack         discussion of stack usage
         pcresyntax        quick syntax reference
         pcretest          description of the pcretest testing command

       In  addition,  in the "man" and HTML formats, there is a short page for
       each C library function, listing its arguments and results.


LIMITATIONS

       There are some size limitations in PCRE but it is hoped that they  will
       never in practice be relevant.

       The  maximum  length of a compiled pattern is 65539 (sic) bytes if PCRE
       is compiled with the default internal linkage size of 2. If you want to
       process  regular  expressions  that are truly enormous, you can compile
       PCRE with an internal linkage size of 3 or 4 (see the  README  file  in
       the  source  distribution and the pcrebuild documentation for details).
       In these cases the limit is substantially larger.  However,  the  speed
       of execution is slower.

       All values in repeating quantifiers must be less than 65536.

       There is no limit to the number of parenthesized subpatterns, but there
       can be no more than 65535 capturing subpatterns.

       The maximum length of name for a named subpattern is 32 characters, and
       the maximum number of named subpatterns is 10000.

       The  maximum  length of a subject string is the largest positive number
       that an integer variable can hold. However, when using the  traditional
       matching function, PCRE uses recursion to handle subpatterns and indef-
       inite repetition.  This means that the available stack space may  limit
       the size of a subject string that can be processed by certain patterns.
       For a discussion of stack issues, see the pcrestack documentation.


UTF-8 AND UNICODE PROPERTY SUPPORT

       From release 3.3, PCRE has  had  some  support  for  character  strings
       encoded  in the UTF-8 format. For release 4.0 this was greatly extended
       to cover most common requirements, and in release 5.0  additional  sup-
       port for Unicode general category properties was added.

       In  order  process  UTF-8 strings, you must build PCRE to include UTF-8
       support in the code, and, in addition,  you  must  call  pcre_compile()
       with  the  PCRE_UTF8  option  flag,  or the pattern must start with the
       sequence (*UTF8). When either of these is the case,  both  the  pattern
       and  any  subject  strings  that  are matched against it are treated as
       UTF-8 strings instead of strings of 1-byte characters.

       If you compile PCRE with UTF-8 support, but do not use it at run  time,
       the  library will be a bit bigger, but the additional run time overhead
       is limited to testing the PCRE_UTF8 flag occasionally, so should not be
       very big.

       If PCRE is built with Unicode character property support (which implies
       UTF-8 support), the escape sequences \p{..}, \P{..}, and  \X  are  sup-
       ported.  The available properties that can be tested are limited to the
       general category properties such as Lu for an upper case letter  or  Nd
       for  a  decimal number, the Unicode script names such as Arabic or Han,
       and the derived properties Any and L&. A full  list  is  given  in  the
       pcrepattern documentation. Only the short names for properties are sup-
       ported. For example, \p{L} matches a letter. Its Perl synonym,  \p{Let-
       ter},  is  not  supported.   Furthermore,  in Perl, many properties may
       optionally be prefixed by "Is", for compatibility with Perl  5.6.  PCRE
       does not support this.

   Validity of UTF-8 strings

       When  you  set  the  PCRE_UTF8 flag, the strings passed as patterns and
       subjects are (by default) checked for validity on entry to the relevant
       functions.  From  release 7.3 of PCRE, the check is according the rules
       of RFC 3629, which are themselves derived from the  Unicode  specifica-
       tion.  Earlier  releases  of PCRE followed the rules of RFC 2279, which
       allows the full range of 31-bit values (0 to 0x7FFFFFFF).  The  current
       check allows only values in the range U+0 to U+10FFFF, excluding U+D800
       to U+DFFF.

       The excluded code points are the "Low Surrogate Area"  of  Unicode,  of
       which  the Unicode Standard says this: "The Low Surrogate Area does not
       contain any  character  assignments,  consequently  no  character  code
       charts or namelists are provided for this area. Surrogates are reserved
       for use with UTF-16 and then must be used in pairs."  The  code  points
       that  are  encoded  by  UTF-16  pairs are available as independent code
       points in the UTF-8 encoding. (In  other  words,  the  whole  surrogate
       thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)

       If  an  invalid  UTF-8  string  is  passed  to  PCRE,  an  error return
       (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
       that your strings are valid, and therefore want to skip these checks in
       order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
       compile  time  or at run time, PCRE assumes that the pattern or subject
       it is given (respectively) contains only valid  UTF-8  codes.  In  this
       case, it does not diagnose an invalid UTF-8 string.

       If  you  pass  an  invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
       what happens depends on why the string is invalid. If the  string  con-
       forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
       string of characters in the range 0  to  0x7FFFFFFF.  In  other  words,
       apart from the initial validity test, PCRE (when in UTF-8 mode) handles
       strings according to the more liberal rules of RFC  2279.  However,  if
       the  string does not even conform to RFC 2279, the result is undefined.
       Your program may crash.

       If you want to process strings  of  values  in  the  full  range  0  to
       0x7FFFFFFF,  encoded in a UTF-8-like manner as per the old RFC, you can
       set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
       this situation, you will have to apply your own validity check.

   General comments about UTF-8 mode

       1.  An  unbraced  hexadecimal  escape sequence (such as \xb3) matches a
       two-byte UTF-8 character if the value is greater than 127.

       2. Octal numbers up to \777 are recognized, and  match  two-byte  UTF-8
       characters for values greater than \177.

       3.  Repeat quantifiers apply to complete UTF-8 characters, not to indi-
       vidual bytes, for example: \x{100}{3}.

       4. The dot metacharacter matches one UTF-8 character instead of a  sin-
       gle byte.

       5.  The  escape sequence \C can be used to match a single byte in UTF-8
       mode, but its use can lead to some strange effects.  This  facility  is
       not available in the alternative matching function, pcre_dfa_exec().

       6.  The  character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
       test characters of any code value, but, by default, the characters that
       PCRE  recognizes  as digits, spaces, or word characters remain the same
       set as before, all with values less than 256. This  remains  true  even
       when  PCRE  is built to include Unicode property support, because to do
       otherwise would slow down PCRE in many common cases. Note in particular
       that this applies to \b and \B, because they are defined in terms of \w
       and \W. If you really want to test for a wider sense of, say,  "digit",
       you  can  use  explicit Unicode property tests such as \p{Nd}. Alterna-
       tively, if you set the PCRE_UCP option,  the  way  that  the  character
       escapes  work  is changed so that Unicode properties are used to deter-
       mine which characters match. There are more details in the  section  on
       generic character types in the pcrepattern documentation.

       7.  Similarly,  characters that match the POSIX named character classes
       are all low-valued characters, unless the PCRE_UCP option is set.

       8. However, the horizontal and  vertical  whitespace  matching  escapes
       (\h,  \H,  \v, and \V) do match all the appropriate Unicode characters,
       whether or not PCRE_UCP is set.

       9. Case-insensitive matching applies only to  characters  whose  values
       are  less than 128, unless PCRE is built with Unicode property support.
       Even when Unicode property support is available, PCRE  still  uses  its
       own  character  tables when checking the case of low-valued characters,
       so as not to degrade performance.  The Unicode property information  is
       used only for characters with higher values. Furthermore, PCRE supports
       case-insensitive matching only  when  there  is  a  one-to-one  mapping
       between  a letter's cases. There are a small number of many-to-one map-
       pings in Unicode; these are not supported by PCRE.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

       Putting an actual email address here seems to have been a spam  magnet,
       so  I've  taken  it away. If you want to email me, use my two initials,
       followed by the two digits 10, at the domain cam.ac.uk.


REVISION

       Last updated: 13 November 2010
       Copyright (c) 1997-2010 University of Cambridge.
------------------------------------------------------------------------------


PCREBUILD(3)                                                      PCREBUILD(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE BUILD-TIME OPTIONS

       This  document  describes  the  optional  features  of PCRE that can be
       selected when the library is compiled. It assumes use of the  configure
       script,  where the optional features are selected or deselected by pro-
       viding options to configure before running the make  command.  However,
       the  same  options  can be selected in both Unix-like and non-Unix-like
       environments using the GUI facility of cmake-gui if you are using CMake
       instead of configure to build PCRE.

       There  is  a  lot more information about building PCRE in non-Unix-like
       environments in the file called NON_UNIX_USE, which is part of the PCRE
       distribution.  You  should consult this file as well as the README file
       if you are building in a non-Unix-like environment.

       The complete list of options for configure (which includes the standard
       ones  such  as  the  selection  of  the  installation directory) can be
       obtained by running

         ./configure --help

       The following sections include  descriptions  of  options  whose  names
       begin with --enable or --disable. These settings specify changes to the
       defaults for the configure command. Because of the way  that  configure
       works,  --enable  and --disable always come in pairs, so the complemen-
       tary option always exists as well, but as it specifies the default,  it
       is not described.


C++ SUPPORT

       By default, the configure script will search for a C++ compiler and C++
       header files. If it finds them, it automatically builds the C++ wrapper
       library for PCRE. You can disable this by adding

         --disable-cpp

       to the configure command.


UTF-8 SUPPORT

       To build PCRE with support for UTF-8 Unicode character strings, add

         --enable-utf8

       to  the  configure  command.  Of  itself, this does not make PCRE treat
       strings as UTF-8. As well as compiling PCRE with this option, you  also
       have  have to set the PCRE_UTF8 option when you call the pcre_compile()
       or pcre_compile2() functions.

       If you set --enable-utf8 when compiling in an EBCDIC environment,  PCRE
       expects its input to be either ASCII or UTF-8 (depending on the runtime
       option). It is not possible to support both EBCDIC and UTF-8  codes  in
       the  same  version  of  the  library.  Consequently,  --enable-utf8 and
       --enable-ebcdic are mutually exclusive.


UNICODE CHARACTER PROPERTY SUPPORT

       UTF-8 support allows PCRE to process character values greater than  255
       in  the  strings that it handles. On its own, however, it does not pro-
       vide any facilities for accessing the properties of such characters. If
       you  want  to  be able to use the pattern escapes \P, \p, and \X, which
       refer to Unicode character properties, you must add

         --enable-unicode-properties

       to the configure command. This implies UTF-8 support, even if you  have
       not explicitly requested it.

       Including  Unicode  property  support  adds around 30K of tables to the
       PCRE library. Only the general category properties such as  Lu  and  Nd
       are supported. Details are given in the pcrepattern documentation.


CODE VALUE OF NEWLINE

       By  default,  PCRE interprets the linefeed (LF) character as indicating
       the end of a line. This is the normal newline  character  on  Unix-like
       systems.  You  can compile PCRE to use carriage return (CR) instead, by
       adding

         --enable-newline-is-cr

       to the  configure  command.  There  is  also  a  --enable-newline-is-lf
       option, which explicitly specifies linefeed as the newline character.

       Alternatively, you can specify that line endings are to be indicated by
       the two character sequence CRLF. If you want this, add

         --enable-newline-is-crlf

       to the configure command. There is a fourth option, specified by

         --enable-newline-is-anycrlf

       which causes PCRE to recognize any of the three sequences  CR,  LF,  or
       CRLF as indicating a line ending. Finally, a fifth option, specified by

         --enable-newline-is-any

       causes PCRE to recognize any Unicode newline sequence.

       Whatever  line  ending convention is selected when PCRE is built can be
       overridden when the library functions are called. At build time  it  is
       conventional to use the standard for your operating system.


WHAT \R MATCHES

       By  default,  the  sequence \R in a pattern matches any Unicode newline
       sequence, whatever has been selected as the line  ending  sequence.  If
       you specify

         --enable-bsr-anycrlf

       the  default  is changed so that \R matches only CR, LF, or CRLF. What-
       ever is selected when PCRE is built can be overridden when the  library
       functions are called.


BUILDING SHARED AND STATIC LIBRARIES

       The  PCRE building process uses libtool to build both shared and static
       Unix libraries by default. You can suppress one of these by adding  one
       of

         --disable-shared
         --disable-static

       to the configure command, as required.


POSIX MALLOC USAGE

       When PCRE is called through the POSIX interface (see the pcreposix doc-
       umentation), additional working storage is  required  for  holding  the
       pointers  to capturing substrings, because PCRE requires three integers
       per substring, whereas the POSIX interface provides only  two.  If  the
       number of expected substrings is small, the wrapper function uses space
       on the stack, because this is faster than using malloc() for each call.
       The default threshold above which the stack is no longer used is 10; it
       can be changed by adding a setting such as

         --with-posix-malloc-threshold=20

       to the configure command.


HANDLING VERY LARGE PATTERNS

       Within a compiled pattern, offset values are used  to  point  from  one
       part  to another (for example, from an opening parenthesis to an alter-
       nation metacharacter). By default, two-byte values are used  for  these
       offsets,  leading  to  a  maximum size for a compiled pattern of around
       64K. This is sufficient to handle all but the most  gigantic  patterns.
       Nevertheless,  some  people do want to process truyl enormous patterns,
       so it is possible to compile PCRE to use three-byte or  four-byte  off-
       sets by adding a setting such as

         --with-link-size=3

       to  the  configure  command.  The value given must be 2, 3, or 4. Using
       longer offsets slows down the operation of PCRE because it has to  load
       additional bytes when handling them.


AVOIDING EXCESSIVE STACK USAGE

       When matching with the pcre_exec() function, PCRE implements backtrack-
       ing by making recursive calls to an internal function  called  match().
       In  environments  where  the size of the stack is limited, this can se-
       verely limit PCRE's operation. (The Unix environment does  not  usually
       suffer from this problem, but it may sometimes be necessary to increase
       the maximum stack size.  There is a discussion in the  pcrestack  docu-
       mentation.)  An alternative approach to recursion that uses memory from
       the heap to remember data, instead of using recursive  function  calls,
       has  been  implemented to work round the problem of limited stack size.
       If you want to build a version of PCRE that works this way, add

         --disable-stack-for-recursion

       to the configure command. With this configuration, PCRE  will  use  the
       pcre_stack_malloc  and pcre_stack_free variables to call memory manage-
       ment functions. By default these point to malloc() and free(), but  you
       can replace the pointers so that your own functions are used instead.

       Separate  functions  are  provided  rather  than  using pcre_malloc and
       pcre_free because the  usage  is  very  predictable:  the  block  sizes
       requested  are  always  the  same,  and  the blocks are always freed in
       reverse order. A calling program might be able to  implement  optimized
       functions  that  perform  better  than  malloc()  and free(). PCRE runs
       noticeably more slowly when built in this way. This option affects only
       the pcre_exec() function; it is not relevant for pcre_dfa_exec().


LIMITING PCRE RESOURCE USAGE

       Internally,  PCRE has a function called match(), which it calls repeat-
       edly  (sometimes  recursively)  when  matching  a  pattern   with   the
       pcre_exec()  function.  By controlling the maximum number of times this
       function may be called during a single matching operation, a limit  can
       be  placed  on  the resources used by a single call to pcre_exec(). The
       limit can be changed at run time, as described in the pcreapi  documen-
       tation.  The default is 10 million, but this can be changed by adding a
       setting such as

         --with-match-limit=500000

       to  the  configure  command.  This  setting  has  no  effect   on   the
       pcre_dfa_exec() matching function.

       In  some  environments  it is desirable to limit the depth of recursive
       calls of match() more strictly than the total number of calls, in order
       to  restrict  the maximum amount of stack (or heap, if --disable-stack-
       for-recursion is specified) that is used. A second limit controls this;
       it  defaults  to  the  value  that is set for --with-match-limit, which
       imposes no additional constraints. However, you can set a  lower  limit
       by adding, for example,

         --with-match-limit-recursion=10000

       to  the  configure  command.  This  value can also be overridden at run
       time.


CREATING CHARACTER TABLES AT BUILD TIME

       PCRE uses fixed tables for processing characters whose code values  are
       less  than 256. By default, PCRE is built with a set of tables that are
       distributed in the file pcre_chartables.c.dist. These  tables  are  for
       ASCII codes only. If you add

         --enable-rebuild-chartables

       to  the  configure  command, the distributed tables are no longer used.
       Instead, a program called dftables is compiled and  run.  This  outputs
       the source for new set of tables, created in the default locale of your
       C runtime system. (This method of replacing the tables does not work if
       you  are cross compiling, because dftables is run on the local host. If
       you need to create alternative tables when cross  compiling,  you  will
       have to do so "by hand".)


USING EBCDIC CODE

       PCRE  assumes  by  default that it will run in an environment where the
       character code is ASCII (or Unicode, which is  a  superset  of  ASCII).
       This  is  the  case for most computer operating systems. PCRE can, how-
       ever, be compiled to run in an EBCDIC environment by adding

         --enable-ebcdic

       to the configure command. This setting implies --enable-rebuild-charta-
       bles.  You  should  only  use  it if you know that you are in an EBCDIC
       environment (for example,  an  IBM  mainframe  operating  system).  The
       --enable-ebcdic option is incompatible with --enable-utf8.


PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT

       By default, pcregrep reads all files as plain text. You can build it so
       that it recognizes files whose names end in .gz or .bz2, and reads them
       with libz or libbz2, respectively, by adding one or both of

         --enable-pcregrep-libz
         --enable-pcregrep-libbz2

       to the configure command. These options naturally require that the rel-
       evant libraries are installed on your system. Configuration  will  fail
       if they are not.


PCRETEST OPTION FOR LIBREADLINE SUPPORT

       If you add

         --enable-pcretest-libreadline

       to  the  configure  command,  pcretest  is  linked with the libreadline
       library, and when its input is from a terminal, it reads it  using  the
       readline() function. This provides line-editing and history facilities.
       Note that libreadline is GPL-licensed, so if you distribute a binary of
       pcretest linked in this way, there may be licensing issues.

       Setting  this  option  causes  the -lreadline option to be added to the
       pcretest build. In many operating environments with  a  sytem-installed
       libreadline this is sufficient. However, in some environments (e.g.  if
       an unmodified distribution version of readline is in use),  some  extra
       configuration  may  be necessary. The INSTALL file for libreadline says
       this:

         "Readline uses the termcap functions, but does not link with the
         termcap or curses library itself, allowing applications which link
         with readline the to choose an appropriate library."

       If your environment has not been set up so that an appropriate  library
       is automatically included, you may need to add something like

         LIBS="-ncurses"

       immediately before the configure command.


SEE ALSO

       pcreapi(3), pcre_config(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 29 September 2009
       Copyright (c) 1997-2009 University of Cambridge.
------------------------------------------------------------------------------


PCREMATCHING(3)                                                PCREMATCHING(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE MATCHING ALGORITHMS

       This document describes the two different algorithms that are available
       in PCRE for matching a compiled regular expression against a given sub-
       ject  string.  The  "standard"  algorithm  is  the  one provided by the
       pcre_exec() function.  This works in the same was  as  Perl's  matching
       function, and provides a Perl-compatible matching operation.

       An  alternative  algorithm is provided by the pcre_dfa_exec() function;
       this operates in a different way, and is not  Perl-compatible.  It  has
       advantages  and disadvantages compared with the standard algorithm, and
       these are described below.

       When there is only one possible way in which a given subject string can
       match  a pattern, the two algorithms give the same answer. A difference
       arises, however, when there are multiple possibilities. For example, if
       the pattern

         ^<.*>

       is matched against the string

         <something> <something else> <something further>

       there are three possible answers. The standard algorithm finds only one
       of them, whereas the alternative algorithm finds all three.


REGULAR EXPRESSIONS AS TREES

       The set of strings that are matched by a regular expression can be rep-
       resented  as  a  tree structure. An unlimited repetition in the pattern
       makes the tree of infinite size, but it is still a tree.  Matching  the
       pattern  to a given subject string (from a given starting point) can be
       thought of as a search of the tree.  There are two  ways  to  search  a
       tree:  depth-first  and  breadth-first, and these correspond to the two
       matching algorithms provided by PCRE.


THE STANDARD MATCHING ALGORITHM

       In the terminology of Jeffrey Friedl's book "Mastering Regular  Expres-
       sions",  the  standard  algorithm  is an "NFA algorithm". It conducts a
       depth-first search of the pattern tree. That is, it  proceeds  along  a
       single path through the tree, checking that the subject matches what is
       required. When there is a mismatch, the algorithm  tries  any  alterna-
       tives  at  the  current point, and if they all fail, it backs up to the
       previous branch point in the  tree,  and  tries  the  next  alternative
       branch  at  that  level.  This often involves backing up (moving to the
       left) in the subject string as well.  The  order  in  which  repetition
       branches  are  tried  is controlled by the greedy or ungreedy nature of
       the quantifier.

       If a leaf node is reached, a matching string has  been  found,  and  at
       that  point the algorithm stops. Thus, if there is more than one possi-
       ble match, this algorithm returns the first one that it finds.  Whether
       this  is the shortest, the longest, or some intermediate length depends
       on the way the greedy and ungreedy repetition quantifiers are specified
       in the pattern.

       Because  it  ends  up  with a single path through the tree, it is rela-
       tively straightforward for this algorithm to keep  track  of  the  sub-
       strings  that  are  matched  by portions of the pattern in parentheses.
       This provides support for capturing parentheses and back references.


THE ALTERNATIVE MATCHING ALGORITHM

       This algorithm conducts a breadth-first search of  the  tree.  Starting
       from  the  first  matching  point  in the subject, it scans the subject
       string from left to right, once, character by character, and as it does
       this,  it remembers all the paths through the tree that represent valid
       matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
       though  it is not implemented as a traditional finite state machine (it
       keeps multiple states active simultaneously).

       Although the general principle of this matching algorithm  is  that  it
       scans  the subject string only once, without backtracking, there is one
       exception: when a lookaround assertion is encountered,  the  characters
       following  or  preceding  the  current  point  have to be independently
       inspected.

       The scan continues until either the end of the subject is  reached,  or
       there  are  no more unterminated paths. At this point, terminated paths
       represent the different matching possibilities (if there are none,  the
       match  has  failed).   Thus,  if there is more than one possible match,
       this algorithm finds all of them, and in particular, it finds the long-
       est.  The  matches are returned in decreasing order of length. There is
       an option to stop the algorithm after the first match (which is  neces-
       sarily the shortest) is found.

       Note that all the matches that are found start at the same point in the
       subject. If the pattern

         cat(er(pillar)?)?

       is matched against the string "the caterpillar catchment",  the  result
       will  be the three strings "caterpillar", "cater", and "cat" that start
       at the fifth character of the subject. The algorithm does not automati-
       cally move on to find matches that start at later positions.

       There are a number of features of PCRE regular expressions that are not
       supported by the alternative matching algorithm. They are as follows:

       1. Because the algorithm finds all  possible  matches,  the  greedy  or
       ungreedy  nature  of repetition quantifiers is not relevant. Greedy and
       ungreedy quantifiers are treated in exactly the same way. However, pos-
       sessive  quantifiers can make a difference when what follows could also
       match what is quantified, for example in a pattern like this:

         ^a++\w!

       This pattern matches "aaab!" but not "aaa!", which would be matched  by
       a  non-possessive quantifier. Similarly, if an atomic group is present,
       it is matched as if it were a standalone pattern at the current  point,
       and  the  longest match is then "locked in" for the rest of the overall
       pattern.

       2. When dealing with multiple paths through the tree simultaneously, it
       is  not  straightforward  to  keep track of captured substrings for the
       different matching possibilities, and  PCRE's  implementation  of  this
       algorithm does not attempt to do this. This means that no captured sub-
       strings are available.

       3. Because no substrings are captured, back references within the  pat-
       tern are not supported, and cause errors if encountered.

       4.  For  the same reason, conditional expressions that use a backrefer-
       ence as the condition or test for a specific group  recursion  are  not
       supported.

       5.  Because  many  paths  through the tree may be active, the \K escape
       sequence, which resets the start of the match when encountered (but may
       be  on  some  paths  and not on others), is not supported. It causes an
       error if encountered.

       6. Callouts are supported, but the value of the  capture_top  field  is
       always 1, and the value of the capture_last field is always -1.

       7.  The \C escape sequence, which (in the standard algorithm) matches a
       single byte, even in UTF-8 mode, is not supported because the  alterna-
       tive  algorithm  moves  through  the  subject string one character at a
       time, for all active paths through the tree.

       8. Except for (*FAIL), the backtracking control verbs such as  (*PRUNE)
       are  not  supported.  (*FAIL)  is supported, and behaves like a failing
       negative assertion.


ADVANTAGES OF THE ALTERNATIVE ALGORITHM

       Using the alternative matching algorithm provides the following  advan-
       tages:

       1. All possible matches (at a single point in the subject) are automat-
       ically found, and in particular, the longest match is  found.  To  find
       more than one match using the standard algorithm, you have to do kludgy
       things with callouts.

       2. Because the alternative algorithm  scans  the  subject  string  just
       once,  and  never  needs to backtrack, it is possible to pass very long
       subject strings to the matching function in  several  pieces,  checking
       for  partial  matching  each time. Although it is possible to do multi-
       segment matching using the standard algorithm (pcre_exec()), by retain-
       ing  partially matched substrings, it is more complicated. The pcrepar-
       tial documentation gives details  of  partial  matching  and  discusses
       multi-segment matching.


DISADVANTAGES OF THE ALTERNATIVE ALGORITHM

       The alternative algorithm suffers from a number of disadvantages:

       1.  It  is  substantially  slower  than the standard algorithm. This is
       partly because it has to search for all possible matches, but  is  also
       because it is less susceptible to optimization.

       2. Capturing parentheses and back references are not supported.

       3. Although atomic groups are supported, their use does not provide the
       performance advantage that it does for the standard algorithm.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 17 November 2010
       Copyright (c) 1997-2010 University of Cambridge.
------------------------------------------------------------------------------


PCREAPI(3)                                                          PCREAPI(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE NATIVE API

       #include <pcre.h>

       pcre *pcre_compile(const char *pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre_extra *pcre_study(const pcre *code, int options,
            const char **errptr);

       int pcre_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);

       int pcre_copy_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            char *buffer, int buffersize);

       int pcre_copy_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber, char *buffer,
            int buffersize);

       int pcre_get_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            const char **stringptr);

       int pcre_get_stringnumber(const pcre *code,
            const char *name);

       int pcre_get_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

       int pcre_get_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber,
            const char **stringptr);

       int pcre_get_substring_list(const char *subject,
            int *ovector, int stringcount, const char ***listptr);

       void pcre_free_substring(const char *stringptr);

       void pcre_free_substring_list(const char **stringptr);

       const unsigned char *pcre_maketables(void);

       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
            int what, void *where);

       int pcre_info(const pcre *code, int *optptr, int *firstcharptr);

       int pcre_refcount(pcre *code, int adjust);

       int pcre_config(int what, void *where);

       char *pcre_version(void);

       void *(*pcre_malloc)(size_t);

       void (*pcre_free)(void *);

       void *(*pcre_stack_malloc)(size_t);

       void (*pcre_stack_free)(void *);

       int (*pcre_callout)(pcre_callout_block *);


PCRE API OVERVIEW

       PCRE has its own native API, which is described in this document. There
       are also some wrapper functions that correspond to  the  POSIX  regular
       expression  API.  These  are  described in the pcreposix documentation.
       Both of these APIs define a set of C function calls. A C++  wrapper  is
       distributed with PCRE. It is documented in the pcrecpp page.

       The  native  API  C  function prototypes are defined in the header file
       pcre.h, and on Unix systems the library itself is called  libpcre.   It
       can normally be accessed by adding -lpcre to the command for linking an
       application  that  uses  PCRE.  The  header  file  defines  the  macros
       PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
       bers for the library.  Applications can use these  to  include  support
       for different releases of PCRE.

       In a Windows environment, if you want to statically link an application
       program against a non-dll pcre.a  file,  you  must  define  PCRE_STATIC
       before  including  pcre.h or pcrecpp.h, because otherwise the pcre_mal-
       loc()   and   pcre_free()   exported   functions   will   be   declared
       __declspec(dllimport), with unwanted results.

       The   functions   pcre_compile(),  pcre_compile2(),  pcre_study(),  and
       pcre_exec() are used for compiling and matching regular expressions  in
       a  Perl-compatible  manner. A sample program that demonstrates the sim-
       plest way of using them is provided in the file  called  pcredemo.c  in
       the PCRE source distribution. A listing of this program is given in the
       pcredemo documentation, and the pcresample documentation describes  how
       to compile and run it.

       A second matching function, pcre_dfa_exec(), which is not Perl-compati-
       ble, is also provided. This uses a different algorithm for  the  match-
       ing.  The  alternative algorithm finds all possible matches (at a given
       point in the subject), and scans the subject just  once  (unless  there
       are  lookbehind  assertions).  However,  this algorithm does not return
       captured substrings. A description of the two matching  algorithms  and
       their  advantages  and disadvantages is given in the pcrematching docu-
       mentation.

       In addition to the main compiling and  matching  functions,  there  are
       convenience functions for extracting captured substrings from a subject
       string that is matched by pcre_exec(). They are:

         pcre_copy_substring()
         pcre_copy_named_substring()
         pcre_get_substring()
         pcre_get_named_substring()
         pcre_get_substring_list()
         pcre_get_stringnumber()
         pcre_get_stringtable_entries()

       pcre_free_substring() and pcre_free_substring_list() are also provided,
       to free the memory used for extracted strings.

       The  function  pcre_maketables()  is  used  to build a set of character
       tables  in  the  current  locale   for   passing   to   pcre_compile(),
       pcre_exec(),  or  pcre_dfa_exec(). This is an optional facility that is
       provided for specialist use.  Most  commonly,  no  special  tables  are
       passed,  in  which case internal tables that are generated when PCRE is
       built are used.

       The function pcre_fullinfo() is used to find out  information  about  a
       compiled  pattern; pcre_info() is an obsolete version that returns only
       some of the available information, but is retained for  backwards  com-
       patibility.   The function pcre_version() returns a pointer to a string
       containing the version of PCRE and its date of release.

       The function pcre_refcount() maintains a  reference  count  in  a  data
       block  containing  a compiled pattern. This is provided for the benefit
       of object-oriented applications.

       The global variables pcre_malloc and pcre_free  initially  contain  the
       entry  points  of  the  standard malloc() and free() functions, respec-
       tively. PCRE calls the memory management functions via these variables,
       so  a  calling  program  can replace them if it wishes to intercept the
       calls. This should be done before calling any PCRE functions.

       The global variables pcre_stack_malloc  and  pcre_stack_free  are  also
       indirections  to  memory  management functions. These special functions
       are used only when PCRE is compiled to use  the  heap  for  remembering
       data, instead of recursive function calls, when running the pcre_exec()
       function. See the pcrebuild documentation for  details  of  how  to  do
       this.  It  is  a non-standard way of building PCRE, for use in environ-
       ments that have limited stacks. Because of the greater  use  of  memory
       management,  it  runs  more  slowly. Separate functions are provided so
       that special-purpose external code can be  used  for  this  case.  When
       used,  these  functions  are always called in a stack-like manner (last
       obtained, first freed), and always for memory blocks of the same  size.
       There  is  a discussion about PCRE's stack usage in the pcrestack docu-
       mentation.

       The global variable pcre_callout initially contains NULL. It can be set
       by  the  caller  to  a "callout" function, which PCRE will then call at
       specified points during a matching operation. Details are given in  the
       pcrecallout documentation.


NEWLINES

       PCRE  supports five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, or any Unicode newline sequence. The Unicode newline  sequences
       are  the  three just mentioned, plus the single characters VT (vertical
       tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS  (line
       separator, U+2028), and PS (paragraph separator, U+2029).

       Each  of  the first three conventions is used by at least one operating
       system as its standard newline sequence. When PCRE is built, a  default
       can  be  specified.  The default default is LF, which is the Unix stan-
       dard. When PCRE is run, the default can be overridden,  either  when  a
       pattern is compiled, or when it is matched.

       At compile time, the newline convention can be specified by the options
       argument of pcre_compile(), or it can be specified by special  text  at
       the start of the pattern itself; this overrides any other settings. See
       the pcrepattern page for details of the special character sequences.

       In the PCRE documentation the word "newline" is used to mean "the char-
       acter  or pair of characters that indicate a line break". The choice of
       newline convention affects the handling of  the  dot,  circumflex,  and
       dollar metacharacters, the handling of #-comments in /x mode, and, when
       CRLF is a recognized line ending sequence, the match position  advance-
       ment for a non-anchored pattern. There is more detail about this in the
       section on pcre_exec() options below.

       The choice of newline convention does not affect the interpretation  of
       the  \n  or  \r  escape  sequences, nor does it affect what \R matches,
       which is controlled in a similar way, but by separate options.


MULTITHREADING

       The PCRE functions can be used in  multi-threading  applications,  with
       the  proviso  that  the  memory  management  functions  pointed  to  by
       pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
       callout function pointed to by pcre_callout, are shared by all threads.

       The  compiled form of a regular expression is not altered during match-
       ing, so the same compiled pattern can safely be used by several threads
       at once.


SAVING PRECOMPILED PATTERNS FOR LATER USE

       The compiled form of a regular expression can be saved and re-used at a
       later time, possibly by a different program, and even on a  host  other
       than  the  one  on  which  it  was  compiled.  Details are given in the
       pcreprecompile documentation. However, compiling a  regular  expression
       with  one version of PCRE for use with a different version is not guar-
       anteed to work and may cause crashes.


CHECKING BUILD-TIME OPTIONS

       int pcre_config(int what, void *where);

       The function pcre_config() makes it possible for a PCRE client to  dis-
       cover which optional features have been compiled into the PCRE library.
       The pcrebuild documentation has more details about these optional  fea-
       tures.

       The  first  argument  for pcre_config() is an integer, specifying which
       information is required; the second argument is a pointer to a variable
       into  which  the  information  is  placed. The following information is
       available:

         PCRE_CONFIG_UTF8

       The output is an integer that is set to one if UTF-8 support is  avail-
       able; otherwise it is set to zero.

         PCRE_CONFIG_UNICODE_PROPERTIES

       The  output  is  an  integer  that is set to one if support for Unicode
       character properties is available; otherwise it is set to zero.

         PCRE_CONFIG_NEWLINE

       The output is an integer whose value specifies  the  default  character
       sequence  that is recognized as meaning "newline". The four values that
       are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
       and  -1  for  ANY.  Though they are derived from ASCII, the same values
       are returned in EBCDIC environments. The default should normally corre-
       spond to the standard sequence for your operating system.

         PCRE_CONFIG_BSR

       The output is an integer whose value indicates what character sequences
       the \R escape sequence matches by default. A value of 0 means  that  \R
       matches  any  Unicode  line ending sequence; a value of 1 means that \R
       matches only CR, LF, or CRLF. The default can be overridden when a pat-
       tern is compiled or matched.

         PCRE_CONFIG_LINK_SIZE

       The  output  is  an  integer that contains the number of bytes used for
       internal linkage in compiled regular expressions. The value is 2, 3, or
       4.  Larger  values  allow larger regular expressions to be compiled, at
       the expense of slower matching. The default value of  2  is  sufficient
       for  all  but  the  most massive patterns, since it allows the compiled
       pattern to be up to 64K in size.

         PCRE_CONFIG_POSIX_MALLOC_THRESHOLD

       The output is an integer that contains the threshold  above  which  the
       POSIX  interface  uses malloc() for output vectors. Further details are
       given in the pcreposix documentation.

         PCRE_CONFIG_MATCH_LIMIT

       The output is a long integer that gives the default limit for the  num-
       ber  of  internal  matching  function calls in a pcre_exec() execution.
       Further details are given with pcre_exec() below.

         PCRE_CONFIG_MATCH_LIMIT_RECURSION

       The output is a long integer that gives the default limit for the depth
       of   recursion  when  calling  the  internal  matching  function  in  a
       pcre_exec() execution.  Further  details  are  given  with  pcre_exec()
       below.

         PCRE_CONFIG_STACKRECURSE

       The  output is an integer that is set to one if internal recursion when
       running pcre_exec() is implemented by recursive function calls that use
       the  stack  to remember their state. This is the usual way that PCRE is
       compiled. The output is zero if PCRE was compiled to use blocks of data
       on  the  heap  instead  of  recursive  function  calls.  In  this case,
       pcre_stack_malloc and  pcre_stack_free  are  called  to  manage  memory
       blocks on the heap, thus avoiding the use of the stack.


COMPILING A PATTERN

       pcre *pcre_compile(const char *pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre *pcre_compile2(const char *pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       Either of the functions pcre_compile() or pcre_compile2() can be called
       to compile a pattern into an internal form. The only difference between
       the  two interfaces is that pcre_compile2() has an additional argument,
       errorcodeptr, via which a numerical error  code  can  be  returned.  To
       avoid  too  much repetition, we refer just to pcre_compile() below, but
       the information applies equally to pcre_compile2().

       The pattern is a C string terminated by a binary zero, and is passed in
       the  pattern  argument.  A  pointer to a single block of memory that is
       obtained via pcre_malloc is returned. This contains the  compiled  code
       and related data. The pcre type is defined for the returned block; this
       is a typedef for a structure whose contents are not externally defined.
       It is up to the caller to free the memory (via pcre_free) when it is no
       longer required.

       Although the compiled code of a PCRE regex is relocatable, that is,  it
       does not depend on memory location, the complete pcre data block is not
       fully relocatable, because it may contain a copy of the tableptr  argu-
       ment, which is an address (see below).

       The options argument contains various bit settings that affect the com-
       pilation. It should be zero if no options are required.  The  available
       options  are  described  below. Some of them (in particular, those that
       are compatible with Perl, but some others as well) can also be set  and
       unset  from  within  the  pattern  (see the detailed description in the
       pcrepattern documentation). For those options that can be different  in
       different  parts  of  the pattern, the contents of the options argument
       specifies their settings at the start of compilation and execution. The
       PCRE_ANCHORED,  PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and
       PCRE_NO_START_OPT options can be set at the time of matching as well as
       at compile time.

       If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
       if compilation of a pattern fails,  pcre_compile()  returns  NULL,  and
       sets the variable pointed to by errptr to point to a textual error mes-
       sage. This is a static string that is part of the library. You must not
       try  to  free  it. The offset from the start of the pattern to the byte
       that was being processed when the error was discovered is placed in the
       variable  pointed to by erroffset, which must not be NULL. If it is, an
       immediate error is given. Some errors are not detected until checks are
       carried  out  when the whole pattern has been scanned; in this case the
       offset is set to the end of the pattern.

       Note that the offset is in bytes, not characters, even in  UTF-8  mode.
       It  may  point  into the middle of a UTF-8 character (for example, when
       PCRE_ERROR_BADUTF8 is returned for an invalid UTF-8 string).

       If pcre_compile2() is used instead of pcre_compile(),  and  the  error-
       codeptr  argument is not NULL, a non-zero error code number is returned
       via this argument in the event of an error. This is in addition to  the
       textual error message. Error codes and messages are listed below.

       If  the  final  argument, tableptr, is NULL, PCRE uses a default set of
       character tables that are  built  when  PCRE  is  compiled,  using  the
       default  C  locale.  Otherwise, tableptr must be an address that is the
       result of a call to pcre_maketables(). This value is  stored  with  the
       compiled  pattern,  and used again by pcre_exec(), unless another table
       pointer is passed to it. For more discussion, see the section on locale
       support below.

       This  code  fragment  shows a typical straightforward call to pcre_com-
       pile():

         pcre *re;
         const char *error;
         int erroffset;
         re = pcre_compile(
           "^A.*Z",          /* the pattern */
           0,                /* default options */
           &error,           /* for error message */
           &erroffset,       /* for error offset */
           NULL);            /* use default character tables */

       The following names for option bits are defined in  the  pcre.h  header
       file:

         PCRE_ANCHORED

       If this bit is set, the pattern is forced to be "anchored", that is, it
       is constrained to match only at the first matching point in the  string
       that  is being searched (the "subject string"). This effect can also be
       achieved by appropriate constructs in the pattern itself, which is  the
       only way to do it in Perl.

         PCRE_AUTO_CALLOUT

       If this bit is set, pcre_compile() automatically inserts callout items,
       all with number 255, before each pattern item. For  discussion  of  the
       callout facility, see the pcrecallout documentation.

         PCRE_BSR_ANYCRLF
         PCRE_BSR_UNICODE

       These options (which are mutually exclusive) control what the \R escape
       sequence matches. The choice is either to match only CR, LF,  or  CRLF,
       or to match any Unicode newline sequence. The default is specified when
       PCRE is built. It can be overridden from within the pattern, or by set-
       ting an option when a compiled pattern is matched.

         PCRE_CASELESS

       If  this  bit is set, letters in the pattern match both upper and lower
       case letters. It is equivalent to Perl's  /i  option,  and  it  can  be
       changed  within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
       always understands the concept of case for characters whose values  are
       less  than 128, so caseless matching is always possible. For characters
       with higher values, the concept of case is supported if  PCRE  is  com-
       piled  with Unicode property support, but not otherwise. If you want to
       use caseless matching for characters 128 and  above,  you  must  ensure
       that  PCRE  is  compiled  with Unicode property support as well as with
       UTF-8 support.

         PCRE_DOLLAR_ENDONLY

       If this bit is set, a dollar metacharacter in the pattern matches  only
       at  the  end  of the subject string. Without this option, a dollar also
       matches immediately before a newline at the end of the string (but  not
       before  any  other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
       if PCRE_MULTILINE is set.  There is no equivalent  to  this  option  in
       Perl, and no way to set it within a pattern.

         PCRE_DOTALL

       If  this bit is set, a dot metacharacter in the pattern matches a char-
       acter of any value, including one that indicates a newline. However, it
       only  ever  matches  one character, even if newlines are coded as CRLF.
       Without this option, a dot does not match when the current position  is
       at a newline. This option is equivalent to Perl's /s option, and it can
       be changed within a pattern by a (?s) option setting. A negative  class
       such as [^a] always matches newline characters, independent of the set-
       ting of this option.

         PCRE_DUPNAMES

       If this bit is set, names used to identify capturing  subpatterns  need
       not be unique. This can be helpful for certain types of pattern when it
       is known that only one instance of the named  subpattern  can  ever  be
       matched.  There  are  more details of named subpatterns below; see also
       the pcrepattern documentation.

         PCRE_EXTENDED

       If this bit is set, whitespace  data  characters  in  the  pattern  are
       totally ignored except when escaped or inside a character class. White-
       space does not include the VT character (code 11). In addition, charac-
       ters between an unescaped # outside a character class and the next new-
       line, inclusive, are also ignored. This  is  equivalent  to  Perl's  /x
       option,  and  it  can be changed within a pattern by a (?x) option set-
       ting.

       Which characters are interpreted  as  newlines  is  controlled  by  the
       options  passed to pcre_compile() or by a special sequence at the start
       of the pattern, as described in the section entitled  "Newline  conven-
       tions" in the pcrepattern documentation. Note that the end of this type
       of comment is  a  literal  newline  sequence  in  the  pattern;  escape
       sequences that happen to represent a newline do not count.

       This  option  makes  it possible to include comments inside complicated
       patterns.  Note, however, that this applies only  to  data  characters.
       Whitespace   characters  may  never  appear  within  special  character
       sequences in a pattern, for example within the sequence (?( that intro-
       duces a conditional subpattern.

         PCRE_EXTRA

       This  option  was invented in order to turn on additional functionality
       of PCRE that is incompatible with Perl, but it  is  currently  of  very
       little  use. When set, any backslash in a pattern that is followed by a
       letter that has no special meaning  causes  an  error,  thus  reserving
       these  combinations  for  future  expansion.  By default, as in Perl, a
       backslash followed by a letter with no special meaning is treated as  a
       literal. (Perl can, however, be persuaded to give an error for this, by
       running it with the -w option.) There are at present no other  features
       controlled  by this option. It can also be set by a (?X) option setting
       within a pattern.

         PCRE_FIRSTLINE

       If this option is set, an  unanchored  pattern  is  required  to  match
       before  or  at  the  first  newline  in  the subject string, though the
       matched text may continue over the newline.

         PCRE_JAVASCRIPT_COMPAT

       If this option is set, PCRE's behaviour is changed in some ways so that
       it  is  compatible with JavaScript rather than Perl. The changes are as
       follows:

       (1) A lone closing square bracket in a pattern  causes  a  compile-time
       error,  because this is illegal in JavaScript (by default it is treated
       as a data character). Thus, the pattern AB]CD becomes illegal when this
       option is set.

       (2)  At run time, a back reference to an unset subpattern group matches
       an empty string (by default this causes the current  matching  alterna-
       tive  to  fail). A pattern such as (\1)(a) succeeds when this option is
       set (assuming it can find an "a" in the subject), whereas it  fails  by
       default, for Perl compatibility.

         PCRE_MULTILINE

       By  default,  PCRE  treats the subject string as consisting of a single
       line of characters (even if it actually contains newlines). The  "start
       of  line"  metacharacter  (^)  matches only at the start of the string,
       while the "end of line" metacharacter ($) matches only at  the  end  of
       the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
       is set). This is the same as Perl.

       When PCRE_MULTILINE it is set, the "start of line" and  "end  of  line"
       constructs  match  immediately following or immediately before internal
       newlines in the subject string, respectively, as well as  at  the  very
       start  and  end.  This is equivalent to Perl's /m option, and it can be
       changed within a pattern by a (?m) option setting. If there are no new-
       lines  in  a  subject string, or no occurrences of ^ or $ in a pattern,
       setting PCRE_MULTILINE has no effect.

         PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

       These options override the default newline definition that  was  chosen
       when  PCRE  was built. Setting the first or the second specifies that a
       newline is indicated by a single character (CR  or  LF,  respectively).
       Setting  PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
       two-character CRLF  sequence.  Setting  PCRE_NEWLINE_ANYCRLF  specifies
       that any of the three preceding sequences should be recognized. Setting
       PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should  be
       recognized. The Unicode newline sequences are the three just mentioned,
       plus the single characters VT (vertical  tab,  U+000B),  FF  (formfeed,
       U+000C),  NEL  (next line, U+0085), LS (line separator, U+2028), and PS
       (paragraph separator, U+2029). The last  two  are  recognized  only  in
       UTF-8 mode.

       The  newline  setting  in  the  options  word  uses three bits that are
       treated as a number, giving eight possibilities. Currently only six are
       used  (default  plus the five values above). This means that if you set
       more than one newline option, the combination may or may not be  sensi-
       ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
       PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers  and
       cause an error.

       The  only  time  that a line break in a pattern is specially recognized
       when compiling is when PCRE_EXTENDED is set. CR and LF  are  whitespace
       characters,  and so are ignored in this mode. Also, an unescaped # out-
       side a character class indicates a comment that lasts until  after  the
       next  line break sequence. In other circumstances, line break sequences
       in patterns are treated as literal data.

       The newline option that is set at compile time becomes the default that
       is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.

         PCRE_NO_AUTO_CAPTURE

       If this option is set, it disables the use of numbered capturing paren-
       theses in the pattern. Any opening parenthesis that is not followed  by
       ?  behaves as if it were followed by ?: but named parentheses can still
       be used for capturing (and they acquire  numbers  in  the  usual  way).
       There is no equivalent of this option in Perl.

         NO_START_OPTIMIZE

       This  is an option that acts at matching time; that is, it is really an
       option for pcre_exec() or pcre_dfa_exec(). If  it  is  set  at  compile
       time,  it is remembered with the compiled pattern and assumed at match-
       ing time. For details  see  the  discussion  of  PCRE_NO_START_OPTIMIZE
       below.

         PCRE_UCP

       This  option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W,
       \w, and some of the POSIX character classes.  By  default,  only  ASCII
       characters  are  recognized, but if PCRE_UCP is set, Unicode properties
       are used instead to classify characters. More details are given in  the
       section  on generic character types in the pcrepattern page. If you set
       PCRE_UCP, matching one of the items it affects takes much  longer.  The
       option  is  available only if PCRE has been compiled with Unicode prop-
       erty support.

         PCRE_UNGREEDY

       This option inverts the "greediness" of the quantifiers  so  that  they
       are  not greedy by default, but become greedy if followed by "?". It is
       not compatible with Perl. It can also be set by a (?U)  option  setting
       within the pattern.

         PCRE_UTF8

       This  option  causes PCRE to regard both the pattern and the subject as
       strings of UTF-8 characters instead of single-byte  character  strings.
       However,  it is available only when PCRE is built to include UTF-8 sup-
       port. If not, the use of this option provokes an error. Details of  how
       this  option  changes the behaviour of PCRE are given in the section on
       UTF-8 support in the main pcre page.

         PCRE_NO_UTF8_CHECK

       When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
       automatically  checked.  There  is  a  discussion about the validity of
       UTF-8 strings in the main pcre page. If an invalid  UTF-8  sequence  of
       bytes  is  found,  pcre_compile() returns an error. If you already know
       that your pattern is valid, and you want to skip this check for perfor-
       mance  reasons,  you  can set the PCRE_NO_UTF8_CHECK option. When it is
       set, the effect of passing an invalid UTF-8  string  as  a  pattern  is
       undefined.  It  may  cause your program to crash. Note that this option
       can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress  the
       UTF-8 validity checking of subject strings.


COMPILATION ERROR CODES

       The  following  table  lists  the  error  codes than may be returned by
       pcre_compile2(), along with the error messages that may be returned  by
       both  compiling functions. As PCRE has developed, some error codes have
       fallen out of use. To avoid confusion, they have not been re-used.

          0  no error
          1  \ at end of pattern
          2  \c at end of pattern
          3  unrecognized character follows \
          4  numbers out of order in {} quantifier
          5  number too big in {} quantifier
          6  missing terminating ] for character class
          7  invalid escape sequence in character class
          8  range out of order in character class
          9  nothing to repeat
         10  [this code is not in use]
         11  internal error: unexpected repeat
         12  unrecognized character after (? or (?-
         13  POSIX named classes are supported only within a class
         14  missing )
         15  reference to non-existent subpattern
         16  erroffset passed as NULL
         17  unknown option bit(s) set
         18  missing ) after comment
         19  [this code is not in use]
         20  regular expression is too large
         21  failed to get memory
         22  unmatched parentheses
         23  internal error: code overflow
         24  unrecognized character after (?<
         25  lookbehind assertion is not fixed length
         26  malformed number or name after (?(
         27  conditional group contains more than two branches
         28  assertion expected after (?(
         29  (?R or (?[+-]digits must be followed by )
         30  unknown POSIX class name
         31  POSIX collating elements are not supported
         32  this version of PCRE is not compiled with PCRE_UTF8 support
         33  [this code is not in use]
         34  character value in \x{...} sequence is too large
         35  invalid condition (?(0)
         36  \C not allowed in lookbehind assertion
         37  PCRE does not support \L, \l, \N, \U, or \u
         38  number after (?C is > 255
         39  closing ) for (?C expected
         40  recursive call could loop indefinitely
         41  unrecognized character after (?P
         42  syntax error in subpattern name (missing terminator)
         43  two named subpatterns have the same name
         44  invalid UTF-8 string
         45  support for \P, \p, and \X has not been compiled
         46  malformed \P or \p sequence
         47  unknown property name after \P or \p
         48  subpattern name is too long (maximum 32 characters)
         49  too many named subpatterns (maximum 10000)
         50  [this code is not in use]
         51  octal value is greater than \377 (not in UTF-8 mode)
         52  internal error: overran compiling workspace
         53  internal error: previously-checked referenced subpattern
               not found
         54  DEFINE group contains more than one branch
         55  repeating a DEFINE group is not allowed
         56  inconsistent NEWLINE options
         57  \g is not followed by a braced, angle-bracketed, or quoted
               name/number or by a plain number
         58  a numbered reference must not be zero
         59  an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
         60  (*VERB) not recognized
         61  number is too big
         62  subpattern name expected
         63  digit expected after (?+
         64  ] is an invalid data character in JavaScript compatibility mode
         65  different names for subpatterns of the same number are
               not allowed
         66  (*MARK) must have an argument
         67  this version of PCRE is not compiled with PCRE_UCP support

       The numbers 32 and 10000 in errors 48 and 49  are  defaults;  different
       values may be used if the limits were changed when PCRE was built.


STUDYING A PATTERN

       pcre_extra *pcre_study(const pcre *code, int options
            const char **errptr);

       If  a  compiled  pattern is going to be used several times, it is worth
       spending more time analyzing it in order to speed up the time taken for
       matching.  The function pcre_study() takes a pointer to a compiled pat-
       tern as its first argument. If studying the pattern produces additional
       information  that  will  help speed up matching, pcre_study() returns a
       pointer to a pcre_extra block, in which the study_data field points  to
       the results of the study.

       The  returned  value  from  pcre_study()  can  be  passed  directly  to
       pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block  also  con-
       tains  other  fields  that can be set by the caller before the block is
       passed; these are described below in the section on matching a pattern.

       If studying the  pattern  does  not  produce  any  useful  information,
       pcre_study() returns NULL. In that circumstance, if the calling program
       wants  to  pass  any  of   the   other   fields   to   pcre_exec()   or
       pcre_dfa_exec(), it must set up its own pcre_extra block.

       The  second  argument of pcre_study() contains option bits. At present,
       no options are defined, and this argument should always be zero.

       The third argument for pcre_study() is a pointer for an error  message.
       If  studying  succeeds  (even  if no data is returned), the variable it
       points to is set to NULL. Otherwise it is set to  point  to  a  textual
       error message. This is a static string that is part of the library. You
       must not try to free it. You should test the  error  pointer  for  NULL
       after calling pcre_study(), to be sure that it has run successfully.

       This is a typical call to pcre_study():

         pcre_extra *pe;
         pe = pcre_study(
           re,             /* result of pcre_compile() */
           0,              /* no options exist */
           &error);        /* set to NULL or points to a message */

       Studying a pattern does two things: first, a lower bound for the length
       of subject string that is needed to match the pattern is computed. This
       does not mean that there are any strings of that length that match, but
       it does guarantee that no shorter strings match. The value is  used  by
       pcre_exec()  and  pcre_dfa_exec()  to  avoid  wasting time by trying to
       match strings that are shorter than the lower bound. You can  find  out
       the value in a calling program via the pcre_fullinfo() function.

       Studying a pattern is also useful for non-anchored patterns that do not
       have a single fixed starting character. A bitmap of  possible  starting
       bytes  is  created. This speeds up finding a position in the subject at
       which to start matching.

       The two optimizations just described can be  disabled  by  setting  the
       PCRE_NO_START_OPTIMIZE    option    when    calling    pcre_exec()   or
       pcre_dfa_exec(). You might want to do this  if  your  pattern  contains
       callouts  or  (*MARK),  and you want to make use of these facilities in
       cases where matching fails. See the discussion  of  PCRE_NO_START_OPTI-
       MIZE below.


LOCALE SUPPORT

       PCRE  handles  caseless matching, and determines whether characters are
       letters, digits, or whatever, by reference to a set of tables,  indexed
       by  character  value.  When running in UTF-8 mode, this applies only to
       characters with codes less than 128. By  default,  higher-valued  codes
       never match escapes such as \w or \d, but they can be tested with \p if
       PCRE is built with Unicode character property  support.  Alternatively,
       the  PCRE_UCP  option  can  be  set at compile time; this causes \w and
       friends to use Unicode property support instead of built-in tables. The
       use of locales with Unicode is discouraged. If you are handling charac-
       ters with codes greater than 128, you should either use UTF-8 and  Uni-
       code, or use locales, but not try to mix the two.

       PCRE  contains  an  internal set of tables that are used when the final
       argument of pcre_compile() is  NULL.  These  are  sufficient  for  many
       applications.  Normally, the internal tables recognize only ASCII char-
       acters. However, when PCRE is built, it is possible to cause the inter-
       nal tables to be rebuilt in the default "C" locale of the local system,
       which may cause them to be different.

       The internal tables can always be overridden by tables supplied by  the
       application that calls PCRE. These may be created in a different locale
       from the default. As more and more applications change  to  using  Uni-
       code, the need for this locale support is expected to die away.

       External  tables  are  built by calling the pcre_maketables() function,
       which has no arguments, in the relevant locale. The result can then  be
       passed  to  pcre_compile()  or  pcre_exec()  as often as necessary. For
       example, to build and use tables that are appropriate  for  the  French
       locale  (where  accented  characters  with  values greater than 128 are
       treated as letters), the following code could be used:

         setlocale(LC_CTYPE, "fr_FR");
         tables = pcre_maketables();
         re = pcre_compile(..., tables);

       The locale name "fr_FR" is used on Linux and other  Unix-like  systems;
       if you are using Windows, the name for the French locale is "french".

       When  pcre_maketables()  runs,  the  tables are built in memory that is
       obtained via pcre_malloc. It is the caller's responsibility  to  ensure
       that  the memory containing the tables remains available for as long as
       it is needed.

       The pointer that is passed to pcre_compile() is saved with the compiled
       pattern,  and the same tables are used via this pointer by pcre_study()
       and normally also by pcre_exec(). Thus, by default, for any single pat-
       tern, compilation, studying and matching all happen in the same locale,
       but different patterns can be compiled in different locales.

       It is possible to pass a table pointer or NULL (indicating the  use  of
       the  internal  tables)  to  pcre_exec(). Although not intended for this
       purpose, this facility could be used to match a pattern in a  different
       locale from the one in which it was compiled. Passing table pointers at
       run time is discussed below in the section on matching a pattern.


INFORMATION ABOUT A PATTERN

       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
            int what, void *where);

       The pcre_fullinfo() function returns information about a compiled  pat-
       tern. It replaces the obsolete pcre_info() function, which is neverthe-
       less retained for backwards compability (and is documented below).

       The first argument for pcre_fullinfo() is a  pointer  to  the  compiled
       pattern.  The second argument is the result of pcre_study(), or NULL if
       the pattern was not studied. The third argument specifies  which  piece
       of  information  is required, and the fourth argument is a pointer to a
       variable to receive the data. The yield of the  function  is  zero  for
       success, or one of the following negative numbers:

         PCRE_ERROR_NULL       the argument code was NULL
                               the argument where was NULL
         PCRE_ERROR_BADMAGIC   the "magic number" was not found
         PCRE_ERROR_BADOPTION  the value of what was invalid

       The  "magic  number" is placed at the start of each compiled pattern as
       an simple check against passing an arbitrary memory pointer. Here is  a
       typical  call  of pcre_fullinfo(), to obtain the length of the compiled
       pattern:

         int rc;
         size_t length;
         rc = pcre_fullinfo(
           re,               /* result of pcre_compile() */
           pe,               /* result of pcre_study(), or NULL */
           PCRE_INFO_SIZE,   /* what is required */
           &length);         /* where to put the data */

       The possible values for the third argument are defined in  pcre.h,  and
       are as follows:

         PCRE_INFO_BACKREFMAX

       Return  the  number  of  the highest back reference in the pattern. The
       fourth argument should point to an int variable. Zero  is  returned  if
       there are no back references.

         PCRE_INFO_CAPTURECOUNT

       Return  the  number of capturing subpatterns in the pattern. The fourth
       argument should point to an int variable.

         PCRE_INFO_DEFAULT_TABLES

       Return a pointer to the internal default character tables within  PCRE.
       The  fourth  argument should point to an unsigned char * variable. This
       information call is provided for internal use by the pcre_study() func-
       tion.  External  callers  can  cause PCRE to use its internal tables by
       passing a NULL table pointer.

         PCRE_INFO_FIRSTBYTE

       Return information about the first byte of any matched  string,  for  a
       non-anchored  pattern. The fourth argument should point to an int vari-
       able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old  name
       is still recognized for backwards compatibility.)

       If  there  is  a  fixed first byte, for example, from a pattern such as
       (cat|cow|coyote), its value is returned. Otherwise, if either

       (a) the pattern was compiled with the PCRE_MULTILINE option, and  every
       branch starts with "^", or

       (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
       set (if it were set, the pattern would be anchored),

       -1 is returned, indicating that the pattern matches only at  the  start
       of  a  subject string or after any newline within the string. Otherwise
       -2 is returned. For anchored patterns, -2 is returned.

         PCRE_INFO_FIRSTTABLE

       If the pattern was studied, and this resulted in the construction of  a
       256-bit table indicating a fixed set of bytes for the first byte in any
       matching string, a pointer to the table is returned. Otherwise NULL  is
       returned.  The fourth argument should point to an unsigned char * vari-
       able.

         PCRE_INFO_HASCRORLF

       Return 1 if the pattern contains any explicit  matches  for  CR  or  LF
       characters,  otherwise  0.  The  fourth argument should point to an int
       variable. An explicit match is either a literal CR or LF character,  or
       \r or \n.

         PCRE_INFO_JCHANGED

       Return  1  if  the (?J) or (?-J) option setting is used in the pattern,
       otherwise 0. The fourth argument should point to an int variable.  (?J)
       and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.

         PCRE_INFO_LASTLITERAL

       Return  the  value of the rightmost literal byte that must exist in any
       matched string, other than at its  start,  if  such  a  byte  has  been
       recorded. The fourth argument should point to an int variable. If there
       is no such byte, -1 is returned. For anchored patterns, a last  literal
       byte  is  recorded only if it follows something of variable length. For
       example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
       /^a\dz\d/ the returned value is -1.

         PCRE_INFO_MINLENGTH

       If  the  pattern  was studied and a minimum length for matching subject
       strings was computed, its value is  returned.  Otherwise  the  returned
       value  is  -1. The value is a number of characters, not bytes (this may
       be relevant in UTF-8 mode). The fourth argument should point to an  int
       variable.  A  non-negative  value is a lower bound to the length of any
       matching string. There may not be any strings of that  length  that  do
       actually match, but every string that does match is at least that long.

         PCRE_INFO_NAMECOUNT
         PCRE_INFO_NAMEENTRYSIZE
         PCRE_INFO_NAMETABLE

       PCRE  supports the use of named as well as numbered capturing parenthe-
       ses. The names are just an additional way of identifying the  parenthe-
       ses, which still acquire numbers. Several convenience functions such as
       pcre_get_named_substring() are provided for  extracting  captured  sub-
       strings  by  name. It is also possible to extract the data directly, by
       first converting the name to a number in order to  access  the  correct
       pointers in the output vector (described with pcre_exec() below). To do
       the conversion, you need  to  use  the  name-to-number  map,  which  is
       described by these three values.

       The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
       gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
       of  each  entry;  both  of  these  return  an int value. The entry size
       depends on the length of the longest name. PCRE_INFO_NAMETABLE  returns
       a  pointer  to  the  first  entry of the table (a pointer to char). The
       first two bytes of each entry are the number of the capturing parenthe-
       sis,  most  significant byte first. The rest of the entry is the corre-
       sponding name, zero terminated.

       The names are in alphabetical order. Duplicate names may appear if  (?|
       is used to create multiple groups with the same number, as described in
       the section on duplicate subpattern numbers in  the  pcrepattern  page.
       Duplicate  names  for  subpatterns with different numbers are permitted
       only if PCRE_DUPNAMES is set. In all cases  of  duplicate  names,  they
       appear  in  the table in the order in which they were found in the pat-
       tern. In the absence of (?| this is the  order  of  increasing  number;
       when (?| is used this is not necessarily the case because later subpat-
       terns may have lower numbers.

       As a simple example of the name/number table,  consider  the  following
       pattern  (assume  PCRE_EXTENDED is set, so white space - including new-
       lines - is ignored):

         (?<date> (?<year>(\d\d)?\d\d) -
         (?<month>\d\d) - (?<day>\d\d) )

       There are four named subpatterns, so the table has  four  entries,  and
       each  entry  in the table is eight bytes long. The table is as follows,
       with non-printing bytes shows in hexadecimal, and undefined bytes shown
       as ??:

         00 01 d  a  t  e  00 ??
         00 05 d  a  y  00 ?? ??
         00 04 m  o  n  t  h  00
         00 02 y  e  a  r  00 ??

       When  writing  code  to  extract  data from named subpatterns using the
       name-to-number map, remember that the length of the entries  is  likely
       to be different for each compiled pattern.

         PCRE_INFO_OKPARTIAL

       Return  1  if  the  pattern  can  be  used  for  partial  matching with
       pcre_exec(), otherwise 0. The fourth argument should point  to  an  int
       variable.  From  release  8.00,  this  always  returns  1,  because the
       restrictions that previously applied  to  partial  matching  have  been
       lifted.  The  pcrepartial documentation gives details of partial match-
       ing.

         PCRE_INFO_OPTIONS

       Return a copy of the options with which the pattern was  compiled.  The
       fourth  argument  should  point to an unsigned long int variable. These
       option bits are those specified in the call to pcre_compile(), modified
       by any top-level option settings at the start of the pattern itself. In
       other words, they are the options that will be in force  when  matching
       starts.  For  example, if the pattern /(?im)abc(?-i)d/ is compiled with
       the PCRE_EXTENDED option, the result is PCRE_CASELESS,  PCRE_MULTILINE,
       and PCRE_EXTENDED.

       A  pattern  is  automatically  anchored by PCRE if all of its top-level
       alternatives begin with one of the following:

         ^     unless PCRE_MULTILINE is set
         \A    always
         \G    always
         .*    if PCRE_DOTALL is set and there are no back
                 references to the subpattern in which .* appears

       For such patterns, the PCRE_ANCHORED bit is set in the options returned
       by pcre_fullinfo().

         PCRE_INFO_SIZE

       Return  the  size  of the compiled pattern, that is, the value that was
       passed as the argument to pcre_malloc() when PCRE was getting memory in
       which to place the compiled data. The fourth argument should point to a
       size_t variable.

         PCRE_INFO_STUDYSIZE

       Return the size of the data block pointed to by the study_data field in
       a  pcre_extra  block.  That  is,  it  is  the  value that was passed to
       pcre_malloc() when PCRE was getting memory into which to place the data
       created  by  pcre_study().  If pcre_extra is NULL, or there is no study
       data, zero is returned. The fourth argument should point  to  a  size_t
       variable.


OBSOLETE INFO FUNCTION

       int pcre_info(const pcre *code, int *optptr, int *firstcharptr);

       The  pcre_info()  function is now obsolete because its interface is too
       restrictive to return all the available data about a compiled  pattern.
       New   programs   should  use  pcre_fullinfo()  instead.  The  yield  of
       pcre_info() is the number of capturing subpatterns, or one of the  fol-
       lowing negative numbers:

         PCRE_ERROR_NULL       the argument code was NULL
         PCRE_ERROR_BADMAGIC   the "magic number" was not found

       If  the  optptr  argument is not NULL, a copy of the options with which
       the pattern was compiled is placed in the integer  it  points  to  (see
       PCRE_INFO_OPTIONS above).

       If  the  pattern  is  not anchored and the firstcharptr argument is not
       NULL, it is used to pass back information about the first character  of
       any matched string (see PCRE_INFO_FIRSTBYTE above).


REFERENCE COUNTS

       int pcre_refcount(pcre *code, int adjust);

       The  pcre_refcount()  function is used to maintain a reference count in
       the data block that contains a compiled pattern. It is provided for the
       benefit  of  applications  that  operate  in an object-oriented manner,
       where different parts of the application may be using the same compiled
       pattern, but you want to free the block when they are all done.

       When a pattern is compiled, the reference count field is initialized to
       zero.  It is changed only by calling this function, whose action is  to
       add  the  adjust  value  (which may be positive or negative) to it. The
       yield of the function is the new value. However, the value of the count
       is  constrained to lie between 0 and 65535, inclusive. If the new value
       is outside these limits, it is forced to the appropriate limit value.

       Except when it is zero, the reference count is not correctly  preserved
       if  a  pattern  is  compiled on one host and then transferred to a host
       whose byte-order is different. (This seems a highly unlikely scenario.)


MATCHING A PATTERN: THE TRADITIONAL FUNCTION

       int pcre_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       The function pcre_exec() is called to match a subject string against  a
       compiled  pattern, which is passed in the code argument. If the pattern
       was studied, the result of the study should  be  passed  in  the  extra
       argument.  This  function is the main matching facility of the library,
       and it operates in a Perl-like manner. For specialist use there is also
       an  alternative matching function, which is described below in the sec-
       tion about the pcre_dfa_exec() function.

       In most applications, the pattern will have been compiled (and  option-
       ally  studied)  in the same process that calls pcre_exec(). However, it
       is possible to save compiled patterns and study data, and then use them
       later  in  different processes, possibly even on different hosts. For a
       discussion about this, see the pcreprecompile documentation.

       Here is an example of a simple call to pcre_exec():

         int rc;
         int ovector[30];
         rc = pcre_exec(
           re,             /* result of pcre_compile() */
           NULL,           /* we didn't study the pattern */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           ovector,        /* vector of integers for substring information */
           30);            /* number of elements (NOT size in bytes) */

   Extra data for pcre_exec()

       If the extra argument is not NULL, it must point to a  pcre_extra  data
       block.  The pcre_study() function returns such a block (when it doesn't
       return NULL), but you can also create one for yourself, and pass  addi-
       tional  information  in it. The pcre_extra block contains the following
       fields (not necessarily in this order):

         unsigned long int flags;
         void *study_data;
         unsigned long int match_limit;
         unsigned long int match_limit_recursion;
         void *callout_data;
         const unsigned char *tables;
         unsigned char **mark;

       The flags field is a bitmap that specifies which of  the  other  fields
       are set. The flag bits are:

         PCRE_EXTRA_STUDY_DATA
         PCRE_EXTRA_MATCH_LIMIT
         PCRE_EXTRA_MATCH_LIMIT_RECURSION
         PCRE_EXTRA_CALLOUT_DATA
         PCRE_EXTRA_TABLES
         PCRE_EXTRA_MARK

       Other  flag  bits should be set to zero. The study_data field is set in
       the pcre_extra block that is returned by  pcre_study(),  together  with
       the appropriate flag bit. You should not set this yourself, but you may
       add to the block by setting the other fields  and  their  corresponding
       flag bits.

       The match_limit field provides a means of preventing PCRE from using up
       a vast amount of resources when running patterns that are not going  to
       match,  but  which  have  a very large number of possibilities in their
       search trees. The classic example is a pattern that uses nested  unlim-
       ited repeats.

       Internally,  PCRE uses a function called match() which it calls repeat-
       edly (sometimes recursively). The limit set by match_limit  is  imposed
       on  the  number  of times this function is called during a match, which
       has the effect of limiting the amount of  backtracking  that  can  take
       place. For patterns that are not anchored, the count restarts from zero
       for each position in the subject string.

       The default value for the limit can be set  when  PCRE  is  built;  the
       default  default  is 10 million, which handles all but the most extreme
       cases. You can override the default  by  suppling  pcre_exec()  with  a
       pcre_extra     block    in    which    match_limit    is    set,    and
       PCRE_EXTRA_MATCH_LIMIT is set in the  flags  field.  If  the  limit  is
       exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.

       The  match_limit_recursion field is similar to match_limit, but instead
       of limiting the total number of times that match() is called, it limits
       the  depth  of  recursion. The recursion depth is a smaller number than
       the total number of calls, because not all calls to match() are  recur-
       sive.  This limit is of use only if it is set smaller than match_limit.

       Limiting  the  recursion  depth  limits the amount of stack that can be
       used, or, when PCRE has been compiled to use memory on the heap instead
       of the stack, the amount of heap memory that can be used.

       The  default  value  for  match_limit_recursion can be set when PCRE is
       built; the default default  is  the  same  value  as  the  default  for
       match_limit.  You can override the default by suppling pcre_exec() with
       a  pcre_extra  block  in  which  match_limit_recursion  is   set,   and
       PCRE_EXTRA_MATCH_LIMIT_RECURSION  is  set  in  the  flags field. If the
       limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.

       The callout_data field is used in conjunction with the  "callout"  fea-
       ture, and is described in the pcrecallout documentation.

       The  tables  field  is  used  to  pass  a  character  tables pointer to
       pcre_exec(); this overrides the value that is stored with the  compiled
       pattern.  A  non-NULL value is stored with the compiled pattern only if
       custom tables were supplied to pcre_compile() via  its  tableptr  argu-
       ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
       PCRE's internal tables to be used. This facility is  helpful  when  re-
       using  patterns  that  have been saved after compiling with an external
       set of tables, because the external tables  might  be  at  a  different
       address  when  pcre_exec() is called. See the pcreprecompile documenta-
       tion for a discussion of saving compiled patterns for later use.

       If PCRE_EXTRA_MARK is set in the flags field, the mark  field  must  be
       set  to  point  to a char * variable. If the pattern contains any back-
       tracking control verbs such as (*MARK:NAME), and the execution ends  up
       with  a  name  to  pass back, a pointer to the name string (zero termi-
       nated) is placed in the variable pointed to  by  the  mark  field.  The
       names  are  within  the  compiled pattern; if you wish to retain such a
       name you must copy it before freeing the memory of a compiled  pattern.
       If  there  is no name to pass back, the variable pointed to by the mark
       field set to NULL. For details of the backtracking control  verbs,  see
       the section entitled "Backtracking control" in the pcrepattern documen-
       tation.

   Option bits for pcre_exec()

       The unused bits of the options argument for pcre_exec() must  be  zero.
       The  only  bits  that  may  be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
       PCRE_NOTBOL,   PCRE_NOTEOL,    PCRE_NOTEMPTY,    PCRE_NOTEMPTY_ATSTART,
       PCRE_NO_START_OPTIMIZE,   PCRE_NO_UTF8_CHECK,   PCRE_PARTIAL_SOFT,  and
       PCRE_PARTIAL_HARD.

         PCRE_ANCHORED

       The PCRE_ANCHORED option limits pcre_exec() to matching  at  the  first
       matching  position.  If  a  pattern was compiled with PCRE_ANCHORED, or
       turned out to be anchored by virtue of its contents, it cannot be  made
       unachored at matching time.

         PCRE_BSR_ANYCRLF
         PCRE_BSR_UNICODE

       These options (which are mutually exclusive) control what the \R escape
       sequence matches. The choice is either to match only CR, LF,  or  CRLF,
       or  to  match  any Unicode newline sequence. These options override the
       choice that was made or defaulted when the pattern was compiled.

         PCRE_NEWLINE_CR
         PCRE_NEWLINE_LF
         PCRE_NEWLINE_CRLF
         PCRE_NEWLINE_ANYCRLF
         PCRE_NEWLINE_ANY

       These options override  the  newline  definition  that  was  chosen  or
       defaulted  when the pattern was compiled. For details, see the descrip-
       tion of pcre_compile()  above.  During  matching,  the  newline  choice
       affects  the  behaviour  of the dot, circumflex, and dollar metacharac-
       ters. It may also alter the way the match position is advanced after  a
       match failure for an unanchored pattern.

       When  PCRE_NEWLINE_CRLF,  PCRE_NEWLINE_ANYCRLF,  or PCRE_NEWLINE_ANY is
       set, and a match attempt for an unanchored pattern fails when the  cur-
       rent  position  is  at  a  CRLF  sequence,  and the pattern contains no
       explicit matches for  CR  or  LF  characters,  the  match  position  is
       advanced by two characters instead of one, in other words, to after the
       CRLF.

       The above rule is a compromise that makes the most common cases work as
       expected.  For  example,  if  the  pattern  is .+A (and the PCRE_DOTALL
       option is not set), it does not match the string "\r\nA" because, after
       failing  at the start, it skips both the CR and the LF before retrying.
       However, the pattern [\r\n]A does match that string,  because  it  con-
       tains an explicit CR or LF reference, and so advances only by one char-
       acter after the first failure.

       An explicit match for CR of LF is either a literal appearance of one of
       those  characters,  or  one  of the \r or \n escape sequences. Implicit
       matches such as [^X] do not count, nor does \s (which includes  CR  and
       LF in the characters that it matches).

       Notwithstanding  the above, anomalous effects may still occur when CRLF
       is a valid newline sequence and explicit \r or \n escapes appear in the
       pattern.

         PCRE_NOTBOL

       This option specifies that first character of the subject string is not
       the beginning of a line, so the  circumflex  metacharacter  should  not
       match  before it. Setting this without PCRE_MULTILINE (at compile time)
       causes circumflex never to match. This option affects only  the  behav-
       iour of the circumflex metacharacter. It does not affect \A.

         PCRE_NOTEOL

       This option specifies that the end of the subject string is not the end
       of a line, so the dollar metacharacter should not match it nor  (except
       in  multiline mode) a newline immediately before it. Setting this with-
       out PCRE_MULTILINE (at compile time) causes dollar never to match. This
       option  affects only the behaviour of the dollar metacharacter. It does
       not affect \Z or \z.

         PCRE_NOTEMPTY

       An empty string is not considered to be a valid match if this option is
       set.  If  there are alternatives in the pattern, they are tried. If all
       the alternatives match the empty string, the entire  match  fails.  For
       example, if the pattern

         a?b?

       is  applied  to  a  string not beginning with "a" or "b", it matches an
       empty string at the start of the subject. With PCRE_NOTEMPTY set,  this
       match is not valid, so PCRE searches further into the string for occur-
       rences of "a" or "b".

         PCRE_NOTEMPTY_ATSTART

       This is like PCRE_NOTEMPTY, except that an empty string match  that  is
       not  at  the  start  of  the  subject  is  permitted. If the pattern is
       anchored, such a match can occur only if the pattern contains \K.

       Perl    has    no    direct    equivalent    of    PCRE_NOTEMPTY     or
       PCRE_NOTEMPTY_ATSTART,  but  it  does  make a special case of a pattern
       match of the empty string within its split() function, and  when  using
       the  /g  modifier.  It  is  possible  to emulate Perl's behaviour after
       matching a null string by first trying the match again at the same off-
       set  with  PCRE_NOTEMPTY_ATSTART  and  PCRE_ANCHORED,  and then if that
       fails, by advancing the starting offset (see below) and trying an ordi-
       nary  match  again. There is some code that demonstrates how to do this
       in the pcredemo sample program. In the most general case, you  have  to
       check  to  see  if the newline convention recognizes CRLF as a newline,
       and if so, and the current character is CR followed by LF, advance  the
       starting offset by two characters instead of one.

         PCRE_NO_START_OPTIMIZE

       There  are a number of optimizations that pcre_exec() uses at the start
       of a match, in order to speed up the process. For  example,  if  it  is
       known that an unanchored match must start with a specific character, it
       searches the subject for that character, and fails  immediately  if  it
       cannot  find  it,  without actually running the main matching function.
       This means that a special item such as (*COMMIT) at the start of a pat-
       tern  is  not  considered until after a suitable starting point for the
       match has been found. When callouts or (*MARK) items are in use,  these
       "start-up" optimizations can cause them to be skipped if the pattern is
       never actually used. The start-up optimizations are in  effect  a  pre-
       scan of the subject that takes place before the pattern is run.

       The  PCRE_NO_START_OPTIMIZE option disables the start-up optimizations,
       possibly causing performance to suffer,  but  ensuring  that  in  cases
       where  the  result is "no match", the callouts do occur, and that items
       such as (*COMMIT) and (*MARK) are considered at every possible starting
       position  in  the  subject  string. If PCRE_NO_START_OPTIMIZE is set at
       compile time, it cannot be unset at matching time.

       Setting PCRE_NO_START_OPTIMIZE can change the  outcome  of  a  matching
       operation.  Consider the pattern

         (*COMMIT)ABC

       When  this  is  compiled, PCRE records the fact that a match must start
       with the character "A". Suppose the subject  string  is  "DEFABC".  The
       start-up  optimization  scans along the subject, finds "A" and runs the
       first match attempt from there. The (*COMMIT) item means that the  pat-
       tern  must  match the current starting position, which in this case, it
       does. However, if the same match  is  run  with  PCRE_NO_START_OPTIMIZE
       set,  the  initial  scan  along the subject string does not happen. The
       first match attempt is run starting  from  "D"  and  when  this  fails,
       (*COMMIT)  prevents  any  further  matches  being tried, so the overall
       result is "no match". If the pattern is studied,  more  start-up  opti-
       mizations  may  be  used. For example, a minimum length for the subject
       may be recorded. Consider the pattern

         (*MARK:A)(X|Y)

       The minimum length for a match is one  character.  If  the  subject  is
       "ABC",  there  will  be  attempts  to  match "ABC", "BC", "C", and then
       finally an empty string.  If the pattern is studied, the final  attempt
       does  not take place, because PCRE knows that the subject is too short,
       and so the (*MARK) is never encountered.  In this  case,  studying  the
       pattern  does  not  affect the overall match result, which is still "no
       match", but it does affect the auxiliary information that is returned.

         PCRE_NO_UTF8_CHECK

       When PCRE_UTF8 is set at compile time, the validity of the subject as a
       UTF-8  string is automatically checked when pcre_exec() is subsequently
       called.  The value of startoffset is also checked  to  ensure  that  it
       points  to  the start of a UTF-8 character. There is a discussion about
       the validity of UTF-8 strings in the section on UTF-8  support  in  the
       main  pcre  page.  If  an  invalid  UTF-8  sequence  of bytes is found,
       pcre_exec() returns  the  error  PCRE_ERROR_BADUTF8  or,  if  PCRE_PAR-
       TIAL_HARD  is set and the problem is a truncated UTF-8 character at the
       end of the subject, PCRE_ERROR_SHORTUTF8.  If  startoffset  contains  a
       value  that does not point to the start of a UTF-8 character (or to the
       end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned.

       If you already know that your subject is valid, and you  want  to  skip
       these    checks    for   performance   reasons,   you   can   set   the
       PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might  want  to
       do  this  for the second and subsequent calls to pcre_exec() if you are
       making repeated calls to find all  the  matches  in  a  single  subject
       string.  However,  you  should  be  sure  that the value of startoffset
       points to the start of a UTF-8 character (or the end of  the  subject).
       When  PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid UTF-8
       string as a subject or an invalid value of  startoffset  is  undefined.
       Your program may crash.

         PCRE_PARTIAL_HARD
         PCRE_PARTIAL_SOFT

       These  options turn on the partial matching feature. For backwards com-
       patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A  partial
       match  occurs if the end of the subject string is reached successfully,
       but there are not enough subject characters to complete the  match.  If
       this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set,
       matching continues by testing any remaining alternatives.  Only  if  no
       complete  match  can be found is PCRE_ERROR_PARTIAL returned instead of
       PCRE_ERROR_NOMATCH. In other words,  PCRE_PARTIAL_SOFT  says  that  the
       caller  is  prepared to handle a partial match, but only if no complete
       match can be found.

       If PCRE_PARTIAL_HARD is set, it overrides  PCRE_PARTIAL_SOFT.  In  this
       case,  if  a  partial  match  is found, pcre_exec() immediately returns
       PCRE_ERROR_PARTIAL, without  considering  any  other  alternatives.  In
       other  words, when PCRE_PARTIAL_HARD is set, a partial match is consid-
       ered to be more important that an alternative complete match.

       In both cases, the portion of the string that was  inspected  when  the
       partial match was found is set as the first matching string. There is a
       more detailed discussion of partial and  multi-segment  matching,  with
       examples, in the pcrepartial documentation.

   The string to be matched by pcre_exec()

       The  subject string is passed to pcre_exec() as a pointer in subject, a
       length (in bytes) in length, and a starting byte offset in startoffset.
       If  this  is  negative  or  greater  than  the  length  of the subject,
       pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting  offset  is
       zero,  the  search  for a match starts at the beginning of the subject,
       and this is by far the most common case. In UTF-8 mode, the byte offset
       must  point  to  the start of a UTF-8 character (or the end of the sub-
       ject). Unlike the pattern string, the subject may contain  binary  zero
       bytes.

       A  non-zero  starting offset is useful when searching for another match
       in the same subject by calling pcre_exec() again after a previous  suc-
       cess.   Setting  startoffset differs from just passing over a shortened
       string and setting PCRE_NOTBOL in the case of  a  pattern  that  begins
       with any kind of lookbehind. For example, consider the pattern

         \Biss\B

       which  finds  occurrences  of "iss" in the middle of words. (\B matches
       only if the current position in the subject is not  a  word  boundary.)
       When  applied  to the string "Mississipi" the first call to pcre_exec()
       finds the first occurrence. If pcre_exec() is called  again  with  just
       the  remainder  of  the  subject,  namely  "issipi", it does not match,
       because \B is always false at the start of the subject, which is deemed
       to  be  a  word  boundary. However, if pcre_exec() is passed the entire
       string again, but with startoffset set to 4, it finds the second occur-
       rence  of "iss" because it is able to look behind the starting point to
       discover that it is preceded by a letter.

       Finding all the matches in a subject is tricky  when  the  pattern  can
       match an empty string. It is possible to emulate Perl's /g behaviour by
       first  trying  the  match  again  at  the   same   offset,   with   the
       PCRE_NOTEMPTY_ATSTART  and  PCRE_ANCHORED  options,  and  then  if that
       fails, advancing the starting  offset  and  trying  an  ordinary  match
       again. There is some code that demonstrates how to do this in the pcre-
       demo sample program. In the most general case, you have to check to see
       if  the newline convention recognizes CRLF as a newline, and if so, and
       the current character is CR followed by LF, advance the starting offset
       by two characters instead of one.

       If  a  non-zero starting offset is passed when the pattern is anchored,
       one attempt to match at the given offset is made. This can only succeed
       if  the  pattern  does  not require the match to be at the start of the
       subject.

   How pcre_exec() returns captured substrings

       In general, a pattern matches a certain portion of the subject, and  in
       addition,  further  substrings  from  the  subject may be picked out by
       parts of the pattern. Following the usage  in  Jeffrey  Friedl's  book,
       this  is  called "capturing" in what follows, and the phrase "capturing
       subpattern" is used for a fragment of a pattern that picks out  a  sub-
       string.  PCRE  supports several other kinds of parenthesized subpattern
       that do not cause substrings to be captured.

       Captured substrings are returned to the caller via a vector of integers
       whose  address is passed in ovector. The number of elements in the vec-
       tor is passed in ovecsize, which must be a non-negative  number.  Note:
       this argument is NOT the size of ovector in bytes.

       The  first  two-thirds of the vector is used to pass back captured sub-
       strings, each substring using a pair of integers. The  remaining  third
       of  the  vector is used as workspace by pcre_exec() while matching cap-
       turing subpatterns, and is not available for passing back  information.
       The  number passed in ovecsize should always be a multiple of three. If
       it is not, it is rounded down.

       When a match is successful, information about  captured  substrings  is
       returned  in  pairs  of integers, starting at the beginning of ovector,
       and continuing up to two-thirds of its length at the  most.  The  first
       element  of  each pair is set to the byte offset of the first character
       in a substring, and the second is set to the byte offset of  the  first
       character  after  the end of a substring. Note: these values are always
       byte offsets, even in UTF-8 mode. They are not character counts.

       The first pair of integers, ovector[0]  and  ovector[1],  identify  the
       portion  of  the subject string matched by the entire pattern. The next
       pair is used for the first capturing subpattern, and so on.  The  value
       returned by pcre_exec() is one more than the highest numbered pair that
       has been set.  For example, if two substrings have been  captured,  the
       returned  value is 3. If there are no capturing subpatterns, the return
       value from a successful match is 1, indicating that just the first pair
       of offsets has been set.

       If a capturing subpattern is matched repeatedly, it is the last portion
       of the string that it matched that is returned.

       If the vector is too small to hold all the captured substring  offsets,
       it is used as far as possible (up to two-thirds of its length), and the
       function returns a value of zero. If the substring offsets are  not  of
       interest,  pcre_exec()  may  be  called with ovector passed as NULL and
       ovecsize as zero. However, if the pattern contains back references  and
       the  ovector is not big enough to remember the related substrings, PCRE
       has to get additional memory for use during matching. Thus it  is  usu-
       ally advisable to supply an ovector.

       The pcre_fullinfo() function can be used to find out how many capturing
       subpatterns there are in a compiled  pattern.  The  smallest  size  for
       ovector  that  will allow for n captured substrings, in addition to the
       offsets of the substring matched by the whole pattern, is (n+1)*3.

       It is possible for capturing subpattern number n+1 to match  some  part
       of the subject when subpattern n has not been used at all. For example,
       if the string "abc" is matched  against  the  pattern  (a|(z))(bc)  the
       return from the function is 4, and subpatterns 1 and 3 are matched, but
       2 is not. When this happens, both values in  the  offset  pairs  corre-
       sponding to unused subpatterns are set to -1.

       Offset  values  that correspond to unused subpatterns at the end of the
       expression are also set to -1. For example,  if  the  string  "abc"  is
       matched  against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
       matched. The return from the function is 2, because  the  highest  used
       capturing  subpattern  number  is 1, and the offsets for for the second
       and third capturing subpatterns (assuming the vector is  large  enough,
       of course) are set to -1.

       Note: Elements of ovector that do not correspond to capturing parenthe-
       ses in the pattern are never changed. That is, if a pattern contains  n
       capturing parentheses, no more than ovector[0] to ovector[2n+1] are set
       by pcre_exec(). The other elements retain whatever values  they  previ-
       ously had.

       Some  convenience  functions  are  provided for extracting the captured
       substrings as separate strings. These are described below.

   Error return values from pcre_exec()

       If pcre_exec() fails, it returns a negative number. The  following  are
       defined in the header file:

         PCRE_ERROR_NOMATCH        (-1)

       The subject string did not match the pattern.

         PCRE_ERROR_NULL           (-2)

       Either  code  or  subject  was  passed as NULL, or ovector was NULL and
       ovecsize was not zero.

         PCRE_ERROR_BADOPTION      (-3)

       An unrecognized bit was set in the options argument.

         PCRE_ERROR_BADMAGIC       (-4)

       PCRE stores a 4-byte "magic number" at the start of the compiled  code,
       to catch the case when it is passed a junk pointer and to detect when a
       pattern that was compiled in an environment of one endianness is run in
       an  environment  with the other endianness. This is the error that PCRE
       gives when the magic number is not present.

         PCRE_ERROR_UNKNOWN_OPCODE (-5)

       While running the pattern match, an unknown item was encountered in the
       compiled  pattern.  This  error  could be caused by a bug in PCRE or by
       overwriting of the compiled pattern.

         PCRE_ERROR_NOMEMORY       (-6)

       If a pattern contains back references, but the ovector that  is  passed
       to pcre_exec() is not big enough to remember the referenced substrings,
       PCRE gets a block of memory at the start of matching to  use  for  this
       purpose.  If the call via pcre_malloc() fails, this error is given. The
       memory is automatically freed at the end of matching.

       This error is also given if pcre_stack_malloc() fails  in  pcre_exec().
       This  can happen only when PCRE has been compiled with --disable-stack-
       for-recursion.

         PCRE_ERROR_NOSUBSTRING    (-7)

       This error is used by the pcre_copy_substring(),  pcre_get_substring(),
       and  pcre_get_substring_list()  functions  (see  below).  It  is  never
       returned by pcre_exec().

         PCRE_ERROR_MATCHLIMIT     (-8)

       The backtracking limit, as specified by  the  match_limit  field  in  a
       pcre_extra  structure  (or  defaulted) was reached. See the description
       above.

         PCRE_ERROR_CALLOUT        (-9)

       This error is never generated by pcre_exec() itself. It is provided for
       use  by  callout functions that want to yield a distinctive error code.
       See the pcrecallout documentation for details.

         PCRE_ERROR_BADUTF8        (-10)

       A string that contains an invalid UTF-8 byte sequence was passed  as  a
       subject.   However,  if  PCRE_PARTIAL_HARD  is set and the problem is a
       truncated UTF-8 character at the end of the subject,  PCRE_ERROR_SHORT-
       UTF8 is used instead.

         PCRE_ERROR_BADUTF8_OFFSET (-11)

       The UTF-8 byte sequence that was passed as a subject was valid, but the
       value of startoffset did not point to the beginning of a UTF-8  charac-
       ter or the end of the subject.

         PCRE_ERROR_PARTIAL        (-12)

       The  subject  string did not match, but it did match partially. See the
       pcrepartial documentation for details of partial matching.

         PCRE_ERROR_BADPARTIAL     (-13)

       This code is no longer in  use.  It  was  formerly  returned  when  the
       PCRE_PARTIAL  option  was used with a compiled pattern containing items
       that were  not  supported  for  partial  matching.  From  release  8.00
       onwards, there are no restrictions on partial matching.

         PCRE_ERROR_INTERNAL       (-14)

       An  unexpected  internal error has occurred. This error could be caused
       by a bug in PCRE or by overwriting of the compiled pattern.

         PCRE_ERROR_BADCOUNT       (-15)

       This error is given if the value of the ovecsize argument is negative.

         PCRE_ERROR_RECURSIONLIMIT (-21)

       The internal recursion limit, as specified by the match_limit_recursion
       field  in  a  pcre_extra  structure (or defaulted) was reached. See the
       description above.

         PCRE_ERROR_BADNEWLINE     (-23)

       An invalid combination of PCRE_NEWLINE_xxx options was given.

         PCRE_ERROR_BADOFFSET      (-24)

       The value of startoffset was negative or greater than the length of the
       subject, that is, the value in length.

         PCRE_ERROR_SHORTUTF8      (-25)

       The  subject  string ended with an incomplete (truncated) UTF-8 charac-
       ter, and the PCRE_PARTIAL_HARD option was  set.  Without  this  option,
       PCRE_ERROR_BADUTF8 is returned in this situation.

       Error numbers -16 to -20 and -22 are not used by pcre_exec().


EXTRACTING CAPTURED SUBSTRINGS BY NUMBER

       int pcre_copy_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber, char *buffer,
            int buffersize);

       int pcre_get_substring(const char *subject, int *ovector,
            int stringcount, int stringnumber,
            const char **stringptr);

       int pcre_get_substring_list(const char *subject,
            int *ovector, int stringcount, const char ***listptr);

       Captured  substrings  can  be  accessed  directly  by using the offsets
       returned by pcre_exec() in  ovector.  For  convenience,  the  functions
       pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
       string_list() are provided for extracting captured substrings  as  new,
       separate,  zero-terminated strings. These functions identify substrings
       by number. The next section describes functions  for  extracting  named
       substrings.

       A  substring that contains a binary zero is correctly extracted and has
       a further zero added on the end, but the result is not, of course, a  C
       string.   However,  you  can  process such a string by referring to the
       length that is  returned  by  pcre_copy_substring()  and  pcre_get_sub-
       string().  Unfortunately, the interface to pcre_get_substring_list() is
       not adequate for handling strings containing binary zeros, because  the
       end of the final string is not independently indicated.

       The  first  three  arguments  are the same for all three of these func-
       tions: subject is the subject string that has  just  been  successfully
       matched, ovector is a pointer to the vector of integer offsets that was
       passed to pcre_exec(), and stringcount is the number of substrings that
       were  captured  by  the match, including the substring that matched the
       entire regular expression. This is the value returned by pcre_exec() if
       it  is greater than zero. If pcre_exec() returned zero, indicating that
       it ran out of space in ovector, the value passed as stringcount  should
       be the number of elements in the vector divided by three.

       The  functions pcre_copy_substring() and pcre_get_substring() extract a
       single substring, whose number is given as  stringnumber.  A  value  of
       zero  extracts  the  substring that matched the entire pattern, whereas
       higher values  extract  the  captured  substrings.  For  pcre_copy_sub-
       string(),  the  string  is  placed  in buffer, whose length is given by
       buffersize, while for pcre_get_substring() a new  block  of  memory  is
       obtained  via  pcre_malloc,  and its address is returned via stringptr.
       The yield of the function is the length of the  string,  not  including
       the terminating zero, or one of these error codes:

         PCRE_ERROR_NOMEMORY       (-6)

       The  buffer  was too small for pcre_copy_substring(), or the attempt to
       get memory failed for pcre_get_substring().

         PCRE_ERROR_NOSUBSTRING    (-7)

       There is no substring whose number is stringnumber.

       The pcre_get_substring_list()  function  extracts  all  available  sub-
       strings  and  builds  a list of pointers to them. All this is done in a
       single block of memory that is obtained via pcre_malloc. The address of
       the  memory  block  is returned via listptr, which is also the start of
       the list of string pointers. The end of the list is marked  by  a  NULL
       pointer.  The  yield  of  the function is zero if all went well, or the
       error code

         PCRE_ERROR_NOMEMORY       (-6)

       if the attempt to get the memory block failed.

       When any of these functions encounter a substring that is unset,  which
       can  happen  when  capturing subpattern number n+1 matches some part of
       the subject, but subpattern n has not been used at all, they return  an
       empty string. This can be distinguished from a genuine zero-length sub-
       string by inspecting the appropriate offset in ovector, which is  nega-
       tive for unset substrings.

       The  two convenience functions pcre_free_substring() and pcre_free_sub-
       string_list() can be used to free the memory  returned  by  a  previous
       call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
       tively. They do nothing more than  call  the  function  pointed  to  by
       pcre_free,  which  of course could be called directly from a C program.
       However, PCRE is used in some situations where it is linked via a  spe-
       cial   interface  to  another  programming  language  that  cannot  use
       pcre_free directly; it is for these cases that the functions  are  pro-
       vided.


EXTRACTING CAPTURED SUBSTRINGS BY NAME

       int pcre_get_stringnumber(const pcre *code,
            const char *name);

       int pcre_copy_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            char *buffer, int buffersize);

       int pcre_get_named_substring(const pcre *code,
            const char *subject, int *ovector,
            int stringcount, const char *stringname,
            const char **stringptr);

       To  extract a substring by name, you first have to find associated num-
       ber.  For example, for this pattern

         (a+)b(?<xxx>\d+)...

       the number of the subpattern called "xxx" is 2. If the name is known to
       be unique (PCRE_DUPNAMES was not set), you can find the number from the
       name by calling pcre_get_stringnumber(). The first argument is the com-
       piled pattern, and the second is the name. The yield of the function is
       the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if  there  is  no
       subpattern of that name.

       Given the number, you can extract the substring directly, or use one of
       the functions described in the previous section. For convenience, there
       are also two functions that do the whole job.

       Most    of    the    arguments   of   pcre_copy_named_substring()   and
       pcre_get_named_substring() are the same  as  those  for  the  similarly
       named  functions  that extract by number. As these are described in the
       previous section, they are not re-described here. There  are  just  two
       differences:

       First,  instead  of a substring number, a substring name is given. Sec-
       ond, there is an extra argument, given at the start, which is a pointer
       to  the compiled pattern. This is needed in order to gain access to the
       name-to-number translation table.

       These functions call pcre_get_stringnumber(), and if it succeeds,  they
       then  call  pcre_copy_substring() or pcre_get_substring(), as appropri-
       ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate  names,  the
       behaviour may not be what you want (see the next section).

       Warning: If the pattern uses the (?| feature to set up multiple subpat-
       terns with the same number, as described in the  section  on  duplicate
       subpattern  numbers  in  the  pcrepattern page, you cannot use names to
       distinguish the different subpatterns, because names are  not  included
       in  the compiled code. The matching process uses only numbers. For this
       reason, the use of different names for subpatterns of the  same  number
       causes an error at compile time.


DUPLICATE SUBPATTERN NAMES

       int pcre_get_stringtable_entries(const pcre *code,
            const char *name, char **first, char **last);

       When  a  pattern  is  compiled with the PCRE_DUPNAMES option, names for
       subpatterns are not required to be unique. (Duplicate names are  always
       allowed  for subpatterns with the same number, created by using the (?|
       feature. Indeed, if such subpatterns are named, they  are  required  to
       use the same names.)

       Normally, patterns with duplicate names are such that in any one match,
       only one of the named subpatterns participates. An example is shown  in
       the pcrepattern documentation.

       When    duplicates   are   present,   pcre_copy_named_substring()   and
       pcre_get_named_substring() return the first substring corresponding  to
       the  given  name  that  is set. If none are set, PCRE_ERROR_NOSUBSTRING
       (-7) is returned; no  data  is  returned.  The  pcre_get_stringnumber()
       function  returns one of the numbers that are associated with the name,
       but it is not defined which it is.

       If you want to get full details of all captured substrings for a  given
       name,  you  must  use  the pcre_get_stringtable_entries() function. The
       first argument is the compiled pattern, and the second is the name. The
       third  and  fourth  are  pointers to variables which are updated by the
       function. After it has run, they point to the first and last entries in
       the  name-to-number  table  for  the  given  name.  The function itself
       returns the length of each entry,  or  PCRE_ERROR_NOSUBSTRING  (-7)  if
       there  are none. The format of the table is described above in the sec-
       tion entitled Information about a  pattern.   Given  all  the  relevant
       entries  for the name, you can extract each of their numbers, and hence
       the captured data, if any.


FINDING ALL POSSIBLE MATCHES

       The traditional matching function uses a  similar  algorithm  to  Perl,
       which stops when it finds the first match, starting at a given point in
       the subject. If you want to find all possible matches, or  the  longest
       possible  match,  consider using the alternative matching function (see
       below) instead. If you cannot use the alternative function,  but  still
       need  to  find all possible matches, you can kludge it up by making use
       of the callout facility, which is described in the pcrecallout documen-
       tation.

       What you have to do is to insert a callout right at the end of the pat-
       tern.  When your callout function is called, extract and save the  cur-
       rent  matched  substring.  Then  return  1, which forces pcre_exec() to
       backtrack and try other alternatives. Ultimately, when it runs  out  of
       matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.


MATCHING A PATTERN: THE ALTERNATIVE FUNCTION

       int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
            const char *subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);

       The  function  pcre_dfa_exec()  is  called  to  match  a subject string
       against a compiled pattern, using a matching algorithm that  scans  the
       subject  string  just  once, and does not backtrack. This has different
       characteristics to the normal algorithm, and  is  not  compatible  with
       Perl.  Some  of the features of PCRE patterns are not supported. Never-
       theless, there are times when this kind of matching can be useful.  For
       a  discussion  of  the  two matching algorithms, and a list of features
       that pcre_dfa_exec() does not support, see the pcrematching  documenta-
       tion.

       The  arguments  for  the  pcre_dfa_exec()  function are the same as for
       pcre_exec(), plus two extras. The ovector argument is used in a differ-
       ent  way,  and  this is described below. The other common arguments are
       used in the same way as for pcre_exec(), so their  description  is  not
       repeated here.

       The  two  additional  arguments provide workspace for the function. The
       workspace vector should contain at least 20 elements. It  is  used  for
       keeping  track  of  multiple  paths  through  the  pattern  tree.  More
       workspace will be needed for patterns and subjects where  there  are  a
       lot of potential matches.

       Here is an example of a simple call to pcre_dfa_exec():

         int rc;
         int ovector[10];
         int wspace[20];
         rc = pcre_dfa_exec(
           re,             /* result of pcre_compile() */
           NULL,           /* we didn't study the pattern */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           ovector,        /* vector of integers for substring information */
           10,             /* number of elements (NOT size in bytes) */
           wspace,         /* working space vector */
           20);            /* number of elements (NOT size in bytes) */

   Option bits for pcre_dfa_exec()

       The  unused  bits  of  the options argument for pcre_dfa_exec() must be
       zero. The only bits  that  may  be  set  are  PCRE_ANCHORED,  PCRE_NEW-
       LINE_xxx,        PCRE_NOTBOL,        PCRE_NOTEOL,        PCRE_NOTEMPTY,
       PCRE_NOTEMPTY_ATSTART,      PCRE_NO_UTF8_CHECK,       PCRE_BSR_ANYCRLF,
       PCRE_BSR_UNICODE,  PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR-
       TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART.  All but  the  last
       four  of  these  are  exactly  the  same  as  for pcre_exec(), so their
       description is not repeated here.

         PCRE_PARTIAL_HARD
         PCRE_PARTIAL_SOFT

       These have the same general effect as they do for pcre_exec(), but  the
       details  are  slightly  different.  When  PCRE_PARTIAL_HARD  is set for
       pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of  the  sub-
       ject  is  reached  and there is still at least one matching possibility
       that requires additional characters. This happens even if some complete
       matches have also been found. When PCRE_PARTIAL_SOFT is set, the return
       code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end
       of  the  subject  is  reached, there have been no complete matches, but
       there is still at least one matching possibility. The  portion  of  the
       string  that  was inspected when the longest partial match was found is
       set as the first matching string  in  both  cases.   There  is  a  more
       detailed  discussion  of partial and multi-segment matching, with exam-
       ples, in the pcrepartial documentation.

         PCRE_DFA_SHORTEST

       Setting the PCRE_DFA_SHORTEST option causes the matching  algorithm  to
       stop as soon as it has found one match. Because of the way the alterna-
       tive algorithm works, this is necessarily the shortest  possible  match
       at the first possible matching point in the subject string.

         PCRE_DFA_RESTART

       When pcre_dfa_exec() returns a partial match, it is possible to call it
       again, with additional subject characters, and have  it  continue  with
       the  same match. The PCRE_DFA_RESTART option requests this action; when
       it is set, the workspace and wscount options must  reference  the  same
       vector  as  before  because data about the match so far is left in them
       after a partial match. There is more discussion of this facility in the
       pcrepartial documentation.

   Successful returns from pcre_dfa_exec()

       When  pcre_dfa_exec()  succeeds, it may have matched more than one sub-
       string in the subject. Note, however, that all the matches from one run
       of  the  function  start  at the same point in the subject. The shorter
       matches are all initial substrings of the longer matches. For  example,
       if the pattern

         <.*>

       is matched against the string

         This is <something> <something else> <something further> no more

       the three matched strings are

         <something>
         <something> <something else>
         <something> <something else> <something further>

       On  success,  the  yield of the function is a number greater than zero,
       which is the number of matched substrings.  The  substrings  themselves
       are  returned  in  ovector. Each string uses two elements; the first is
       the offset to the start, and the second is the offset to  the  end.  In
       fact,  all  the  strings  have the same start offset. (Space could have
       been saved by giving this only once, but it was decided to retain  some
       compatibility  with  the  way pcre_exec() returns data, even though the
       meaning of the strings is different.)

       The strings are returned in reverse order of length; that is, the long-
       est  matching  string is given first. If there were too many matches to
       fit into ovector, the yield of the function is zero, and the vector  is
       filled with the longest matches.

   Error returns from pcre_dfa_exec()

       The  pcre_dfa_exec()  function returns a negative number when it fails.
       Many of the errors are the same  as  for  pcre_exec(),  and  these  are
       described  above.   There are in addition the following errors that are
       specific to pcre_dfa_exec():

         PCRE_ERROR_DFA_UITEM      (-16)

       This return is given if pcre_dfa_exec() encounters an item in the  pat-
       tern  that  it  does not support, for instance, the use of \C or a back
       reference.

         PCRE_ERROR_DFA_UCOND      (-17)

       This return is given if pcre_dfa_exec()  encounters  a  condition  item
       that  uses  a back reference for the condition, or a test for recursion
       in a specific group. These are not supported.

         PCRE_ERROR_DFA_UMLIMIT    (-18)

       This return is given if pcre_dfa_exec() is called with an  extra  block
       that contains a setting of the match_limit field. This is not supported
       (it is meaningless).

         PCRE_ERROR_DFA_WSSIZE     (-19)

       This return is given if  pcre_dfa_exec()  runs  out  of  space  in  the
       workspace vector.

         PCRE_ERROR_DFA_RECURSE    (-20)

       When  a  recursive subpattern is processed, the matching function calls
       itself recursively, using private vectors for  ovector  and  workspace.
       This  error  is  given  if  the output vector is not large enough. This
       should be extremely rare, as a vector of size 1000 is used.


SEE ALSO

       pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3),  pcrepar-
       tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 21 November 2010
       Copyright (c) 1997-2010 University of Cambridge.
------------------------------------------------------------------------------


PCRECALLOUT(3)                                                  PCRECALLOUT(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE CALLOUTS

       int (*pcre_callout)(pcre_callout_block *);

       PCRE provides a feature called "callout", which is a means of temporar-
       ily passing control to the caller of PCRE  in  the  middle  of  pattern
       matching.  The  caller of PCRE provides an external function by putting
       its entry point in the global variable pcre_callout. By  default,  this
       variable contains NULL, which disables all calling out.

       Within  a  regular  expression,  (?C) indicates the points at which the
       external function is to be called.  Different  callout  points  can  be
       identified  by  putting  a number less than 256 after the letter C. The
       default value is zero.  For  example,  this  pattern  has  two  callout
       points:

         (?C1)abc(?C2)def

       If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() or
       pcre_compile2() is called, PCRE  automatically  inserts  callouts,  all
       with  number  255,  before  each  item  in the pattern. For example, if
       PCRE_AUTO_CALLOUT is used with the pattern

         A(\d{2}|--)

       it is processed as if it were

       (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

       Notice that there is a callout before and after  each  parenthesis  and
       alternation  bar.  Automatic  callouts  can  be  used  for tracking the
       progress of pattern matching. The pcretest command has an  option  that
       sets  automatic callouts; when it is used, the output indicates how the
       pattern is matched. This is useful information when you are  trying  to
       optimize the performance of a particular pattern.


MISSING CALLOUTS

       You  should  be  aware  that,  because of optimizations in the way PCRE
       matches patterns by default, callouts  sometimes  do  not  happen.  For
       example, if the pattern is

         ab(?C4)cd

       PCRE knows that any matching string must contain the letter "d". If the
       subject string is "abyz", the lack of "d" means that  matching  doesn't
       ever  start,  and  the  callout is never reached. However, with "abyd",
       though the result is still no match, the callout is obeyed.

       If the pattern is studied, PCRE knows the minimum length of a  matching
       string,  and will immediately give a "no match" return without actually
       running a match if the subject is not long enough, or,  for  unanchored
       patterns, if it has been scanned far enough.

       You  can disable these optimizations by passing the PCRE_NO_START_OPTI-
       MIZE option to pcre_compile(), pcre_exec(), or pcre_dfa_exec(),  or  by
       starting the pattern with (*NO_START_OPT). This slows down the matching
       process, but does ensure that callouts such as the  example  above  are
       obeyed.


THE CALLOUT INTERFACE

       During  matching, when PCRE reaches a callout point, the external func-
       tion defined by pcre_callout is called (if it is set). This applies  to
       both  the  pcre_exec()  and the pcre_dfa_exec() matching functions. The
       only argument to the callout function is a pointer  to  a  pcre_callout
       block. This structure contains the following fields:

         int          version;
         int          callout_number;
         int         *offset_vector;
         const char  *subject;
         int          subject_length;
         int          start_match;
         int          current_position;
         int          capture_top;
         int          capture_last;
         void        *callout_data;
         int          pattern_position;
         int          next_item_length;

       The  version  field  is an integer containing the version number of the
       block format. The initial version was 0; the current version is 1.  The
       version  number  will  change  again in future if additional fields are
       added, but the intention is never to remove any of the existing fields.

       The callout_number field contains the number of the  callout,  as  com-
       piled  into  the pattern (that is, the number after ?C for manual call-
       outs, and 255 for automatically generated callouts).

       The offset_vector field is a pointer to the vector of offsets that  was
       passed   by   the   caller  to  pcre_exec()  or  pcre_dfa_exec().  When
       pcre_exec() is used, the contents can be inspected in order to  extract
       substrings  that  have  been  matched  so  far,  in the same way as for
       extracting substrings after a match has completed. For  pcre_dfa_exec()
       this field is not useful.

       The subject and subject_length fields contain copies of the values that
       were passed to pcre_exec().

       The start_match field normally contains the offset within  the  subject
       at  which  the  current  match  attempt started. However, if the escape
       sequence \K has been encountered, this value is changed to reflect  the
       modified  starting  point.  If the pattern is not anchored, the callout
       function may be called several times from the same point in the pattern
       for different starting points in the subject.

       The  current_position  field  contains the offset within the subject of
       the current match pointer.

       When the pcre_exec() function is used, the capture_top  field  contains
       one  more than the number of the highest numbered captured substring so
       far. If no substrings have been captured, the value of  capture_top  is
       one.  This  is always the case when pcre_dfa_exec() is used, because it
       does not support captured substrings.

       The capture_last field contains the number of the  most  recently  cap-
       tured  substring. If no substrings have been captured, its value is -1.
       This is always the case when pcre_dfa_exec() is used.

       The callout_data field contains a value that is passed  to  pcre_exec()
       or  pcre_dfa_exec() specifically so that it can be passed back in call-
       outs. It is passed in the pcre_callout field  of  the  pcre_extra  data
       structure.  If  no such data was passed, the value of callout_data in a
       pcre_callout block is NULL. There is a description  of  the  pcre_extra
       structure in the pcreapi documentation.

       The  pattern_position field is present from version 1 of the pcre_call-
       out structure. It contains the offset to the next item to be matched in
       the pattern string.

       The  next_item_length field is present from version 1 of the pcre_call-
       out structure. It contains the length of the next item to be matched in
       the  pattern  string. When the callout immediately precedes an alterna-
       tion bar, a closing parenthesis, or the end of the pattern, the  length
       is  zero.  When the callout precedes an opening parenthesis, the length
       is that of the entire subpattern.

       The pattern_position and next_item_length fields are intended  to  help
       in  distinguishing between different automatic callouts, which all have
       the same callout number. However, they are set for all callouts.


RETURN VALUES

       The external callout function returns an integer to PCRE. If the  value
       is  zero,  matching  proceeds  as  normal. If the value is greater than
       zero, matching fails at the current point, but  the  testing  of  other
       matching possibilities goes ahead, just as if a lookahead assertion had
       failed. If the value is less than zero, the  match  is  abandoned,  and
       pcre_exec() or pcre_dfa_exec() returns the negative value.

       Negative   values   should   normally   be   chosen  from  the  set  of
       PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
       dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
       reserved for use by callout functions; it will never be  used  by  PCRE
       itself.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 21 November 2010
       Copyright (c) 1997-2010 University of Cambridge.
------------------------------------------------------------------------------


PCRECOMPAT(3)                                                    PCRECOMPAT(3)


NAME
       PCRE - Perl-compatible regular expressions


DIFFERENCES BETWEEN PCRE AND PERL

       This  document describes the differences in the ways that PCRE and Perl
       handle regular expressions. The differences  described  here  are  with
       respect to Perl versions 5.10 and above.

       1.  PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
       of what it does have are given in the section on UTF-8 support  in  the
       main pcre page.

       2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
       permits them, but they do not mean what you might think.  For  example,
       (?!a){3} does not assert that the next three characters are not "a". It
       just asserts that the next character is not "a" three times.

       3. Capturing subpatterns that occur inside  negative  lookahead  asser-
       tions  are  counted,  but their entries in the offsets vector are never
       set. Perl sets its numerical variables from any such patterns that  are
       matched before the assertion fails to match something (thereby succeed-
       ing), but only if the negative lookahead assertion  contains  just  one
       branch.

       4.  Though  binary zero characters are supported in the subject string,
       they are not allowed in a pattern string because it is passed as a nor-
       mal C string, terminated by zero. The escape sequence \0 can be used in
       the pattern to represent a binary zero.

       5. The following Perl escape sequences are not supported: \l,  \u,  \L,
       \U, and \N. In fact these are implemented by Perl's general string-han-
       dling and are not part of its pattern matching engine. If any of  these
       are encountered by PCRE, an error is generated.

       6.  The Perl escape sequences \p, \P, and \X are supported only if PCRE
       is built with Unicode character property support. The  properties  that
       can  be tested with \p and \P are limited to the general category prop-
       erties such as Lu and Nd, script names such as Greek or  Han,  and  the
       derived  properties  Any  and  L&. PCRE does support the Cs (surrogate)
       property, which Perl does not; the  Perl  documentation  says  "Because
       Perl hides the need for the user to understand the internal representa-
       tion of Unicode characters, there is no need to implement the  somewhat
       messy concept of surrogates."

       7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
       ters in between are treated as literals.  This  is  slightly  different
       from  Perl  in  that  $  and  @ are also handled as literals inside the
       quotes. In Perl, they cause variable interpolation (but of course  PCRE
       does not have variables). Note the following examples:

           Pattern            PCRE matches      Perl matches

           \Qabc$xyz\E        abc$xyz           abc followed by the
                                                  contents of $xyz
           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz

       The  \Q...\E  sequence  is recognized both inside and outside character
       classes.

       8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
       constructions.  However,  there is support for recursive patterns. This
       is not available in Perl 5.8, but it is in Perl 5.10.  Also,  the  PCRE
       "callout"  feature allows an external function to be called during pat-
       tern matching. See the pcrecallout documentation for details.

       9. Subpatterns that are called  recursively  or  as  "subroutines"  are
       always  treated  as  atomic  groups  in  PCRE. This is like Python, but
       unlike Perl. There is a discussion of an example that explains this  in
       more  detail  in  the section on recursion differences from Perl in the
       pcrepattern page.

       10. There are some differences that are concerned with the settings  of
       captured  strings  when  part  of  a  pattern is repeated. For example,
       matching "aba" against the  pattern  /^(a(b)?)+$/  in  Perl  leaves  $2
       unset, but in PCRE it is set to "b".

       11.  PCRE's handling of duplicate subpattern numbers and duplicate sub-
       pattern names is not as general as Perl's. This is a consequence of the
       fact the PCRE works internally just with numbers, using an external ta-
       ble to translate between numbers and names. In  particular,  a  pattern
       such  as  (?|(?<a>A)|(?<b)B),  where the two capturing parentheses have
       the same number but different names, is not supported,  and  causes  an
       error  at compile time. If it were allowed, it would not be possible to
       distinguish which parentheses matched, because both names map  to  cap-
       turing subpattern number 1. To avoid this confusing situation, an error
       is given at compile time.

       12. Perl recognizes comments in some  places  that  PCRE  doesn't,  for
       example, between the ( and ? at the start of a subpattern.

       13. PCRE provides some extensions to the Perl regular expression facil-
       ities.  Perl 5.10 includes new features that are not  in  earlier  ver-
       sions  of  Perl, some of which (such as named parentheses) have been in
       PCRE for some time. This list is with respect to Perl 5.10:

       (a) Although lookbehind assertions in  PCRE  must  match  fixed  length
       strings,  each alternative branch of a lookbehind assertion can match a
       different length of string. Perl requires them all  to  have  the  same
       length.

       (b)  If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
       meta-character matches only at the very end of the string.

       (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
       cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
       ignored.  (Perl can be made to issue a warning.)

       (d) If PCRE_UNGREEDY is set, the greediness of the  repetition  quanti-
       fiers is inverted, that is, by default they are not greedy, but if fol-
       lowed by a question mark they are.

       (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
       tried only at the first matching position in the subject string.

       (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
       and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no  Perl  equiva-
       lents.

       (g)  The  \R escape sequence can be restricted to match only CR, LF, or
       CRLF by the PCRE_BSR_ANYCRLF option.

       (h) The callout facility is PCRE-specific.

       (i) The partial matching facility is PCRE-specific.

       (j) Patterns compiled by PCRE can be saved and re-used at a later time,
       even on different hosts that have the other endianness.

       (k)  The  alternative  matching function (pcre_dfa_exec()) matches in a
       different way and is not Perl-compatible.

       (l) PCRE recognizes some special sequences such as (*CR) at  the  start
       of a pattern that set overall options that cannot be changed within the
       pattern.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 31 October 2010
       Copyright (c) 1997-2010 University of Cambridge.
------------------------------------------------------------------------------


PCREPATTERN(3)                                                  PCREPATTERN(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE REGULAR EXPRESSION DETAILS

       The  syntax and semantics of the regular expressions that are supported
       by PCRE are described in detail below. There is a quick-reference  syn-
       tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
       semantics as closely as it can. PCRE  also  supports  some  alternative
       regular  expression  syntax (which does not conflict with the Perl syn-
       tax) in order to provide some compatibility with regular expressions in
       Python, .NET, and Oniguruma.

       Perl's  regular expressions are described in its own documentation, and
       regular expressions in general are covered in a number of  books,  some
       of  which  have  copious  examples. Jeffrey Friedl's "Mastering Regular
       Expressions", published by  O'Reilly,  covers  regular  expressions  in
       great  detail.  This  description  of  PCRE's  regular  expressions  is
       intended as reference material.

       The original operation of PCRE was on strings of  one-byte  characters.
       However,  there is now also support for UTF-8 character strings. To use
       this, PCRE must be built to include UTF-8 support, and  you  must  call
       pcre_compile()  or  pcre_compile2() with the PCRE_UTF8 option. There is
       also a special sequence that can be given at the start of a pattern:

         (*UTF8)

       Starting a pattern with this sequence  is  equivalent  to  setting  the
       PCRE_UTF8  option.  This  feature  is  not Perl-compatible. How setting
       UTF-8 mode affects pattern matching  is  mentioned  in  several  places
       below.  There  is  also  a  summary of UTF-8 features in the section on
       UTF-8 support in the main pcre page.

       Another special sequence that may appear at the start of a  pattern  or
       in combination with (*UTF8) is:

         (*UCP)

       This  has  the  same  effect  as setting the PCRE_UCP option: it causes
       sequences such as \d and \w to  use  Unicode  properties  to  determine
       character types, instead of recognizing only characters with codes less
       than 128 via a lookup table.

       If a pattern starts with (*NO_START_OPT), it has  the  same  effect  as
       setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
       time. There are also some more of these special sequences that are con-
       cerned with the handling of newlines; they are described below.

       The  remainder  of  this  document discusses the patterns that are sup-
       ported by PCRE when its main matching function, pcre_exec(),  is  used.
       From   release   6.0,   PCRE   offers   a   second  matching  function,
       pcre_dfa_exec(), which matches using a different algorithm that is  not
       Perl-compatible. Some of the features discussed below are not available
       when pcre_dfa_exec() is used. The advantages and disadvantages  of  the
       alternative  function, and how it differs from the normal function, are
       discussed in the pcrematching page.


NEWLINE CONVENTIONS

       PCRE supports five different conventions for indicating line breaks  in
       strings:  a  single  CR (carriage return) character, a single LF (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding,  or  any Unicode newline sequence. The pcreapi page has further
       discussion about newlines, and shows how to set the newline  convention
       in the options arguments for the compiling and matching functions.

       It  is also possible to specify a newline convention by starting a pat-
       tern string with one of the following five sequences:

         (*CR)        carriage return
         (*LF)        linefeed
         (*CRLF)      carriage return, followed by linefeed
         (*ANYCRLF)   any of the three above
         (*ANY)       all Unicode newline sequences

       These override the default and the options given to  pcre_compile()  or
       pcre_compile2().  For example, on a Unix system where LF is the default
       newline sequence, the pattern

         (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no  longer  a  newline. Note that these special settings, which are not
       Perl-compatible, are recognized only at the very start  of  a  pattern,
       and  that  they  must  be  in  upper  case. If more than one of them is
       present, the last one is used.

       The newline convention affects the interpretation of the dot  metachar-
       acter  when  PCRE_DOTALL is not set, and also the behaviour of \N. How-
       ever, it does not affect  what  the  \R  escape  sequence  matches.  By
       default,  this is any Unicode newline sequence, for Perl compatibility.
       However, this can be changed; see the description of \R in the  section
       entitled  "Newline sequences" below. A change of \R setting can be com-
       bined with a change of newline convention.


CHARACTERS AND METACHARACTERS

       A regular expression is a pattern that is  matched  against  a  subject
       string  from  left  to right. Most characters stand for themselves in a
       pattern, and match the corresponding characters in the  subject.  As  a
       trivial example, the pattern

         The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless matching is specified (the PCRE_CASELESS option), letters  are
       matched  independently  of case. In UTF-8 mode, PCRE always understands
       the concept of case for characters whose values are less than  128,  so
       caseless  matching  is always possible. For characters with higher val-
       ues, the concept of case is supported if PCRE is compiled with  Unicode
       property  support,  but  not  otherwise.   If  you want to use caseless
       matching for characters 128 and above, you must  ensure  that  PCRE  is
       compiled with Unicode property support as well as with UTF-8 support.

       The  power  of  regular  expressions  comes from the ability to include
       alternatives and repetitions in the pattern. These are encoded  in  the
       pattern by the use of metacharacters, which do not stand for themselves
       but instead are interpreted in some special way.

       There are two different sets of metacharacters: those that  are  recog-
       nized  anywhere in the pattern except within square brackets, and those
       that are recognized within square brackets.  Outside  square  brackets,
       the metacharacters are as follows:

         \      general escape character with several uses
         ^      assert start of string (or line, in multiline mode)
         $      assert end of string (or line, in multiline mode)
         .      match any character except newline (by default)
         [      start character class definition
         |      start of alternative branch
         (      start subpattern
         )      end subpattern
         ?      extends the meaning of (
                also 0 or 1 quantifier
                also quantifier minimizer
         *      0 or more quantifier
         +      1 or more quantifier
                also "possessive quantifier"
         {      start min/max quantifier

       Part  of  a  pattern  that is in square brackets is called a "character
       class". In a character class the only metacharacters are:

         \      general escape character
         ^      negate the class, but only if the first character
         -      indicates character range
         [      POSIX character class (only if followed by POSIX
                  syntax)
         ]      terminates the character class

       The following sections describe the use of each of the metacharacters.


BACKSLASH

       The backslash character has several uses. Firstly, if it is followed by
       a character that is not a number or a letter, it takes away any special
       meaning that character may have. This use of  backslash  as  an  escape
       character applies both inside and outside character classes.

       For  example,  if  you want to match a * character, you write \* in the
       pattern.  This escaping action applies whether  or  not  the  following
       character  would  otherwise be interpreted as a metacharacter, so it is
       always safe to precede a non-alphanumeric  with  backslash  to  specify
       that  it stands for itself. In particular, if you want to match a back-
       slash, you write \\.

       In UTF-8 mode, only ASCII numbers and letters have any special  meaning
       after  a  backslash.  All  other characters (in particular, those whose
       codepoints are greater than 127) are treated as literals.

       If a pattern is compiled with the PCRE_EXTENDED option,  whitespace  in
       the  pattern (other than in a character class) and characters between a
       # outside a character class and the next newline are ignored. An escap-
       ing  backslash  can  be  used to include a whitespace or # character as
       part of the pattern.

       If you want to remove the special meaning from a  sequence  of  charac-
       ters,  you can do so by putting them between \Q and \E. This is differ-
       ent from Perl in that $ and  @  are  handled  as  literals  in  \Q...\E
       sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola-
       tion. Note the following examples:

         Pattern            PCRE matches   Perl matches

         \Qabc$xyz\E        abc$xyz        abc followed by the
                                             contents of $xyz
         \Qabc\$xyz\E       abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes.  An isolated \E that is not preceded by \Q is ignored.

   Non-printing characters

       A second use of backslash provides a way of encoding non-printing char-
       acters in patterns in a visible manner. There is no restriction on  the
       appearance  of non-printing characters, apart from the binary zero that
       terminates a pattern, but when a pattern  is  being  prepared  by  text
       editing,  it  is  often  easier  to  use  one  of  the following escape
       sequences than the binary character it represents:

         \a        alarm, that is, the BEL character (hex 07)
         \cx       "control-x", where x is any ASCII character
         \e        escape (hex 1B)
         \f        formfeed (hex 0C)
         \n        linefeed (hex 0A)
         \r        carriage return (hex 0D)
         \t        tab (hex 09)
         \ddd      character with octal code ddd, or back reference
         \xhh      character with hex code hh
         \x{hhh..} character with hex code hhh..

       The precise effect of \cx is as follows: if x is a lower  case  letter,
       it  is converted to upper case. Then bit 6 of the character (hex 40) is
       inverted.  Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
       is  7B),  while  \c; becomes hex 7B (; is 3B). If the byte following \c
       has a value greater than 127, a compile-time error occurs.  This  locks
       out  non-ASCII  characters in both byte mode and UTF-8 mode. (When PCRE
       is compiled in EBCDIC mode, all byte values are  valid.  A  lower  case
       letter is converted to upper case, and then the 0xc0 bits are flipped.)

       After  \x, from zero to two hexadecimal digits are read (letters can be
       in upper or lower case). Any number of hexadecimal  digits  may  appear
       between  \x{  and  },  but the value of the character code must be less
       than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
       the  maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
       than the largest Unicode code point, which is 10FFFF.

       If characters other than hexadecimal digits appear between \x{  and  },
       or if there is no terminating }, this form of escape is not recognized.
       Instead, the initial \x will be  interpreted  as  a  basic  hexadecimal
       escape,  with  no  following  digits, giving a character whose value is
       zero.

       Characters whose value is less than 256 can be defined by either of the
       two  syntaxes  for  \x. There is no difference in the way they are han-
       dled. For example, \xdc is exactly the same as \x{dc}.

       After \0 up to two further octal digits are read. If  there  are  fewer
       than  two  digits,  just  those  that  are  present  are used. Thus the
       sequence \0\x\07 specifies two binary zeros followed by a BEL character
       (code  value 7). Make sure you supply two digits after the initial zero
       if the pattern character that follows is itself an octal digit.

       The handling of a backslash followed by a digit other than 0 is compli-
       cated.  Outside a character class, PCRE reads it and any following dig-
       its as a decimal number. If the number is less than  10,  or  if  there
       have been at least that many previous capturing left parentheses in the
       expression, the entire  sequence  is  taken  as  a  back  reference.  A
       description  of how this works is given later, following the discussion
       of parenthesized subpatterns.

       Inside a character class, or if the decimal number is  greater  than  9
       and  there have not been that many capturing subpatterns, PCRE re-reads
       up to three octal digits following the backslash, and uses them to gen-
       erate  a data character. Any subsequent digits stand for themselves. In
       non-UTF-8 mode, the value of a character specified  in  octal  must  be
       less  than  \400.  In  UTF-8 mode, values up to \777 are permitted. For
       example:

         \040   is another way of writing a space
         \40    is the same, provided there are fewer than 40
                   previous capturing subpatterns
         \7     is always a back reference
         \11    might be a back reference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a back reference, otherwise the
                   character with octal code 113
         \377   might be a back reference, otherwise
                   the byte consisting entirely of 1 bits
         \81    is either a back reference, or a binary zero
                   followed by the two characters "8" and "1"

       Note that octal values of 100 or greater must not be  introduced  by  a
       leading zero, because no more than three octal digits are ever read.

       All the sequences that define a single character value can be used both
       inside and outside character classes. In addition, inside  a  character
       class,  the  sequence \b is interpreted as the backspace character (hex
       08). The sequences \B, \N, \R, and \X are not special inside a  charac-
       ter  class.  Like  any  other  unrecognized  escape sequences, they are
       treated as the literal characters "B", "N", "R", and  "X"  by  default,
       but cause an error if the PCRE_EXTRA option is set. Outside a character
       class, these sequences have different meanings.

   Absolute and relative back references

       The sequence \g followed by an unsigned or a negative  number,  option-
       ally  enclosed  in braces, is an absolute or relative back reference. A
       named back reference can be coded as \g{name}. Back references are dis-
       cussed later, following the discussion of parenthesized subpatterns.

   Absolute and relative subroutine calls

       For  compatibility with Oniguruma, the non-Perl syntax \g followed by a
       name or a number enclosed either in angle brackets or single quotes, is
       an  alternative  syntax for referencing a subpattern as a "subroutine".
       Details are discussed later.   Note  that  \g{...}  (Perl  syntax)  and
       \g<...>  (Oniguruma  syntax)  are  not synonymous. The former is a back
       reference; the latter is a subroutine call.

   Generic character types

       Another use of backslash is for specifying generic character types:

         \d     any decimal digit
         \D     any character that is not a decimal digit
         \h     any horizontal whitespace character
         \H     any character that is not a horizontal whitespace character
         \s     any whitespace character
         \S     any character that is not a whitespace character
         \v     any vertical whitespace character
         \V     any character that is not a vertical whitespace character
         \w     any "word" character
         \W     any "non-word" character

       There is also the single sequence \N, which matches a non-newline char-
       acter.   This  is the same as the "." metacharacter when PCRE_DOTALL is
       not set.

       Each pair of lower and upper case escape sequences partitions the  com-
       plete  set  of  characters  into two disjoint sets. Any given character
       matches one, and only one, of each pair. The sequences can appear  both
       inside  and outside character classes. They each match one character of
       the appropriate type. If the current matching point is at  the  end  of
       the  subject string, all of them fail, because there is no character to
       match.

       For compatibility with Perl, \s does not match the VT  character  (code
       11).   This makes it different from the the POSIX "space" class. The \s
       characters are HT (9), LF (10), FF (12), CR (13), and  space  (32).  If
       "use locale;" is included in a Perl script, \s may match the VT charac-
       ter. In PCRE, it never does.

       A "word" character is an underscore or any character that is  a  letter
       or  digit.   By  default,  the definition of letters and digits is con-
       trolled by PCRE's low-valued character tables, and may vary if  locale-
       specific  matching is taking place (see "Locale support" in the pcreapi
       page). For example, in a French locale such  as  "fr_FR"  in  Unix-like
       systems,  or "french" in Windows, some character codes greater than 128
       are used for accented letters, and these are then matched  by  \w.  The
       use of locales with Unicode is discouraged.

       By  default,  in  UTF-8  mode,  characters with values greater than 128
       never match \d, \s, or \w, and always  match  \D,  \S,  and  \W.  These
       sequences  retain their original meanings from before UTF-8 support was
       available, mainly for efficiency reasons. However, if PCRE is  compiled
       with  Unicode property support, and the PCRE_UCP option is set, the be-
       haviour is changed so that Unicode properties  are  used  to  determine
       character types, as follows:

         \d  any character that \p{Nd} matches (decimal digit)
         \s  any character that \p{Z} matches, plus HT, LF, FF, CR
         \w  any character that \p{L} or \p{N} matches, plus underscore

       The  upper case escapes match the inverse sets of characters. Note that
       \d matches only decimal digits, whereas \w matches any  Unicode  digit,
       as  well as any Unicode letter, and underscore. Note also that PCRE_UCP
       affects \b, and \B because they are defined in  terms  of  \w  and  \W.
       Matching these sequences is noticeably slower when PCRE_UCP is set.

       The  sequences  \h, \H, \v, and \V are features that were added to Perl
       at release 5.10. In contrast to the other sequences, which  match  only
       ASCII  characters  by  default,  these always match certain high-valued
       codepoints in UTF-8 mode, whether or not PCRE_UCP is set. The  horizon-
       tal space characters are:

         U+0009     Horizontal tab
         U+0020     Space
         U+00A0     Non-break space
         U+1680     Ogham space mark
         U+180E     Mongolian vowel separator
         U+2000     En quad
         U+2001     Em quad
         U+2002     En space
         U+2003     Em space
         U+2004     Three-per-em space
         U+2005     Four-per-em space
         U+2006     Six-per-em space
         U+2007     Figure space
         U+2008     Punctuation space
         U+2009     Thin space
         U+200A     Hair space
         U+202F     Narrow no-break space
         U+205F     Medium mathematical space
         U+3000     Ideographic space

       The vertical space characters are:

         U+000A     Linefeed
         U+000B     Vertical tab
         U+000C     Formfeed
         U+000D     Carriage return
         U+0085     Next line
         U+2028     Line separator
         U+2029     Paragraph separator

   Newline sequences

       Outside  a  character class, by default, the escape sequence \R matches
       any Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the
       following:

         (?>\r\n|\n|\x0b|\f|\r|\x85)

       This  is  an  example  of an "atomic group", details of which are given
       below.  This particular group matches either the two-character sequence
       CR  followed  by  LF,  or  one  of  the single characters LF (linefeed,
       U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
       return, U+000D), or NEL (next line, U+0085). The two-character sequence
       is treated as a single unit that cannot be split.

       In UTF-8 mode, two additional characters whose codepoints  are  greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
       rator, U+2029).  Unicode character property support is not  needed  for
       these characters to be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set  of  Unicode  line  endings)  by  setting  the  option
       PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
       (BSR is an abbrevation for "backslash R".) This can be made the default
       when  PCRE  is  built;  if this is the case, the other behaviour can be
       requested via the PCRE_BSR_UNICODE option.   It  is  also  possible  to
       specify  these  settings  by  starting a pattern string with one of the
       following sequences:

         (*BSR_ANYCRLF)   CR, LF, or CRLF only
         (*BSR_UNICODE)   any Unicode newline sequence

       These override the default and the options given to  pcre_compile()  or
       pcre_compile2(),  but  they  can  be  overridden  by  options  given to
       pcre_exec() or pcre_dfa_exec(). Note that these special settings, which
       are  not  Perl-compatible,  are  recognized only at the very start of a
       pattern, and that they must be in upper case. If more than one of  them
       is present, the last one is used. They can be combined with a change of
       newline convention; for example, a pattern can start with:

         (*ANY)(*BSR_ANYCRLF)

       They can also be combined with the (*UTF8) or (*UCP) special sequences.
       Inside  a  character  class,  \R  is  treated as an unrecognized escape
       sequence, and so matches the letter "R" by default, but causes an error
       if PCRE_EXTRA is set.

   Unicode character properties

       When PCRE is built with Unicode character property support, three addi-
       tional escape sequences that match characters with specific  properties
       are  available.   When not in UTF-8 mode, these sequences are of course
       limited to testing characters whose codepoints are less than  256,  but
       they do work in this mode.  The extra escape sequences are:

         \p{xx}   a character with the xx property
         \P{xx}   a character without the xx property
         \X       an extended Unicode sequence

       The  property  names represented by xx above are limited to the Unicode
       script names, the general category properties, "Any", which matches any
       character   (including  newline),  and  some  special  PCRE  properties
       (described in the next section).  Other Perl properties such as  "InMu-
       sicalSymbols"  are  not  currently supported by PCRE. Note that \P{Any}
       does not match any characters, so always causes a match failure.

       Sets of Unicode characters are defined as belonging to certain scripts.
       A  character from one of these sets can be matched using a script name.
       For example:

         \p{Greek}
         \P{Han}

       Those that are not part of an identified script are lumped together  as
       "Common". The current list of scripts is:

       Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
       Buginese, Buhid, Canadian_Aboriginal, Carian, Cham,  Cherokee,  Common,
       Coptic,   Cuneiform,  Cypriot,  Cyrillic,  Deseret,  Devanagari,  Egyp-
       tian_Hieroglyphs,  Ethiopic,  Georgian,  Glagolitic,   Gothic,   Greek,
       Gujarati,  Gurmukhi,  Han,  Hangul,  Hanunoo,  Hebrew,  Hiragana, Impe-
       rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
       Javanese,  Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
       Latin,  Lepcha,  Limbu,  Linear_B,  Lisu,  Lycian,  Lydian,  Malayalam,
       Meetei_Mayek,  Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
       Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki,  Oriya,  Osmanya,
       Phags_Pa,  Phoenician,  Rejang,  Runic, Samaritan, Saurashtra, Shavian,
       Sinhala, Sundanese, Syloti_Nagri, Syriac,  Tagalog,  Tagbanwa,  Tai_Le,
       Tai_Tham,  Tai_Viet,  Tamil,  Telugu,  Thaana, Thai, Tibetan, Tifinagh,
       Ugaritic, Vai, Yi.

       Each character has exactly one Unicode general category property, spec-
       ified  by a two-letter abbreviation. For compatibility with Perl, nega-
       tion can be specified by including a  circumflex  between  the  opening
       brace  and  the  property  name.  For  example,  \p{^Lu} is the same as
       \P{Lu}.

       If only one letter is specified with \p or \P, it includes all the gen-
       eral  category properties that start with that letter. In this case, in
       the absence of negation, the curly brackets in the escape sequence  are
       optional; these two examples have the same effect:

         \p{L}
         \pL

       The following general category property codes are supported:

         C     Other
         Cc    Control
         Cf    Format
         Cn    Unassigned
         Co    Private use
         Cs    Surrogate

         L     Letter
         Ll    Lower case letter
         Lm    Modifier letter
         Lo    Other letter
         Lt    Title case letter
         Lu    Upper case letter

         M     Mark
         Mc    Spacing mark
         Me    Enclosing mark
         Mn    Non-spacing mark

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

         P     Punctuation
         Pc    Connector punctuation
         Pd    Dash punctuation
         Pe    Close punctuation
         Pf    Final punctuation
         Pi    Initial punctuation
         Po    Other punctuation
         Ps    Open punctuation

         S     Symbol
         Sc    Currency symbol
         Sk    Modifier symbol
         Sm    Mathematical symbol
         So    Other symbol

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

       The  special property L& is also supported: it matches a character that
       has the Lu, Ll, or Lt property, in other words, a letter  that  is  not
       classified as a modifier or "other".

       The  Cs  (Surrogate)  property  applies only to characters in the range
       U+D800 to U+DFFF. Such characters are not valid in UTF-8  strings  (see
       RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
       ing has been turned off (see the discussion  of  PCRE_NO_UTF8_CHECK  in
       the pcreapi page). Perl does not support the Cs property.

       The  long  synonyms  for  property  names  that  Perl supports (such as
       \p{Letter}) are not supported by PCRE, nor is it  permitted  to  prefix
       any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned) prop-
       erty.  Instead, this property is assumed for any code point that is not
       in the Unicode table.

       Specifying  caseless  matching  does not affect these escape sequences.
       For example, \p{Lu} always matches only upper case letters.

       The \X escape matches any number of Unicode  characters  that  form  an
       extended Unicode sequence. \X is equivalent to

         (?>\PM\pM*)

       That  is,  it matches a character without the "mark" property, followed
       by zero or more characters with the "mark"  property,  and  treats  the
       sequence  as  an  atomic group (see below).  Characters with the "mark"
       property are typically accents that  affect  the  preceding  character.
       None  of  them  have  codepoints less than 256, so in non-UTF-8 mode \X
       matches any one character.

       Matching characters by Unicode property is not fast, because  PCRE  has
       to  search  a  structure  that  contains data for over fifteen thousand
       characters. That is why the traditional escape sequences such as \d and
       \w  do  not  use  Unicode properties in PCRE by default, though you can
       make them do so by setting the PCRE_UCP option for pcre_compile() or by
       starting the pattern with (*UCP).

   PCRE's additional properties

       As  well  as  the standard Unicode properties described in the previous
       section, PCRE supports four more that make it possible to convert  tra-
       ditional escape sequences such as \w and \s and POSIX character classes
       to use Unicode properties. PCRE uses these non-standard, non-Perl prop-
       erties internally when PCRE_UCP is set. They are:

         Xan   Any alphanumeric character
         Xps   Any POSIX space character
         Xsp   Any Perl space character
         Xwd   Any Perl "word" character

       Xan  matches  characters that have either the L (letter) or the N (num-
       ber) property. Xps matches the characters tab, linefeed, vertical  tab,
       formfeed,  or  carriage  return, and any other character that has the Z
       (separator) property.  Xsp is the same as Xps, except that vertical tab
       is excluded. Xwd matches the same characters as Xan, plus underscore.

   Resetting the match start

       The  escape sequence \K causes any previously matched characters not to
       be included in the final matched sequence. For example, the pattern:

         foo\Kbar

       matches "foobar", but reports that it has matched "bar".  This  feature
       is  similar  to  a lookbehind assertion (described below).  However, in
       this case, the part of the subject before the real match does not  have
       to  be of fixed length, as lookbehind assertions do. The use of \K does
       not interfere with the setting of captured  substrings.   For  example,
       when the pattern

         (foo)\Kbar

       matches "foobar", the first substring is still set to "foo".

       Perl  documents  that  the  use  of  \K  within assertions is "not well
       defined". In PCRE, \K is acted upon  when  it  occurs  inside  positive
       assertions, but is ignored in negative assertions.

   Simple assertions

       The  final use of backslash is for certain simple assertions. An asser-
       tion specifies a condition that has to be met at a particular point  in
       a  match, without consuming any characters from the subject string. The
       use of subpatterns for more complicated assertions is described  below.
       The backslashed assertions are:

         \b     matches at a word boundary
         \B     matches when not at a word boundary
         \A     matches at the start of the subject
         \Z     matches at the end of the subject
                 also matches before a newline at the end of the subject
         \z     matches only at the end of the subject
         \G     matches at the first matching position in the subject

       Inside  a  character  class, \b has a different meaning; it matches the
       backspace character. If any other of  these  assertions  appears  in  a
       character  class, by default it matches the corresponding literal char-
       acter  (for  example,  \B  matches  the  letter  B).  However,  if  the
       PCRE_EXTRA  option is set, an "invalid escape sequence" error is gener-
       ated instead.

       A word boundary is a position in the subject string where  the  current
       character  and  the previous character do not both match \w or \W (i.e.
       one matches \w and the other matches \W), or the start or  end  of  the
       string  if  the  first  or  last character matches \w, respectively. In
       UTF-8 mode, the meanings of \w and \W can be  changed  by  setting  the
       PCRE_UCP  option. When this is done, it also affects \b and \B. Neither
       PCRE nor Perl has a separate "start of word" or "end of  word"  metase-
       quence.  However,  whatever follows \b normally determines which it is.
       For example, the fragment \ba matches "a" at the start of a word.

       The \A, \Z, and \z assertions differ from  the  traditional  circumflex
       and dollar (described in the next section) in that they only ever match
       at the very start and end of the subject string, whatever  options  are
       set.  Thus,  they are independent of multiline mode. These three asser-
       tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
       affect  only the behaviour of the circumflex and dollar metacharacters.
       However, if the startoffset argument of pcre_exec() is non-zero,  indi-
       cating that matching is to start at a point other than the beginning of
       the subject, \A can never match. The difference between \Z  and  \z  is
       that \Z matches before a newline at the end of the string as well as at
       the very end, whereas \z matches only at the end.

       The \G assertion is true only when the current matching position is  at
       the  start point of the match, as specified by the startoffset argument
       of pcre_exec(). It differs from \A when the  value  of  startoffset  is
       non-zero.  By calling pcre_exec() multiple times with appropriate argu-
       ments, you can mimic Perl's /g option, and it is in this kind of imple-
       mentation where \G can be useful.

       Note,  however,  that  PCRE's interpretation of \G, as the start of the
       current match, is subtly different from Perl's, which defines it as the
       end  of  the  previous  match. In Perl, these can be different when the
       previously matched string was empty. Because PCRE does just  one  match
       at a time, it cannot reproduce this behaviour.

       If  all  the alternatives of a pattern begin with \G, the expression is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.


CIRCUMFLEX AND DOLLAR

       Outside a character class, in the default matching mode, the circumflex
       character is an assertion that is true only  if  the  current  matching
       point  is  at the start of the subject string. If the startoffset argu-
       ment of pcre_exec() is non-zero, circumflex  can  never  match  if  the
       PCRE_MULTILINE  option  is  unset. Inside a character class, circumflex
       has an entirely different meaning (see below).

       Circumflex need not be the first character of the pattern if  a  number
       of  alternatives are involved, but it should be the first thing in each
       alternative in which it appears if the pattern is ever  to  match  that
       branch.  If all possible alternatives start with a circumflex, that is,
       if the pattern is constrained to match only at the start  of  the  sub-
       ject,  it  is  said  to be an "anchored" pattern. (There are also other
       constructs that can cause a pattern to be anchored.)

       A dollar character is an assertion that is true  only  if  the  current
       matching  point  is  at  the  end of the subject string, or immediately
       before a newline at the end of the string (by default). Dollar need not
       be  the  last  character of the pattern if a number of alternatives are
       involved, but it should be the last item in  any  branch  in  which  it
       appears. Dollar has no special meaning in a character class.

       The  meaning  of  dollar  can be changed so that it matches only at the
       very end of the string, by setting the  PCRE_DOLLAR_ENDONLY  option  at
       compile time. This does not affect the \Z assertion.

       The meanings of the circumflex and dollar characters are changed if the
       PCRE_MULTILINE option is set. When  this  is  the  case,  a  circumflex
       matches  immediately after internal newlines as well as at the start of
       the subject string. It does not match after a  newline  that  ends  the
       string.  A dollar matches before any newlines in the string, as well as
       at the very end, when PCRE_MULTILINE is set. When newline is  specified
       as  the  two-character  sequence CRLF, isolated CR and LF characters do
       not indicate newlines.

       For example, the pattern /^abc$/ matches the subject string  "def\nabc"
       (where  \n  represents a newline) in multiline mode, but not otherwise.
       Consequently, patterns that are anchored in single  line  mode  because
       all  branches  start  with  ^ are not anchored in multiline mode, and a
       match for circumflex is  possible  when  the  startoffset  argument  of
       pcre_exec()  is  non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
       PCRE_MULTILINE is set.

       Note that the sequences \A, \Z, and \z can be used to match  the  start
       and  end of the subject in both modes, and if all branches of a pattern
       start with \A it is always anchored, whether or not  PCRE_MULTILINE  is
       set.


FULL STOP (PERIOD, DOT) AND \N

       Outside a character class, a dot in the pattern matches any one charac-
       ter in the subject string except (by default) a character  that  signi-
       fies  the  end  of  a line. In UTF-8 mode, the matched character may be
       more than one byte long.

       When a line ending is defined as a single character, dot never  matches
       that  character; when the two-character sequence CRLF is used, dot does
       not match CR if it is immediately followed  by  LF,  but  otherwise  it
       matches  all characters (including isolated CRs and LFs). When any Uni-
       code line endings are being recognized, dot does not match CR or LF  or
       any of the other line ending characters.

       The  behaviour  of  dot  with regard to newlines can be changed. If the
       PCRE_DOTALL option is set, a dot matches  any  one  character,  without
       exception. If the two-character sequence CRLF is present in the subject
       string, it takes two dots to match it.

       The handling of dot is entirely independent of the handling of  circum-
       flex  and  dollar,  the  only relationship being that they both involve
       newlines. Dot has no special meaning in a character class.

       The escape sequence \N behaves like  a  dot,  except  that  it  is  not
       affected  by  the  PCRE_DOTALL  option.  In other words, it matches any
       character except one that signifies the end of a line.


MATCHING A SINGLE BYTE

       Outside a character class, the escape sequence \C matches any one byte,
       both  in  and  out  of  UTF-8 mode. Unlike a dot, it always matches any
       line-ending characters. The feature is provided in  Perl  in  order  to
       match  individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
       acters into individual bytes, the rest of the string may start  with  a
       malformed  UTF-8  character. For this reason, the \C escape sequence is
       best avoided.

       PCRE does not allow \C to appear in  lookbehind  assertions  (described
       below),  because  in UTF-8 mode this would make it impossible to calcu-
       late the length of the lookbehind.


SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial by default.  However, if the PCRE_JAVASCRIPT_COMPAT option is set,
       a lone closing square bracket causes a compile-time error. If a closing
       square bracket is required as a member of the class, it should  be  the
       first  data  character  in  the  class (after an initial circumflex, if
       present) or escaped with a backslash.

       A character class matches a single character in the subject.  In  UTF-8
       mode, the character may be more than one byte long. A matched character
       must be in the set of characters defined by the class, unless the first
       character  in  the  class definition is a circumflex, in which case the
       subject character must not be in the set defined by  the  class.  If  a
       circumflex  is actually required as a member of the class, ensure it is
       not the first character, or escape it with a backslash.

       For example, the character class [aeiou] matches any lower case  vowel,
       while  [^aeiou]  matches  any character that is not a lower case vowel.
       Note that a circumflex is just a convenient notation for specifying the
       characters  that  are in the class by enumerating those that are not. A
       class that starts with a circumflex is not an assertion; it still  con-
       sumes  a  character  from the subject string, and therefore it fails if
       the current pointer is at the end of the string.

       In UTF-8 mode, characters with values greater than 255 can be  included
       in  a  class as a literal string of bytes, or by using the \x{ escaping
       mechanism.

       When caseless matching is set, any letters in a  class  represent  both
       their  upper  case  and lower case versions, so for example, a caseless
       [aeiou] matches "A" as well as "a", and a caseless  [^aeiou]  does  not
       match  "A", whereas a caseful version would. In UTF-8 mode, PCRE always
       understands the concept of case for characters whose  values  are  less
       than  128, so caseless matching is always possible. For characters with
       higher values, the concept of case is supported  if  PCRE  is  compiled
       with  Unicode  property support, but not otherwise.  If you want to use
       caseless matching in UTF8-mode for characters 128 and above,  you  must
       ensure  that  PCRE is compiled with Unicode property support as well as
       with UTF-8 support.

       Characters that might indicate line breaks are  never  treated  in  any
       special  way  when  matching  character  classes,  whatever line-ending
       sequence is in  use,  and  whatever  setting  of  the  PCRE_DOTALL  and
       PCRE_MULTILINE options is used. A class such as [^a] always matches one
       of these characters.

       The minus (hyphen) character can be used to specify a range of  charac-
       ters  in  a  character  class.  For  example,  [d-m] matches any letter
       between d and m, inclusive. If a  minus  character  is  required  in  a
       class,  it  must  be  escaped  with a backslash or appear in a position
       where it cannot be interpreted as indicating a range, typically as  the
       first or last character in the class.

       It is not possible to have the literal character "]" as the end charac-
       ter of a range. A pattern such as [W-]46] is interpreted as a class  of
       two  characters ("W" and "-") followed by a literal string "46]", so it
       would match "W46]" or "-46]". However, if the "]"  is  escaped  with  a
       backslash  it is interpreted as the end of range, so [W-\]46] is inter-
       preted as a class containing a range followed by two other  characters.
       The  octal or hexadecimal representation of "]" can also be used to end
       a range.

       Ranges operate in the collating sequence of character values. They  can
       also   be  used  for  characters  specified  numerically,  for  example
       [\000-\037]. In UTF-8 mode, ranges can include characters whose  values
       are greater than 255, for example [\x{100}-\x{2ff}].

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to  [][\\^_`wxyzabc],  matched  caselessly,  and  in non-UTF-8 mode, if
       character tables for a French locale are in  use,  [\xc8-\xcb]  matches
       accented  E  characters in both cases. In UTF-8 mode, PCRE supports the
       concept of case for characters with values greater than 128  only  when
       it is compiled with Unicode property support.

       The  character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
       \w, and \W may appear in a character class, and add the characters that
       they  match to the class. For example, [\dABCDEF] matches any hexadeci-
       mal digit. In UTF-8 mode, the PCRE_UCP option affects the  meanings  of
       \d,  \s,  \w  and  their upper case partners, just as it does when they
       appear outside a character class, as described in the section  entitled
       "Generic character types" above. The escape sequence \b has a different
       meaning inside a character class; it matches the  backspace  character.
       The  sequences  \B,  \N,  \R, and \X are not special inside a character
       class. Like any other unrecognized escape sequences, they  are  treated
       as  the literal characters "B", "N", "R", and "X" by default, but cause
       an error if the PCRE_EXTRA option is set.

       A circumflex can conveniently be used with  the  upper  case  character
       types  to specify a more restricted set of characters than the matching
       lower case type.  For example, the class [^\W_] matches any  letter  or
       digit, but not underscore, whereas [\w] includes underscore. A positive
       character class should be read as "something OR something OR ..." and a
       negative class as "NOT something AND NOT something AND NOT ...".

       The  only  metacharacters  that are recognized in character classes are
       backslash, hyphen (only where it can be  interpreted  as  specifying  a
       range),  circumflex  (only  at the start), opening square bracket (only
       when it can be interpreted as introducing a POSIX class name - see  the
       next  section),  and  the  terminating closing square bracket. However,
       escaping other non-alphanumeric characters does no harm.


POSIX CHARACTER CLASSES

       Perl supports the POSIX notation for character classes. This uses names
       enclosed  by  [: and :] within the enclosing square brackets. PCRE also
       supports this notation. For example,

         [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class
       names are:

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits and space
         space    white space (not quite the same as \s)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The  "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
       and space (32). Notice that this list includes the VT  character  (code
       11). This makes "space" different to \s, which does not include VT (for
       Perl compatibility).

       The name "word" is a Perl extension, and "blank"  is  a  GNU  extension
       from  Perl  5.8. Another Perl extension is negation, which is indicated
       by a ^ character after the colon. For example,

         [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE (and Perl) also recognize  the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

       By default, in UTF-8 mode, characters with values greater than  128  do
       not  match any of the POSIX character classes. However, if the PCRE_UCP
       option is passed to pcre_compile(), some of the classes are changed  so
       that Unicode character properties are used. This is achieved by replac-
       ing the POSIX classes by other sequences, as follows:

         [:alnum:]  becomes  \p{Xan}
         [:alpha:]  becomes  \p{L}
         [:blank:]  becomes  \h
         [:digit:]  becomes  \p{Nd}
         [:lower:]  becomes  \p{Ll}
         [:space:]  becomes  \p{Xps}
         [:upper:]  becomes  \p{Lu}
         [:word:]   becomes  \p{Xwd}

       Negated versions, such as [:^alpha:] use \P instead of  \p.  The  other
       POSIX classes are unchanged, and match only characters with code points
       less than 128.


VERTICAL BAR

       Vertical bar characters are used to separate alternative patterns.  For
       example, the pattern

         gilbert|sullivan

       matches  either "gilbert" or "sullivan". Any number of alternatives may
       appear, and an empty  alternative  is  permitted  (matching  the  empty
       string). The matching process tries each alternative in turn, from left
       to right, and the first one that succeeds is used. If the  alternatives
       are  within a subpattern (defined below), "succeeds" means matching the
       rest of the main pattern as well as the alternative in the subpattern.


INTERNAL OPTION SETTING

       The settings of the  PCRE_CASELESS,  PCRE_MULTILINE,  PCRE_DOTALL,  and
       PCRE_EXTENDED  options  (which are Perl-compatible) can be changed from
       within the pattern by  a  sequence  of  Perl  option  letters  enclosed
       between "(?" and ")".  The option letters are

         i  for PCRE_CASELESS
         m  for PCRE_MULTILINE
         s  for PCRE_DOTALL
         x  for PCRE_EXTENDED

       For example, (?im) sets caseless, multiline matching. It is also possi-
       ble to unset these options by preceding the letter with a hyphen, and a
       combined  setting and unsetting such as (?im-sx), which sets PCRE_CASE-
       LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and  PCRE_EXTENDED,
       is  also  permitted.  If  a  letter  appears  both before and after the
       hyphen, the option is unset.

       The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and  PCRE_EXTRA
       can  be changed in the same way as the Perl-compatible options by using
       the characters J, U and X respectively.

       When one of these option changes occurs at  top  level  (that  is,  not
       inside  subpattern parentheses), the change applies to the remainder of
       the pattern that follows. If the change is placed right at the start of
       a pattern, PCRE extracts it into the global options (and it will there-
       fore show up in data extracted by the pcre_fullinfo() function).

       An option change within a subpattern (see below for  a  description  of
       subpatterns)  affects only that part of the subpattern that follows it,
       so

         (a(?i)b)c

       matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
       used).   By  this means, options can be made to have different settings
       in different parts of the pattern. Any changes made in one  alternative
       do  carry  on  into subsequent branches within the same subpattern. For
       example,

         (a(?i)b|c)

       matches "ab", "aB", "c", and "C", even though  when  matching  "C"  the
       first  branch  is  abandoned before the option setting. This is because
       the effects of option settings happen at compile time. There  would  be
       some very weird behaviour otherwise.

       Note:  There  are  other  PCRE-specific  options that can be set by the
       application when the compile or match functions  are  called.  In  some
       cases the pattern can contain special leading sequences such as (*CRLF)
       to override what the application has set or what  has  been  defaulted.
       Details  are  given  in the section entitled "Newline sequences" above.
       There are also the (*UTF8) and (*UCP) leading  sequences  that  can  be
       used  to  set  UTF-8 and Unicode property modes; they are equivalent to
       setting the PCRE_UTF8 and the PCRE_UCP options, respectively.


SUBPATTERNS

       Subpatterns are delimited by parentheses (round brackets), which can be
       nested.  Turning part of a pattern into a subpattern does two things:

       1. It localizes a set of alternatives. For example, the pattern

         cat(aract|erpillar|)

       matches  "cataract",  "caterpillar", or "cat". Without the parentheses,
       it would match "cataract", "erpillar" or an empty string.

       2. It sets up the subpattern as  a  capturing  subpattern.  This  means
       that,  when  the  whole  pattern  matches,  that portion of the subject
       string that matched the subpattern is passed back to the caller via the
       ovector  argument  of pcre_exec(). Opening parentheses are counted from
       left to right (starting from 1) to obtain  numbers  for  the  capturing
       subpatterns.  For  example,  if  the  string  "the red king" is matched
       against the pattern

         the ((red|white) (king|queen))

       the captured substrings are "red king", "red", and "king", and are num-
       bered 1, 2, and 3, respectively.

       The  fact  that  plain  parentheses  fulfil two functions is not always
       helpful.  There are often times when a grouping subpattern is  required
       without  a capturing requirement. If an opening parenthesis is followed
       by a question mark and a colon, the subpattern does not do any  captur-
       ing,  and  is  not  counted when computing the number of any subsequent
       capturing subpatterns. For example, if the string "the white queen"  is
       matched against the pattern

         the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and are numbered
       1 and 2. The maximum number of capturing subpatterns is 65535.

       As a convenient shorthand, if any option settings are required  at  the
       start  of  a  non-capturing  subpattern,  the option letters may appear
       between the "?" and the ":". Thus the two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are
       tried  from  left  to right, and options are not reset until the end of
       the subpattern is reached, an option setting in one branch does  affect
       subsequent  branches,  so  the above patterns match "SUNDAY" as well as
       "Saturday".


DUPLICATE SUBPATTERN NUMBERS

       Perl 5.10 introduced a feature whereby each alternative in a subpattern
       uses  the same numbers for its capturing parentheses. Such a subpattern
       starts with (?| and is itself a non-capturing subpattern. For  example,
       consider this pattern:

         (?|(Sat)ur|(Sun))day

       Because  the two alternatives are inside a (?| group, both sets of cap-
       turing parentheses are numbered one. Thus, when  the  pattern  matches,
       you  can  look  at captured substring number one, whichever alternative
       matched. This construct is useful when you want to  capture  part,  but
       not all, of one of a number of alternatives. Inside a (?| group, paren-
       theses are numbered as usual, but the number is reset at the  start  of
       each  branch.  The numbers of any capturing parentheses that follow the
       subpattern start after the highest number used in any branch. The  fol-
       lowing example is taken from the Perl documentation. The numbers under-
       neath show in which buffer the captured content will be stored.

         # before  ---------------branch-reset----------- after
         / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
         # 1            2         2  3        2     3     4

       A back reference to a numbered subpattern uses the  most  recent  value
       that  is  set  for that number by any subpattern. The following pattern
       matches "abcabc" or "defdef":

         /(?|(abc)|(def))\1/

       In contrast, a recursive or "subroutine" call to a numbered  subpattern
       always  refers  to  the first one in the pattern with the given number.
       The following pattern matches "abcabc" or "defabc":

         /(?|(abc)|(def))(?1)/

       If a condition test for a subpattern's having matched refers to a  non-
       unique  number, the test is true if any of the subpatterns of that num-
       ber have matched.

       An alternative approach to using this "branch reset" feature is to  use
       duplicate named subpatterns, as described in the next section.


NAMED SUBPATTERNS

       Identifying  capturing  parentheses  by number is simple, but it can be
       very hard to keep track of the numbers in complicated  regular  expres-
       sions.  Furthermore,  if  an  expression  is  modified, the numbers may
       change. To help with this difficulty, PCRE supports the naming of  sub-
       patterns. This feature was not added to Perl until release 5.10. Python
       had the feature earlier, and PCRE introduced it at release  4.0,  using
       the  Python syntax. PCRE now supports both the Perl and the Python syn-
       tax. Perl allows identically numbered  subpatterns  to  have  different
       names, but PCRE does not.

       In  PCRE,  a subpattern can be named in one of three ways: (?<name>...)
       or (?'name'...) as in Perl, or (?P<name>...) as in  Python.  References
       to  capturing parentheses from other parts of the pattern, such as back
       references, recursion, and conditions, can be made by name as  well  as
       by number.

       Names  consist  of  up  to  32 alphanumeric characters and underscores.
       Named capturing parentheses are still  allocated  numbers  as  well  as
       names,  exactly as if the names were not present. The PCRE API provides
       function calls for extracting the name-to-number translation table from
       a compiled pattern. There is also a convenience function for extracting
       a captured substring by name.

       By default, a name must be unique within a pattern, but it is  possible
       to relax this constraint by setting the PCRE_DUPNAMES option at compile
       time. (Duplicate names are also always permitted for  subpatterns  with
       the  same  number, set up as described in the previous section.) Dupli-
       cate names can be useful for patterns where only one  instance  of  the
       named  parentheses  can  match. Suppose you want to match the name of a
       weekday, either as a 3-letter abbreviation or as the full name, and  in
       both cases you want to extract the abbreviation. This pattern (ignoring
       the line breaks) does the job:

         (?<DN>Mon|Fri|Sun)(?:day)?|
         (?<DN>Tue)(?:sday)?|
         (?<DN>Wed)(?:nesday)?|
         (?<DN>Thu)(?:rsday)?|
         (?<DN>Sat)(?:urday)?

       There are five capturing substrings, but only one is ever set  after  a
       match.  (An alternative way of solving this problem is to use a "branch
       reset" subpattern, as described in the previous section.)

       The convenience function for extracting the data by  name  returns  the
       substring  for  the first (and in this example, the only) subpattern of
       that name that matched. This saves searching  to  find  which  numbered
       subpattern it was.

       If  you  make  a  back  reference to a non-unique named subpattern from
       elsewhere in the pattern, the one that corresponds to the first  occur-
       rence of the name is used. In the absence of duplicate numbers (see the
       previous section) this is the one with the lowest number. If you use  a
       named  reference  in a condition test (see the section about conditions
       below), either to check whether a subpattern has matched, or  to  check
       for  recursion,  all  subpatterns with the same name are tested. If the
       condition is true for any one of them, the overall condition  is  true.
       This is the same behaviour as testing by number. For further details of
       the interfaces for handling named subpatterns, see the pcreapi documen-
       tation.

       Warning: You cannot use different names to distinguish between two sub-
       patterns with the same number because PCRE uses only the  numbers  when
       matching. For this reason, an error is given at compile time if differ-
       ent names are given to subpatterns with the same number.  However,  you
       can  give  the same name to subpatterns with the same number, even when
       PCRE_DUPNAMES is not set.


REPETITION

       Repetition is specified by quantifiers, which can  follow  any  of  the
       following items:

         a literal data character
         the dot metacharacter
         the \C escape sequence
         the \X escape sequence (in UTF-8 mode with Unicode properties)
         the \R escape sequence
         an escape such as \d or \pL that matches a single character
         a character class
         a back reference (see next section)
         a parenthesized subpattern (unless it is an assertion)
         a recursive or "subroutine" call to a subpattern

       The  general repetition quantifier specifies a minimum and maximum num-
       ber of permitted matches, by giving the two numbers in  curly  brackets
       (braces),  separated  by  a comma. The numbers must be less than 65536,
       and the first must be less than or equal to the second. For example:

         z{2,4}

       matches "zz", "zzz", or "zzzz". A closing brace on its  own  is  not  a
       special  character.  If  the second number is omitted, but the comma is
       present, there is no upper limit; if the second number  and  the  comma
       are  both omitted, the quantifier specifies an exact number of required
       matches. Thus

         [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, while

         \d{8}

       matches exactly 8 digits. An opening curly bracket that  appears  in  a
       position  where a quantifier is not allowed, or one that does not match
       the syntax of a quantifier, is taken as a literal character. For  exam-
       ple, {,6} is not a quantifier, but a literal string of four characters.

       In  UTF-8  mode,  quantifiers  apply to UTF-8 characters rather than to
       individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
       acters, each of which is represented by a two-byte sequence. Similarly,
       when Unicode property support is available, \X{3} matches three Unicode
       extended  sequences,  each of which may be several bytes long (and they
       may be of different lengths).

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be use-
       ful for subpatterns that are referenced as subroutines  from  elsewhere
       in the pattern (but see also the section entitled "Defining subpatterns
       for use by reference only" below). Items other  than  subpatterns  that
       have a {0} quantifier are omitted from the compiled pattern.

       For  convenience, the three most common quantifiers have single-charac-
       ter abbreviations:

         *    is equivalent to {0,}
         +    is equivalent to {1,}
         ?    is equivalent to {0,1}

       It is possible to construct infinite loops by  following  a  subpattern
       that can match no characters with a quantifier that has no upper limit,
       for example:

         (a?)*

       Earlier versions of Perl and PCRE used to give an error at compile time
       for  such  patterns. However, because there are cases where this can be
       useful, such patterns are now accepted, but if any  repetition  of  the
       subpattern  does in fact match no characters, the loop is forcibly bro-
       ken.

       By default, the quantifiers are "greedy", that is, they match  as  much
       as  possible  (up  to  the  maximum number of permitted times), without
       causing the rest of the pattern to fail. The classic example  of  where
       this gives problems is in trying to match comments in C programs. These
       appear between /* and */ and within the comment,  individual  *  and  /
       characters  may  appear. An attempt to match C comments by applying the
       pattern

         /\*.*\*/

       to the string

         /* first comment */  not comment  /* second comment */

       fails, because it matches the entire string owing to the greediness  of
       the .*  item.

       However,  if  a quantifier is followed by a question mark, it ceases to
       be greedy, and instead matches the minimum number of times possible, so
       the pattern

         /\*.*?\*/

       does  the  right  thing with the C comments. The meaning of the various
       quantifiers is not otherwise changed,  just  the  preferred  number  of
       matches.   Do  not  confuse this use of question mark with its use as a
       quantifier in its own right. Because it has two uses, it can  sometimes
       appear doubled, as in

         \d??\d

       which matches one digit by preference, but can match two if that is the
       only way the rest of the pattern matches.

       If the PCRE_UNGREEDY option is set (an option that is not available  in
       Perl),  the  quantifiers are not greedy by default, but individual ones
       can be made greedy by following them with a  question  mark.  In  other
       words, it inverts the default behaviour.

       When  a  parenthesized  subpattern  is quantified with a minimum repeat
       count that is greater than 1 or with a limited maximum, more memory  is
       required  for  the  compiled  pattern, in proportion to the size of the
       minimum or maximum.

       If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
       alent  to  Perl's  /s) is set, thus allowing the dot to match newlines,
       the pattern is implicitly anchored, because whatever  follows  will  be
       tried  against every character position in the subject string, so there
       is no point in retrying the overall match at  any  position  after  the
       first.  PCRE  normally treats such a pattern as though it were preceded
       by \A.

       In cases where it is known that the subject  string  contains  no  new-
       lines,  it  is  worth setting PCRE_DOTALL in order to obtain this opti-
       mization, or alternatively using ^ to indicate anchoring explicitly.

       However, there is one situation where the optimization cannot be  used.
       When .*  is inside capturing parentheses that are the subject of a back
       reference elsewhere in the pattern, a match at the start may fail where
       a later one succeeds. Consider, for example:

         (.*)abc\1

       If  the subject is "xyz123abc123" the match point is the fourth charac-
       ter. For this reason, such a pattern is not implicitly anchored.

       When a capturing subpattern is repeated, the value captured is the sub-
       string that matched the final iteration. For example, after

         (tweedle[dume]{3}\s*)+

       has matched "tweedledum tweedledee" the value of the captured substring
       is "tweedledee". However, if there are  nested  capturing  subpatterns,
       the  corresponding captured values may have been set in previous itera-
       tions. For example, after

         /(a|(b))+/

       matches "aba" the value of the second captured substring is "b".


ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

       With both maximizing ("greedy") and minimizing ("ungreedy"  or  "lazy")
       repetition,  failure  of what follows normally causes the repeated item
       to be re-evaluated to see if a different number of repeats  allows  the
       rest  of  the pattern to match. Sometimes it is useful to prevent this,
       either to change the nature of the match, or to cause it  fail  earlier
       than  it otherwise might, when the author of the pattern knows there is
       no point in carrying on.

       Consider, for example, the pattern \d+foo when applied to  the  subject
       line

         123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       action of the matcher is to try again with only 5 digits  matching  the
       \d+  item,  and  then  with  4,  and  so on, before ultimately failing.
       "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides
       the  means for specifying that once a subpattern has matched, it is not
       to be re-evaluated in this way.

       If we use atomic grouping for the previous example, the  matcher  gives
       up  immediately  on failing to match "foo" the first time. The notation
       is a kind of special parenthesis, starting with (?> as in this example:

         (?>\d+)foo

       This kind of parenthesis "locks up" the  part of the  pattern  it  con-
       tains  once  it  has matched, and a failure further into the pattern is
       prevented from backtracking into it. Backtracking past it  to  previous
       items, however, works as normal.

       An  alternative  description  is that a subpattern of this type matches
       the string of characters that an  identical  standalone  pattern  would
       match, if anchored at the current point in the subject string.

       Atomic grouping subpatterns are not capturing subpatterns. Simple cases
       such as the above example can be thought of as a maximizing repeat that
       must  swallow  everything  it can. So, while both \d+ and \d+? are pre-
       pared to adjust the number of digits they match in order  to  make  the
       rest of the pattern match, (?>\d+) can only match an entire sequence of
       digits.

       Atomic groups in general can of course contain arbitrarily  complicated
       subpatterns,  and  can  be  nested. However, when the subpattern for an
       atomic group is just a single repeated item, as in the example above, a
       simpler  notation,  called  a "possessive quantifier" can be used. This
       consists of an additional + character  following  a  quantifier.  Using
       this notation, the previous example can be rewritten as

         \d++foo

       Note that a possessive quantifier can be used with an entire group, for
       example:

         (abc|xyz){2,3}+

       Possessive  quantifiers  are  always  greedy;  the   setting   of   the
       PCRE_UNGREEDY option is ignored. They are a convenient notation for the
       simpler forms of atomic group. However, there is no difference  in  the
       meaning  of  a  possessive  quantifier and the equivalent atomic group,
       though there may be a performance  difference;  possessive  quantifiers
       should be slightly faster.

       The  possessive  quantifier syntax is an extension to the Perl 5.8 syn-
       tax.  Jeffrey Friedl originated the idea (and the name)  in  the  first
       edition of his book. Mike McCloskey liked it, so implemented it when he
       built Sun's Java package, and PCRE copied it from there. It  ultimately
       found its way into Perl at release 5.10.

       PCRE has an optimization that automatically "possessifies" certain sim-
       ple pattern constructs. For example, the sequence  A+B  is  treated  as
       A++B  because  there is no point in backtracking into a sequence of A's
       when B must follow.

       When a pattern contains an unlimited repeat inside  a  subpattern  that
       can  itself  be  repeated  an  unlimited number of times, the use of an
       atomic group is the only way to avoid some  failing  matches  taking  a
       very long time indeed. The pattern

         (\D+|<\d+>)*[!?]

       matches  an  unlimited number of substrings that either consist of non-
       digits, or digits enclosed in <>, followed by either ! or  ?.  When  it
       matches, it runs quickly. However, if it is applied to

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it  takes  a  long  time  before reporting failure. This is because the
       string can be divided between the internal \D+ repeat and the  external
       *  repeat  in  a  large  number of ways, and all have to be tried. (The
       example uses [!?] rather than a single character at  the  end,  because
       both  PCRE  and  Perl have an optimization that allows for fast failure
       when a single character is used. They remember the last single  charac-
       ter  that  is required for a match, and fail early if it is not present
       in the string.) If the pattern is changed so that  it  uses  an  atomic
       group, like this:

         ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens quickly.


BACK REFERENCES

       Outside a character class, a backslash followed by a digit greater than
       0 (and possibly further digits) is a back reference to a capturing sub-
       pattern  earlier  (that is, to its left) in the pattern, provided there
       have been that many previous capturing left parentheses.

       However, if the decimal number following the backslash is less than 10,
       it  is  always  taken  as a back reference, and causes an error only if
       there are not that many capturing left parentheses in the  entire  pat-
       tern.  In  other words, the parentheses that are referenced need not be
       to the left of the reference for numbers less than 10. A "forward  back
       reference"  of  this  type can make sense when a repetition is involved
       and the subpattern to the right has participated in an  earlier  itera-
       tion.

       It  is  not  possible to have a numerical "forward back reference" to a
       subpattern whose number is 10 or  more  using  this  syntax  because  a
       sequence  such  as  \50 is interpreted as a character defined in octal.
       See the subsection entitled "Non-printing characters" above for further
       details  of  the  handling of digits following a backslash. There is no
       such problem when named parentheses are used. A back reference  to  any
       subpattern is possible using named parentheses (see below).

       Another  way  of  avoiding  the ambiguity inherent in the use of digits
       following a backslash is to use the \g  escape  sequence.  This  escape
       must be followed by an unsigned number or a negative number, optionally
       enclosed in braces. These examples are all identical:

         (ring), \1
         (ring), \g1
         (ring), \g{1}

       An unsigned number specifies an absolute reference without the  ambigu-
       ity that is present in the older syntax. It is also useful when literal
       digits follow the reference. A negative number is a relative reference.
       Consider this example:

         (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a reference to the most recently started captur-
       ing subpattern before \g, that is, is it equivalent to \2 in this exam-
       ple.   Similarly, \g{-2} would be equivalent to \1. The use of relative
       references can be helpful in long patterns, and also in  patterns  that
       are  created  by  joining  together  fragments  that contain references
       within themselves.

       A back reference matches whatever actually matched the  capturing  sub-
       pattern  in  the  current subject string, rather than anything matching
       the subpattern itself (see "Subpatterns as subroutines" below for a way
       of doing that). So the pattern

         (sens|respons)e and \1ibility

       matches  "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If caseful matching is in force at  the
       time  of the back reference, the case of letters is relevant. For exam-
       ple,

         ((?i)rah)\s+\1

       matches "rah rah" and "RAH RAH", but not "RAH  rah",  even  though  the
       original capturing subpattern is matched caselessly.

       There  are  several  different ways of writing back references to named
       subpatterns. The .NET syntax \k{name} and the Perl syntax  \k<name>  or
       \k'name'  are supported, as is the Python syntax (?P=name). Perl 5.10's
       unified back reference syntax, in which \g can be used for both numeric
       and  named  references,  is  also supported. We could rewrite the above
       example in any of the following ways:

         (?<p1>(?i)rah)\s+\k<p1>
         (?'p1'(?i)rah)\s+\k{p1}
         (?P<p1>(?i)rah)\s+(?P=p1)
         (?<p1>(?i)rah)\s+\g{p1}

       A subpattern that is referenced by  name  may  appear  in  the  pattern
       before or after the reference.

       There  may be more than one back reference to the same subpattern. If a
       subpattern has not actually been used in a particular match,  any  back
       references to it always fail by default. For example, the pattern

         (a|(bc))\2

       always  fails  if  it starts to match "a" rather than "bc". However, if
       the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
       ence to an unset value matches an empty string.

       Because  there may be many capturing parentheses in a pattern, all dig-
       its following a backslash are taken as part of a potential back  refer-
       ence  number.   If  the  pattern continues with a digit character, some
       delimiter must  be  used  to  terminate  the  back  reference.  If  the
       PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{
       syntax or an empty comment (see "Comments" below) can be used.

   Recursive back references

       A back reference that occurs inside the parentheses to which it  refers
       fails  when  the subpattern is first used, so, for example, (a\1) never
       matches.  However, such references can be useful inside  repeated  sub-
       patterns. For example, the pattern

         (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
       ation of the subpattern,  the  back  reference  matches  the  character
       string  corresponding  to  the previous iteration. In order for this to
       work, the pattern must be such that the first iteration does  not  need
       to  match the back reference. This can be done using alternation, as in
       the example above, or by a quantifier with a minimum of zero.

       Back references of this type cause the group that they reference to  be
       treated  as  an atomic group.  Once the whole group has been matched, a
       subsequent matching failure cannot cause backtracking into  the  middle
       of the group.


ASSERTIONS

       An  assertion  is  a  test on the characters following or preceding the
       current matching point that does not actually consume  any  characters.
       The  simple  assertions  coded  as  \b, \B, \A, \G, \Z, \z, ^ and $ are
       described above.

       More complicated assertions are coded as  subpatterns.  There  are  two
       kinds:  those  that  look  ahead of the current position in the subject
       string, and those that look  behind  it.  An  assertion  subpattern  is
       matched  in  the  normal way, except that it does not cause the current
       matching position to be changed.

       Assertion subpatterns are not capturing subpatterns,  and  may  not  be
       repeated,  because  it  makes no sense to assert the same thing several
       times. If any kind of assertion contains capturing  subpatterns  within
       it,  these are counted for the purposes of numbering the capturing sub-
       patterns in the whole pattern.  However, substring capturing is carried
       out  only  for  positive assertions, because it does not make sense for
       negative assertions.

   Lookahead assertions

       Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

         \w+(?=;)

       matches  a word followed by a semicolon, but does not include the semi-
       colon in the match, and

         foo(?!bar)

       matches any occurrence of "foo" that is not  followed  by  "bar".  Note
       that the apparently similar pattern

         (?!foo)bar

       does  not  find  an  occurrence  of "bar" that is preceded by something
       other than "foo"; it finds any occurrence of "bar" whatsoever,  because
       the assertion (?!foo) is always true when the next three characters are
       "bar". A lookbehind assertion is needed to achieve the other effect.

       If you want to force a matching failure at some point in a pattern, the
       most  convenient  way  to  do  it  is with (?!) because an empty string
       always matches, so an assertion that requires there not to be an  empty
       string must always fail.  The backtracking control verb (*FAIL) or (*F)
       is a synonym for (?!).

   Lookbehind assertions

       Lookbehind assertions start with (?<= for positive assertions and  (?<!
       for negative assertions. For example,

         (?<!foo)bar

       does  find  an  occurrence  of "bar" that is not preceded by "foo". The
       contents of a lookbehind assertion are restricted  such  that  all  the
       strings it matches must have a fixed length. However, if there are sev-
       eral top-level alternatives, they do not all  have  to  have  the  same
       fixed length. Thus

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

       causes  an  error at compile time. Branches that match different length
       strings are permitted only at the top level of a lookbehind  assertion.
       This is an extension compared with Perl, which requires all branches to
       match the same length of string. An assertion such as

         (?<=ab(c|de))

       is not permitted, because its single top-level  branch  can  match  two
       different lengths, but it is acceptable to PCRE if rewritten to use two
       top-level branches:

         (?<=abc|abde)

       In some cases, the escape sequence \K (see above) can be  used  instead
       of a lookbehind assertion to get round the fixed-length restriction.

       The  implementation  of lookbehind assertions is, for each alternative,
       to temporarily move the current position back by the fixed  length  and
       then try to match. If there are insufficient characters before the cur-
       rent position, the assertion fails.

       PCRE does not allow the \C escape (which matches a single byte in UTF-8
       mode)  to appear in lookbehind assertions, because it makes it impossi-
       ble to calculate the length of the lookbehind. The \X and  \R  escapes,
       which can match different numbers of bytes, are also not permitted.

       "Subroutine"  calls  (see below) such as (?2) or (?&X) are permitted in
       lookbehinds, as long as the subpattern matches a  fixed-length  string.
       Recursion, however, is not supported.

       Possessive  quantifiers  can  be  used  in  conjunction with lookbehind
       assertions to specify efficient matching of fixed-length strings at the
       end of subject strings. Consider a simple pattern such as

         abcd$

       when  applied  to  a  long string that does not match. Because matching
       proceeds from left to right, PCRE will look for each "a" in the subject
       and  then  see  if what follows matches the rest of the pattern. If the
       pattern is specified as

         ^.*abcd$

       the initial .* matches the entire string at first, but when this  fails
       (because there is no following "a"), it backtracks to match all but the
       last character, then all but the last two characters, and so  on.  Once
       again  the search for "a" covers the entire string, from right to left,
       so we are no better off. However, if the pattern is written as

         ^.*+(?<=abcd)

       there can be no backtracking for the .*+ item; it can  match  only  the
       entire  string.  The subsequent lookbehind assertion does a single test
       on the last four characters. If it fails, the match fails  immediately.
       For  long  strings, this approach makes a significant difference to the
       processing time.

   Using multiple assertions

       Several assertions (of any sort) may occur in succession. For example,

         (?<=\d{3})(?<!999)foo

       matches "foo" preceded by three digits that are not "999". Notice  that
       each  of  the  assertions is applied independently at the same point in
       the subject string. First there is a  check  that  the  previous  three
       characters  are  all  digits,  and  then there is a check that the same
       three characters are not "999".  This pattern does not match "foo" pre-
       ceded  by  six  characters,  the first of which are digits and the last
       three of which are not "999". For example, it  doesn't  match  "123abc-
       foo". A pattern to do that is

         (?<=\d{3}...)(?<!999)foo

       This  time  the  first assertion looks at the preceding six characters,
       checking that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

       Assertions can be nested in any combination. For example,

         (?<=(?<!foo)bar)baz

       matches  an occurrence of "baz" that is preceded by "bar" which in turn
       is not preceded by "foo", while

         (?<=\d{3}(?!999)...)foo

       is another pattern that matches "foo" preceded by three digits and  any
       three characters that are not "999".


CONDITIONAL SUBPATTERNS

       It  is possible to cause the matching process to obey a subpattern con-
       ditionally or to choose between two alternative subpatterns,  depending
       on  the result of an assertion, or whether a specific capturing subpat-
       tern has already been matched. The two possible  forms  of  conditional
       subpattern are:

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

       If  the  condition is satisfied, the yes-pattern is used; otherwise the
       no-pattern (if present) is used. If there are more  than  two  alterna-
       tives  in  the subpattern, a compile-time error occurs. Each of the two
       alternatives may itself contain nested subpatterns of any form, includ-
       ing  conditional  subpatterns;  the  restriction  to  two  alternatives
       applies only at the level of the condition. This pattern fragment is an
       example where the alternatives are complex:

         (?(1) (A|B|C) | (D | (?(2)E|F) | E) )


       There  are  four  kinds of condition: references to subpatterns, refer-
       ences to recursion, a pseudo-condition called DEFINE, and assertions.

   Checking for a used subpattern by number

       If the text between the parentheses consists of a sequence  of  digits,
       the condition is true if a capturing subpattern of that number has pre-
       viously matched. If there is more than one  capturing  subpattern  with
       the  same  number  (see  the earlier section about duplicate subpattern
       numbers), the condition is true if any of them have matched. An  alter-
       native  notation is to precede the digits with a plus or minus sign. In
       this case, the subpattern number is relative rather than absolute.  The
       most  recently opened parentheses can be referenced by (?(-1), the next
       most recent by (?(-2), and so on. Inside loops it can also  make  sense
       to refer to subsequent groups. The next parentheses to be opened can be
       referenced as (?(+1), and so on. (The value zero in any of these  forms
       is not used; it provokes a compile-time error.)

       Consider  the  following  pattern, which contains non-significant white
       space to make it more readable (assume the PCRE_EXTENDED option) and to
       divide it into three parts for ease of discussion:

         ( \( )?    [^()]+    (?(1) \) )

       The  first  part  matches  an optional opening parenthesis, and if that
       character is present, sets it as the first captured substring. The sec-
       ond  part  matches one or more characters that are not parentheses. The
       third part is a conditional subpattern that tests whether  or  not  the
       first  set  of  parentheses  matched.  If they did, that is, if subject
       started with an opening parenthesis, the condition is true, and so  the
       yes-pattern  is  executed and a closing parenthesis is required. Other-
       wise, since no-pattern is not present, the subpattern matches  nothing.
       In  other  words,  this  pattern matches a sequence of non-parentheses,
       optionally enclosed in parentheses.

       If you were embedding this pattern in a larger one,  you  could  use  a
       relative reference:

         ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

       This  makes  the  fragment independent of the parentheses in the larger
       pattern.

   Checking for a used subpattern by name

       Perl uses the syntax (?(<name>)...) or (?('name')...)  to  test  for  a
       used  subpattern  by  name.  For compatibility with earlier versions of
       PCRE, which had this facility before Perl, the syntax  (?(name)...)  is
       also  recognized. However, there is a possible ambiguity with this syn-
       tax, because subpattern names may  consist  entirely  of  digits.  PCRE
       looks  first for a named subpattern; if it cannot find one and the name
       consists entirely of digits, PCRE looks for a subpattern of  that  num-
       ber,  which must be greater than zero. Using subpattern names that con-
       sist entirely of digits is not recommended.

       Rewriting the above example to use a named subpattern gives this:

         (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )

       If the name used in a condition of this kind is a duplicate,  the  test
       is  applied to all subpatterns of the same name, and is true if any one
       of them has matched.

   Checking for pattern recursion

       If the condition is the string (R), and there is no subpattern with the
       name  R, the condition is true if a recursive call to the whole pattern
       or any subpattern has been made. If digits or a name preceded by amper-
       sand follow the letter R, for example:

         (?(R3)...) or (?(R&name)...)

       the condition is true if the most recent recursion is into a subpattern
       whose number or name is given. This condition does not check the entire
       recursion  stack.  If  the  name  used in a condition of this kind is a
       duplicate, the test is applied to all subpatterns of the same name, and
       is true if any one of them is the most recent recursion.

       At  "top  level",  all  these recursion test conditions are false.  The
       syntax for recursive patterns is described below.

   Defining subpatterns for use by reference only

       If the condition is the string (DEFINE), and  there  is  no  subpattern
       with  the  name  DEFINE,  the  condition is always false. In this case,
       there may be only one alternative  in  the  subpattern.  It  is  always
       skipped  if  control  reaches  this  point  in the pattern; the idea of
       DEFINE is that it can be used to define "subroutines" that can be  ref-
       erenced  from elsewhere. (The use of "subroutines" is described below.)
       For  example,  a  pattern  to   match   an   IPv4   address   such   as
       "192.168.23.245" could be written like this (ignore whitespace and line
       breaks):

         (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
         \b (?&byte) (\.(?&byte)){3} \b

       The first part of the pattern is a DEFINE group inside which a  another
       group  named "byte" is defined. This matches an individual component of
       an IPv4 address (a number less than 256). When  matching  takes  place,
       this  part  of  the pattern is skipped because DEFINE acts like a false
       condition. The rest of the pattern uses references to the  named  group
       to  match the four dot-separated components of an IPv4 address, insist-
       ing on a word boundary at each end.

   Assertion conditions

       If the condition is not in any of the above  formats,  it  must  be  an
       assertion.   This may be a positive or negative lookahead or lookbehind
       assertion. Consider  this  pattern,  again  containing  non-significant
       white space, and with the two alternatives on the second line:

         (?(?=[^a-z]*[a-z])
         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The  condition  is  a  positive  lookahead  assertion  that  matches an
       optional sequence of non-letters followed by a letter. In other  words,
       it  tests  for the presence of at least one letter in the subject. If a
       letter is found, the subject is matched against the first  alternative;
       otherwise  it  is  matched  against  the  second.  This pattern matches
       strings in one of the two forms dd-aaa-dd or dd-dd-dd,  where  aaa  are
       letters and dd are digits.


COMMENTS

       There are two ways of including comments in patterns that are processed
       by PCRE. In both cases, the start of the comment must not be in a char-
       acter class, nor in the middle of any other sequence of related charac-
       ters such as (?: or a subpattern name or number.  The  characters  that
       make up a comment play no part in the pattern matching.

       The  sequence (?# marks the start of a comment that continues up to the
       next closing parenthesis. Nested parentheses are not permitted. If  the
       PCRE_EXTENDED option is set, an unescaped # character also introduces a
       comment, which in this case continues to  immediately  after  the  next
       newline  character  or character sequence in the pattern. Which charac-
       ters are interpreted as newlines is controlled by the options passed to
       pcre_compile() or by a special sequence at the start of the pattern, as
       described in the section entitled  "Newline  conventions"  above.  Note
       that  the  end of this type of comment is a literal newline sequence in
       the pattern; escape sequences that happen to represent a newline do not
       count.  For  example,  consider this pattern when PCRE_EXTENDED is set,
       and the default newline convention is in force:

         abc #comment \n still comment

       On encountering the # character, pcre_compile()  skips  along,  looking
       for  a newline in the pattern. The sequence \n is still literal at this
       stage, so it does not terminate the comment. Only an  actual  character
       with the code value 0x0a (the default newline) does so.


RECURSIVE PATTERNS

       Consider  the problem of matching a string in parentheses, allowing for
       unlimited nested parentheses. Without the use of  recursion,  the  best
       that  can  be  done  is  to use a pattern that matches up to some fixed
       depth of nesting. It is not possible to  handle  an  arbitrary  nesting
       depth.

       For some time, Perl has provided a facility that allows regular expres-
       sions to recurse (amongst other things). It does this by  interpolating
       Perl  code in the expression at run time, and the code can refer to the
       expression itself. A Perl pattern using code interpolation to solve the
       parentheses problem can be created like this:

         $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears.

       Obviously, PCRE cannot support the interpolation of Perl code. Instead,
       it  supports  special  syntax  for recursion of the entire pattern, and
       also for individual subpattern recursion.  After  its  introduction  in
       PCRE  and  Python,  this  kind of recursion was subsequently introduced
       into Perl at release 5.10.

       A special item that consists of (? followed by a  number  greater  than
       zero and a closing parenthesis is a recursive call of the subpattern of
       the given number, provided that it occurs inside that  subpattern.  (If
       not,  it  is  a  "subroutine" call, which is described in the next sec-
       tion.) The special item (?R) or (?0) is a recursive call of the  entire
       regular expression.

       This  PCRE  pattern  solves  the nested parentheses problem (assume the
       PCRE_EXTENDED option is set so that white space is ignored):

         \( ( [^()]++ | (?R) )* \)

       First it matches an opening parenthesis. Then it matches any number  of
       substrings  which  can  either  be  a sequence of non-parentheses, or a
       recursive match of the pattern itself (that is, a  correctly  parenthe-
       sized substring).  Finally there is a closing parenthesis. Note the use
       of a possessive quantifier to avoid backtracking into sequences of non-
       parentheses.

       If  this  were  part of a larger pattern, you would not want to recurse
       the entire pattern, so instead you could use this:

         ( \( ( [^()]++ | (?1) )* \) )

       We have put the pattern into parentheses, and caused the  recursion  to
       refer to them instead of the whole pattern.

       In  a  larger  pattern,  keeping  track  of  parenthesis numbers can be
       tricky. This is made easier by the use of relative references.  Instead
       of (?1) in the pattern above you can write (?-2) to refer to the second
       most recently opened parentheses  preceding  the  recursion.  In  other
       words,  a  negative  number counts capturing parentheses leftwards from
       the point at which it is encountered.

       It is also possible to refer to  subsequently  opened  parentheses,  by
       writing  references  such  as (?+2). However, these cannot be recursive
       because the reference is not inside the  parentheses  that  are  refer-
       enced.  They  are  always  "subroutine" calls, as described in the next
       section.

       An alternative approach is to use named parentheses instead.  The  Perl
       syntax  for  this  is (?&name); PCRE's earlier syntax (?P>name) is also
       supported. We could rewrite the above example as follows:

         (?<pn> \( ( [^()]++ | (?&pn) )* \) )

       If there is more than one subpattern with the same name,  the  earliest
       one is used.

       This  particular  example pattern that we have been looking at contains
       nested unlimited repeats, and so the use of a possessive quantifier for
       matching strings of non-parentheses is important when applying the pat-
       tern to strings that do not match. For example, when  this  pattern  is
       applied to

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it  yields  "no  match" quickly. However, if a possessive quantifier is
       not used, the match runs for a very long time indeed because there  are
       so  many  different  ways the + and * repeats can carve up the subject,
       and all have to be tested before failure can be reported.

       At the end of a match, the values of capturing  parentheses  are  those
       from  the outermost level. If you want to obtain intermediate values, a
       callout function can be used (see below and the pcrecallout  documenta-
       tion). If the pattern above is matched against

         (ab(cd)ef)

       the  value  for  the  inner capturing parentheses (numbered 2) is "ef",
       which is the last value taken on at the top level. If a capturing  sub-
       pattern is not matched at the top level, its final value is unset, even
       if it is (temporarily) set at a deeper level.

       If there are more than 15 capturing parentheses in a pattern, PCRE  has
       to  obtain extra memory to store data during a recursion, which it does
       by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
       can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.

       Do  not  confuse  the (?R) item with the condition (R), which tests for
       recursion.  Consider this pattern, which matches text in  angle  brack-
       ets,  allowing for arbitrary nesting. Only digits are allowed in nested
       brackets (that is, when recursing), whereas any characters are  permit-
       ted at the outer level.

         < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

       In  this  pattern, (?(R) is the start of a conditional subpattern, with
       two different alternatives for the recursive and  non-recursive  cases.
       The (?R) item is the actual recursive call.

   Recursion difference from Perl

       In  PCRE (like Python, but unlike Perl), a recursive subpattern call is
       always treated as an atomic group. That is, once it has matched some of
       the subject string, it is never re-entered, even if it contains untried
       alternatives and there is a subsequent matching failure.  This  can  be
       illustrated  by the following pattern, which purports to match a palin-
       dromic string that contains an odd number of characters  (for  example,
       "a", "aba", "abcba", "abcdcba"):

         ^(.|(.)(?1)\2)$

       The idea is that it either matches a single character, or two identical
       characters surrounding a sub-palindrome. In Perl, this  pattern  works;
       in  PCRE  it  does  not if the pattern is longer than three characters.
       Consider the subject string "abcba":

       At the top level, the first character is matched, but as it is  not  at
       the end of the string, the first alternative fails; the second alterna-
       tive is taken and the recursion kicks in. The recursive call to subpat-
       tern  1  successfully  matches the next character ("b"). (Note that the
       beginning and end of line tests are not part of the recursion).

       Back at the top level, the next character ("c") is compared  with  what
       subpattern  2 matched, which was "a". This fails. Because the recursion
       is treated as an atomic group, there are now  no  backtracking  points,
       and  so  the  entire  match fails. (Perl is able, at this point, to re-
       enter the recursion and try the second alternative.)  However,  if  the
       pattern is written with the alternatives in the other order, things are
       different:

         ^((.)(?1)\2|.)$

       This time, the recursing alternative is tried first, and  continues  to
       recurse  until  it runs out of characters, at which point the recursion
       fails. But this time we do have  another  alternative  to  try  at  the
       higher  level.  That  is  the  big difference: in the previous case the
       remaining alternative is at a deeper recursion level, which PCRE cannot
       use.

       To  change  the pattern so that it matches all palindromic strings, not
       just those with an odd number of characters, it is tempting  to  change
       the pattern to this:

         ^((.)(?1)\2|.?)$

       Again,  this  works  in Perl, but not in PCRE, and for the same reason.
       When a deeper recursion has matched a single character,  it  cannot  be
       entered  again  in  order  to match an empty string. The solution is to
       separate the two cases, and write out the odd and even cases as  alter-
       natives at the higher level:

         ^(?:((.)(?1)\2|)|((.)(?3)\4|.))

       If  you  want  to match typical palindromic phrases, the pattern has to
       ignore all non-word characters, which can be done like this:

         ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$

       If run with the PCRE_CASELESS option, this pattern matches phrases such
       as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
       Perl. Note the use of the possessive quantifier *+ to avoid  backtrack-
       ing  into  sequences of non-word characters. Without this, PCRE takes a
       great deal longer (ten times or more) to  match  typical  phrases,  and
       Perl takes so long that you think it has gone into a loop.

       WARNING:  The  palindrome-matching patterns above work only if the sub-
       ject string does not start with a palindrome that is shorter  than  the
       entire  string.  For example, although "abcba" is correctly matched, if
       the subject is "ababa", PCRE finds the palindrome "aba" at  the  start,
       then  fails at top level because the end of the string does not follow.
       Once again, it cannot jump back into the recursion to try other  alter-
       natives, so the entire match fails.


SUBPATTERNS AS SUBROUTINES

       If the syntax for a recursive subpattern reference (either by number or
       by name) is used outside the parentheses to which it refers,  it  oper-
       ates  like a subroutine in a programming language. The "called" subpat-
       tern may be defined before or after the reference. A numbered reference
       can be absolute or relative, as in these examples:

         (...(absolute)...)...(?2)...
         (...(relative)...)...(?-1)...
         (...(?+1)...(relative)...

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

       matches  "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If instead the pattern

         (sens|respons)e and (?1)ibility

       is used, it does match "sense and responsibility" as well as the  other
       two  strings.  Another  example  is  given  in the discussion of DEFINE
       above.

       Like recursive subpatterns, a subroutine call is always treated  as  an
       atomic  group. That is, once it has matched some of the subject string,
       it is never re-entered, even if it contains  untried  alternatives  and
       there  is a subsequent matching failure. Any capturing parentheses that
       are set during the subroutine call  revert  to  their  previous  values
       afterwards.

       When  a  subpattern is used as a subroutine, processing options such as
       case-independence are fixed when the subpattern is defined. They cannot
       be changed for different calls. For example, consider this pattern:

         (abc)(?i:(?-1))

       It  matches  "abcabc". It does not match "abcABC" because the change of
       processing option does not affect the called subpattern.


ONIGURUMA SUBROUTINE SYNTAX

       For compatibility with Oniguruma, the non-Perl syntax \g followed by  a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for referencing a  subpattern  as  a  subroutine,
       possibly  recursively. Here are two of the examples used above, rewrit-
       ten using this syntax:

         (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
         (sens|respons)e and \g'1'ibility

       PCRE supports an extension to Oniguruma: if a number is preceded  by  a
       plus or a minus sign it is taken as a relative reference. For example:

         (abc)(?i:\g<-1>)

       Note  that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
       synonymous. The former is a back reference; the latter is a  subroutine
       call.


CALLOUTS

       Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl code to be obeyed in the middle of matching a regular  expression.
       This makes it possible, amongst other things, to extract different sub-
       strings that match the same pair of parentheses when there is a repeti-
       tion.

       PCRE provides a similar feature, but of course it cannot obey arbitrary
       Perl code. The feature is called "callout". The caller of PCRE provides
       an  external function by putting its entry point in the global variable
       pcre_callout.  By default, this variable contains NULL, which  disables
       all calling out.

       Within  a  regular  expression,  (?C) indicates the points at which the
       external function is to be called. If you want  to  identify  different
       callout  points, you can put a number less than 256 after the letter C.
       The default value is zero.  For example, this pattern has  two  callout
       points:

         (?C1)abc(?C2)def

       If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
       automatically installed before each item in the pattern. They  are  all
       numbered 255.

       During matching, when PCRE reaches a callout point (and pcre_callout is
       set), the external function is called. It is provided with  the  number
       of  the callout, the position in the pattern, and, optionally, one item
       of data originally supplied by the caller of pcre_exec().  The  callout
       function  may cause matching to proceed, to backtrack, or to fail alto-
       gether. A complete description of the interface to the callout function
       is given in the pcrecallout documentation.


BACKTRACKING CONTROL

       Perl  5.10 introduced a number of "Special Backtracking Control Verbs",
       which are described in the Perl documentation as "experimental and sub-
       ject  to  change or removal in a future version of Perl". It goes on to
       say: "Their usage in production code should be noted to avoid  problems
       during upgrades." The same remarks apply to the PCRE features described
       in this section.

       Since these verbs are specifically related  to  backtracking,  most  of
       them  can  be  used  only  when  the  pattern  is  to  be matched using
       pcre_exec(), which uses a backtracking algorithm. With the exception of
       (*FAIL), which behaves like a failing negative assertion, they cause an
       error if encountered by pcre_dfa_exec().

       If any of these verbs are used in an assertion or subroutine subpattern
       (including  recursive  subpatterns),  their  effect is confined to that
       subpattern; it does not extend to the surrounding  pattern.  Note  that
       such  subpatterns are processed as anchored at the point where they are
       tested.

       The new verbs make use of what was previously invalid syntax: an  open-
       ing parenthesis followed by an asterisk. They are generally of the form
       (*VERB) or (*VERB:NAME). Some may take either form, with differing  be-
       haviour, depending on whether or not an argument is present. An name is
       a sequence of letters, digits, and underscores. If the name  is  empty,
       that  is, if the closing parenthesis immediately follows the colon, the
       effect is as if the colon were not there. Any number of these verbs may
       occur in a pattern.

       PCRE  contains some optimizations that are used to speed up matching by
       running some checks at the start of each match attempt. For example, it
       may  know  the minimum length of matching subject, or that a particular
       character must be present. When one of these  optimizations  suppresses
       the  running  of  a match, any included backtracking verbs will not, of
       course, be processed. You can suppress the start-of-match optimizations
       by  setting  the  PCRE_NO_START_OPTIMIZE  option when calling pcre_com-
       pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT).

   Verbs that act immediately

       The following verbs act as soon as they are encountered. They  may  not
       be followed by a name.

          (*ACCEPT)

       This  verb causes the match to end successfully, skipping the remainder
       of the pattern. When inside a recursion, only the innermost pattern  is
       ended  immediately.  If  (*ACCEPT) is inside capturing parentheses, the
       data so far is captured. (This feature was added  to  PCRE  at  release
       8.00.) For example:

         A((?:A|B(*ACCEPT)|C)D)

       This  matches  "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
       tured by the outer parentheses.

         (*FAIL) or (*F)

       This verb causes the match to fail, forcing backtracking to  occur.  It
       is  equivalent to (?!) but easier to read. The Perl documentation notes
       that it is probably useful only when combined  with  (?{})  or  (??{}).
       Those  are,  of course, Perl features that are not present in PCRE. The
       nearest equivalent is the callout feature, as for example in this  pat-
       tern:

         a+(?C)(*FAIL)

       A  match  with the string "aaaa" always fails, but the callout is taken
       before each backtrack happens (in this example, 10 times).

   Recording which path was taken

       There is one verb whose main purpose  is  to  track  how  a  match  was
       arrived  at,  though  it  also  has a secondary use in conjunction with
       advancing the match starting point (see (*SKIP) below).

         (*MARK:NAME) or (*:NAME)

       A name is always  required  with  this  verb.  There  may  be  as  many
       instances  of  (*MARK) as you like in a pattern, and their names do not
       have to be unique.

       When a match succeeds, the name  of  the  last-encountered  (*MARK)  is
       passed  back  to  the  caller  via  the  pcre_extra  data structure, as
       described in the section on pcre_extra in the pcreapi documentation. No
       data  is  returned  for a partial match. Here is an example of pcretest
       output, where the /K modifier requests the retrieval and outputting  of
       (*MARK) data:

         /X(*MARK:A)Y|X(*MARK:B)Z/K
         XY
          0: XY
         MK: A
         XZ
          0: XZ
         MK: B

       The (*MARK) name is tagged with "MK:" in this output, and in this exam-
       ple it indicates which of the two alternatives matched. This is a  more
       efficient  way of obtaining this information than putting each alterna-
       tive in its own capturing parentheses.

       A name may also be returned after a failed  match  if  the  final  path
       through  the  pattern involves (*MARK). However, unless (*MARK) used in
       conjunction with (*COMMIT), this is unlikely to  happen  for  an  unan-
       chored pattern because, as the starting point for matching is advanced,
       the final check is often with an empty string, causing a failure before
       (*MARK) is reached. For example:

         /X(*MARK:A)Y|X(*MARK:B)Z/K
         XP
         No match

       There are three potential starting points for this match (starting with
       X, starting with P, and with  an  empty  string).  If  the  pattern  is
       anchored, the result is different:

         /^X(*MARK:A)Y|^X(*MARK:B)Z/K
         XP
         No match, mark = B

       PCRE's  start-of-match  optimizations can also interfere with this. For
       example, if, as a result of a call to pcre_study(), it knows the  mini-
       mum  subject  length for a match, a shorter subject will not be scanned
       at all.

       Note that similar anomalies (though different in detail) exist in Perl,
       no  doubt  for the same reasons. The use of (*MARK) data after a failed
       match of an unanchored pattern is not recommended, unless (*COMMIT)  is
       involved.

   Verbs that act after backtracking

       The following verbs do nothing when they are encountered. Matching con-
       tinues with what follows, but if there is no subsequent match,  causing
       a  backtrack  to  the  verb, a failure is forced. That is, backtracking
       cannot pass to the left of the verb. However, when one of  these  verbs
       appears  inside  an atomic group, its effect is confined to that group,
       because once the group has been matched, there is never any  backtrack-
       ing  into  it.  In  this situation, backtracking can "jump back" to the
       left of the entire atomic group. (Remember also, as stated above,  that
       this localization also applies in subroutine calls and assertions.)

       These  verbs  differ  in exactly what kind of failure occurs when back-
       tracking reaches them.

         (*COMMIT)

       This verb, which may not be followed by a name, causes the whole  match
       to fail outright if the rest of the pattern does not match. Even if the
       pattern is unanchored, no further attempts to find a match by advancing
       the  starting  point  take  place.  Once  (*COMMIT)  has  been  passed,
       pcre_exec() is committed to finding a match  at  the  current  starting
       point, or not at all. For example:

         a+(*COMMIT)b

       This  matches  "xxaab" but not "aacaab". It can be thought of as a kind
       of dynamic anchor, or "I've started, so I must finish." The name of the
       most  recently passed (*MARK) in the path is passed back when (*COMMIT)
       forces a match failure.

       Note that (*COMMIT) at the start of a pattern is not  the  same  as  an
       anchor,  unless  PCRE's start-of-match optimizations are turned off, as
       shown in this pcretest example:

         /(*COMMIT)abc/
         xyzabc
          0: abc
         xyzabc\Y
         No match

       PCRE knows that any match must start  with  "a",  so  the  optimization
       skips  along the subject to "a" before running the first match attempt,
       which succeeds. When the optimization is disabled by the \Y  escape  in
       the second subject, the match starts at "x" and so the (*COMMIT) causes
       it to fail without trying any other starting points.

         (*PRUNE) or (*PRUNE:NAME)

       This verb causes the match to fail at the current starting position  in
       the  subject  if the rest of the pattern does not match. If the pattern
       is unanchored, the normal "bumpalong"  advance  to  the  next  starting
       character  then happens. Backtracking can occur as usual to the left of
       (*PRUNE), before it is reached,  or  when  matching  to  the  right  of
       (*PRUNE),  but  if  there is no match to the right, backtracking cannot
       cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an  alter-
       native  to an atomic group or possessive quantifier, but there are some
       uses of (*PRUNE) that cannot be expressed in any other way.  The behav-
       iour  of  (*PRUNE:NAME)  is  the  same as (*MARK:NAME)(*PRUNE) when the
       match fails completely; the name is passed back if this  is  the  final
       attempt.   (*PRUNE:NAME)  does  not  pass back a name if the match suc-
       ceeds. In an anchored pattern (*PRUNE) has the same  effect  as  (*COM-
       MIT).

         (*SKIP)

       This  verb, when given without a name, is like (*PRUNE), except that if
       the pattern is unanchored, the "bumpalong" advance is not to  the  next
       character, but to the position in the subject where (*SKIP) was encoun-
       tered. (*SKIP) signifies that whatever text was matched leading  up  to
       it cannot be part of a successful match. Consider:

         a+(*SKIP)b

       If  the  subject  is  "aaaac...",  after  the first match attempt fails
       (starting at the first character in the  string),  the  starting  point
       skips on to start the next attempt at "c". Note that a possessive quan-
       tifer does not have the same effect as this example; although it  would
       suppress  backtracking  during  the  first  match  attempt,  the second
       attempt would start at the second character instead of skipping  on  to
       "c".

         (*SKIP:NAME)

       When  (*SKIP) has an associated name, its behaviour is modified. If the
       following pattern fails to match, the previous path through the pattern
       is  searched for the most recent (*MARK) that has the same name. If one
       is found, the "bumpalong" advance is to the subject position that  cor-
       responds  to  that (*MARK) instead of to where (*SKIP) was encountered.
       If no (*MARK) with a matching name is found, normal "bumpalong" of  one
       character happens (the (*SKIP) is ignored).

         (*THEN) or (*THEN:NAME)

       This  verb  causes  a  skip  to  the  next alternation in the innermost
       enclosing group if the rest of the pattern does not match. That is,  it
       cancels  pending backtracking, but only within the current alternation.
       Its name comes from the observation that it can be used for a  pattern-
       based if-then-else block:

         ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       If  the COND1 pattern matches, FOO is tried (and possibly further items
       after the end of the group if FOO succeeds);  on  failure  the  matcher
       skips  to  the second alternative and tries COND2, without backtracking
       into COND1. The behaviour  of  (*THEN:NAME)  is  exactly  the  same  as
       (*MARK:NAME)(*THEN)  if  the  overall  match  fails.  If (*THEN) is not
       directly inside an alternation, it acts like (*PRUNE).

       The above verbs provide four different "strengths" of control when sub-
       sequent  matching  fails. (*THEN) is the weakest, carrying on the match
       at the next alternation. (*PRUNE) comes next, failing the match at  the
       current  starting position, but allowing an advance to the next charac-
       ter (for an unanchored pattern). (*SKIP) is similar,  except  that  the
       advance  may  be  more  than one character. (*COMMIT) is the strongest,
       causing the entire match to fail.

       If more than one is present in a pattern, the "stongest" one wins.  For
       example,  consider  this  pattern, where A, B, etc. are complex pattern
       fragments:

         (A(*COMMIT)B(*THEN)C|D)

       Once A has matched, PCRE is committed to this  match,  at  the  current
       starting  position. If subsequently B matches, but C does not, the nor-
       mal (*THEN) action of trying the next alternation (that is, D) does not
       happen because (*COMMIT) overrides.


SEE ALSO

       pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION

       Last updated: 21 November 2010
       Copyright (c) 1997-2010 University of Cambridge.
------------------------------------------------------------------------------


PCRESYNTAX(3)                                                    PCRESYNTAX(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE REGULAR EXPRESSION SYNTAX SUMMARY

       The  full syntax and semantics of the regular expressions that are sup-
       ported by PCRE are described in  the  pcrepattern  documentation.  This
       document contains just a quick-reference summary of the syntax.


QUOTING

         \x         where x is non-alphanumeric is a literal x
         \Q...\E    treat enclosed characters as literal


CHARACTERS

         \a         alarm, that is, the BEL character (hex 07)
         \cx        "control-x", where x is any ASCII character
         \e         escape (hex 1B)
         \f         formfeed (hex 0C)
         \n         newline (hex 0A)
         \r         carriage return (hex 0D)
         \t         tab (hex 09)
         \ddd       character with octal code ddd, or backreference
         \xhh       character with hex code hh
         \x{hhh..}  character with hex code hhh..


CHARACTER TYPES

         .          any character except newline;
                      in dotall mode, any character whatsoever
         \C         one byte, even in UTF-8 mode (best avoided)
         \d         a decimal digit
         \D         a character that is not a decimal digit
         \h         a horizontal whitespace character
         \H         a character that is not a horizontal whitespace character
         \N         a character that is not a newline
         \p{xx}     a character with the xx property
         \P{xx}     a character without the xx property
         \R         a newline sequence
         \s         a whitespace character
         \S         a character that is not a whitespace character
         \v         a vertical whitespace character
         \V         a character that is not a vertical whitespace character
         \w         a "word" character
         \W         a "non-word" character
         \X         an extended Unicode sequence

       In  PCRE,  by  default, \d, \D, \s, \S, \w, and \W recognize only ASCII
       characters, even in UTF-8 mode. However, this can be changed by setting
       the PCRE_UCP option.


GENERAL CATEGORY PROPERTIES FOR \p and \P

         C          Other
         Cc         Control
         Cf         Format
         Cn         Unassigned
         Co         Private use
         Cs         Surrogate

         L          Letter
         Ll         Lower case letter
         Lm         Modifier letter
         Lo         Other letter
         Lt         Title case letter
         Lu         Upper case letter
         L&         Ll, Lu, or Lt

         M          Mark
         Mc         Spacing mark
         Me         Enclosing mark
         Mn         Non-spacing mark

         N          Number
         Nd         Decimal number
         Nl         Letter number
         No         Other number

         P          Punctuation
         Pc         Connector punctuation
         Pd         Dash punctuation
         Pe         Close punctuation
         Pf         Final punctuation
         Pi         Initial punctuation
         Po         Other punctuation
         Ps         Open punctuation

         S          Symbol
         Sc         Currency symbol
         Sk         Modifier symbol
         Sm         Mathematical symbol
         So         Other symbol

         Z          Separator
         Zl         Line separator
         Zp         Paragraph separator
         Zs         Space separator


PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P

         Xan        Alphanumeric: union of properties L and N
         Xps        POSIX space: property Z or tab, NL, VT, FF, CR
         Xsp        Perl space: property Z or tab, NL, FF, CR
         Xwd        Perl word: property Xan or underscore


SCRIPT NAMES FOR \p AND \P

       Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
       Buginese, Buhid, Canadian_Aboriginal, Carian, Cham,  Cherokee,  Common,
       Coptic,   Cuneiform,  Cypriot,  Cyrillic,  Deseret,  Devanagari,  Egyp-
       tian_Hieroglyphs,  Ethiopic,  Georgian,  Glagolitic,   Gothic,   Greek,
       Gujarati,  Gurmukhi,  Han,  Hangul,  Hanunoo,  Hebrew,  Hiragana, Impe-
       rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
       Javanese,  Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
       Latin,  Lepcha,  Limbu,  Linear_B,  Lisu,  Lycian,  Lydian,  Malayalam,
       Meetei_Mayek,  Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
       Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki,  Oriya,  Osmanya,
       Phags_Pa,  Phoenician,  Rejang,  Runic, Samaritan, Saurashtra, Shavian,
       Sinhala, Sundanese, Syloti_Nagri, Syriac,  Tagalog,  Tagbanwa,  Tai_Le,
       Tai_Tham,  Tai_Viet,  Tamil,  Telugu,  Thaana, Thai, Tibetan, Tifinagh,
       Ugaritic, Vai, Yi.


CHARACTER CLASSES

         [...]       positive character class
         [^...]      negative character class
         [x-y]       range (can be used for hex characters)
         [[:xxx:]]   positive POSIX named set
         [[:^xxx:]]  negative POSIX named set

         alnum       alphanumeric
         alpha       alphabetic
         ascii       0-127
         blank       space or tab
         cntrl       control character
         digit       decimal digit
         graph       printing, excluding space
         lower       lower case letter
         print       printing, including space
         punct       printing, excluding alphanumeric
         space       whitespace
         upper       upper case letter
         word        same as \w
         xdigit      hexadecimal digit

       In PCRE, POSIX character set names recognize only ASCII  characters  by
       default,  but  some  of them use Unicode properties if PCRE_UCP is set.
       You can use \Q...\E inside a character class.


QUANTIFIERS

         ?           0 or 1, greedy
         ?+          0 or 1, possessive
         ??          0 or 1, lazy
         *           0 or more, greedy
         *+          0 or more, possessive
         *?          0 or more, lazy
         +           1 or more, greedy
         ++          1 or more, possessive
         +?          1 or more, lazy
         {n}         exactly n
         {n,m}       at least n, no more than m, greedy
         {n,m}+      at least n, no more than m, possessive
         {n,m}?      at least n, no more than m, lazy
         {n,}        n or more, greedy
         {n,}+       n or more, possessive
         {n,}?       n or more, lazy


ANCHORS AND SIMPLE ASSERTIONS

         \b          word boundary
         \B          not a word boundary
         ^           start of subject
                      also after internal newline in multiline mode
         \A          start of subject
         $           end of subject
                      also before newline at end of subject
                      also before internal newline in multiline mode
         \Z          end of subject
                      also before newline at end of subject
         \z          end of subject
         \G          first matching position in subject


MATCH POINT RESET

         \K          reset start of match


ALTERNATION

         expr|expr|expr...


CAPTURING

         (...)           capturing group
         (?<name>...)    named capturing group (Perl)
         (?'name'...)    named capturing group (Perl)
         (?P<name>...)   named capturing group (Python)
         (?:...)         non-capturing group
         (?|...)         non-capturing group; reset group numbers for
                          capturing groups in each alternative


ATOMIC GROUPS

         (?>...)         atomic, non-capturing group


COMMENT

         (?#....)        comment (not nestable)


OPTION SETTING

         (?i)            caseless
         (?J)            allow duplicate names
         (?m)            multiline
         (?s)            single line (dotall)
         (?U)            default ungreedy (lazy)
         (?x)            extended (ignore white space)
         (?-...)         unset option(s)

       The following are recognized only at the start of a  pattern  or  after
       one of the newline-setting options with similar syntax:

         (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE)
         (*UTF8)         set UTF-8 mode (PCRE_UTF8)
         (*UCP)          set PCRE_UCP (use Unicode properties for \d etc)


LOOKAHEAD AND LOOKBEHIND ASSERTIONS

         (?=...)         positive look ahead
         (?!...)         negative look ahead
         (?<=...)        positive look behind
         (?<!...)        negative look behind

       Each top-level branch of a look behind must be of a fixed length.


BACKREFERENCES

         \n              reference by number (can be ambiguous)
         \gn             reference by number
         \g{n}           reference by number
         \g{-n}          relative reference by number
         \k<name>        reference by name (Perl)
         \k'name'        reference by name (Perl)
         \g{name}        reference by name (Perl)
         \k{name}        reference by name (.NET)
         (?P=name)       reference by name (Python)


SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)

         (?R)            recurse whole pattern
         (?n)            call subpattern by absolute number
         (?+n)           call subpattern by relative number
         (?-n)           call subpattern by relative number
         (?&name)        call subpattern by name (Perl)
         (?P>name)       call subpattern by name (Python)
         \g<name>        call subpattern by name (Oniguruma)
         \g'name'        call subpattern by name (Oniguruma)
         \g<n>           call subpattern by absolute number (Oniguruma)
         \g'n'           call subpattern by absolute number (Oniguruma)
         \g<+n>          call subpattern by relative number (PCRE extension)
         \g'+n'          call subpattern by relative number (PCRE extension)
         \g<-n>          call subpattern by relative number (PCRE extension)
         \g'-n'          call subpattern by relative number (PCRE extension)


CONDITIONAL PATTERNS

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

         (?(n)...        absolute reference condition
         (?(+n)...       relative reference condition
         (?(-n)...       relative reference condition
         (?(<name>)...   named reference condition (Perl)
         (?('name')...   named reference condition (Perl)
         (?(name)...     named reference condition (PCRE)
         (?(R)...        overall recursion condition
         (?(Rn)...       specific group recursion condition
         (?(R&name)...   specific recursion condition
         (?(DEFINE)...   define subpattern for reference
         (?(assert)...   assertion condition


BACKTRACKING CONTROL

       The following act immediately they are reached:

         (*ACCEPT)       force successful match
         (*FAIL)         force backtrack; synonym (*F)

       The  following  act only when a subsequent match failure causes a back-
       track to reach them. They all force a match failure, but they differ in
       what happens afterwards. Those that advance the start-of-match point do
       so only if the pattern is not anchored.

         (*COMMIT)       overall failure, no advance of starting point
         (*PRUNE)        advance to next starting character
         (*SKIP)         advance start to current matching position
         (*THEN)         local failure, backtrack to next alternation


NEWLINE CONVENTIONS

       These are recognized only at the very start of the pattern or  after  a
       (*BSR_...) or (*UTF8) or (*UCP) option.

         (*CR)           carriage return only
         (*LF)           linefeed only
         (*CRLF)         carriage return followed by linefeed
         (*ANYCRLF)      all three of the above