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.

       Starting with release 8.30, it is possible to compile two separate PCRE
       libraries:  the  original,  which  supports  8-bit  character   strings
       (including  UTF-8  strings),  and a second library that supports 16-bit
       character strings (including UTF-16 strings). The build process  allows
       either  one  or both to be built. The majority of the work to make this
       possible was done by Zoltan Herczeg.

       Starting with release 8.32 it is possible to compile a  third  separate
       PCRE library, which supports 32-bit character strings (including UTF-32
       strings). The build process allows any set of the 8-,  16-  and  32-bit
       libraries. The work to make this possible was done by Christian Persch.

       The  three  libraries  contain identical sets of functions, except that
       the names in the 16-bit library start with pcre16_  instead  of  pcre_,
       and  the  names  in  the  32-bit  library start with pcre32_ instead of
       pcre_. To avoid over-complication and reduce the documentation  mainte-
       nance load, most of the documentation describes the 8-bit library, with
       the differences for the 16-bit and  32-bit  libraries  described  sepa-
       rately  in  the  pcre16  and  pcre32  pages. References to functions or
       structures of the  form  pcre[16|32]_xxx  should  be  read  as  meaning
       "pcre_xxx  when  using  the  8-bit  library,  pcre16_xxx when using the
       16-bit library, or pcre32_xxx when using the 32-bit library".

       The current implementation of PCRE corresponds approximately with  Perl
       5.12,  including  support  for  UTF-8/16/32 encoded strings and Unicode
       general category properties. However, UTF-8/16/32 and  Unicode  support
       has to be explicitly enabled; it is not the default. The Unicode tables
       correspond to Unicode release 6.2.0.

       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  for  the  8-bit
       library.  This  is  now  included as part of the PCRE distribution. The
       pcrecpp page has details of this interface.  Other  people's  contribu-
       tions  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-AUTOTOOLS_BUILD files in the source distribution.

       The libraries 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_" or "_pcre16_" or "_pcre32_", 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.


SECURITY CONSIDERATIONS

       If  you  are  using PCRE in a non-UTF application that permits users to
       supply arbitrary patterns for compilation, you should  be  aware  of  a
       feature that allows users to turn on UTF support from within a pattern,
       provided that PCRE was built with UTF support. For  example,  an  8-bit
       pattern  that  begins  with  "(*UTF8)" or "(*UTF)" turns on UTF-8 mode,
       which interprets patterns and subjects as strings of  UTF-8  characters
       instead  of  individual 8-bit characters.  This causes both the pattern
       and any data against which it is matched to be checked for UTF-8 valid-
       ity.  If  the  data  string is very long, such a check might use suffi-
       ciently many resources as to cause your  application  to  lose  perfor-
       mance.

       The  best  way  of  guarding  against  this  possibility  is to use the
       pcre_fullinfo() function to check the compiled  pattern's  options  for
       UTF.

       If  your  application  is one that supports UTF, be aware that validity
       checking can take time. If the same data string is to be  matched  many
       times, you can use the PCRE_NO_UTF[8|16|32]_CHECK option for the second
       and subsequent matches to save redundant checks.

       Another way that performance can be hit is by running  a  pattern  that
       has  a  very  large search tree against a string that will never match.
       Nested unlimited repeats in a pattern are a common example.  PCRE  pro-
       vides some protection against this: see the PCRE_EXTRA_MATCH_LIMIT fea-
       ture in the pcreapi page.


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
         pcre16            details of the 16-bit library
         pcre32            details of the 32-bit library
         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 for the 8-bit library
         pcredemo          a demonstration C program that uses PCRE
         pcregrep          description of the pcregrep command (8-bit only)
         pcrejit           discussion of the just-in-time optimization support
         pcrelimits        details of size and other limits
         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 for the 8-bit library
         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
         pcreunicode       discussion of Unicode and UTF-8/16/32 support

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


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: 11 November 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRE(3)                                                                PCRE(3)


NAME
       PCRE - Perl-compatible regular expressions

       #include <pcre.h>


PCRE 16-BIT API BASIC FUNCTIONS

       pcre16 *pcre16_compile(PCRE_SPTR16 pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre16 *pcre16_compile2(PCRE_SPTR16 pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre16_extra *pcre16_study(const pcre16 *code, int options,
            const char **errptr);

       void pcre16_free_study(pcre16_extra *extra);

       int pcre16_exec(const pcre16 *code, const pcre16_extra *extra,
            PCRE_SPTR16 subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       int pcre16_dfa_exec(const pcre16 *code, const pcre16_extra *extra,
            PCRE_SPTR16 subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);


PCRE 16-BIT API STRING EXTRACTION FUNCTIONS

       int pcre16_copy_named_substring(const pcre16 *code,
            PCRE_SPTR16 subject, int *ovector,
            int stringcount, PCRE_SPTR16 stringname,
            PCRE_UCHAR16 *buffer, int buffersize);

       int pcre16_copy_substring(PCRE_SPTR16 subject, int *ovector,
            int stringcount, int stringnumber, PCRE_UCHAR16 *buffer,
            int buffersize);

       int pcre16_get_named_substring(const pcre16 *code,
            PCRE_SPTR16 subject, int *ovector,
            int stringcount, PCRE_SPTR16 stringname,
            PCRE_SPTR16 *stringptr);

       int pcre16_get_stringnumber(const pcre16 *code,
            PCRE_SPTR16 name);

       int pcre16_get_stringtable_entries(const pcre16 *code,
            PCRE_SPTR16 name, PCRE_UCHAR16 **first, PCRE_UCHAR16 **last);

       int pcre16_get_substring(PCRE_SPTR16 subject, int *ovector,
            int stringcount, int stringnumber,
            PCRE_SPTR16 *stringptr);

       int pcre16_get_substring_list(PCRE_SPTR16 subject,
            int *ovector, int stringcount, PCRE_SPTR16 **listptr);

       void pcre16_free_substring(PCRE_SPTR16 stringptr);

       void pcre16_free_substring_list(PCRE_SPTR16 *stringptr);


PCRE 16-BIT API AUXILIARY FUNCTIONS

       pcre16_jit_stack *pcre16_jit_stack_alloc(int startsize, int maxsize);

       void pcre16_jit_stack_free(pcre16_jit_stack *stack);

       void pcre16_assign_jit_stack(pcre16_extra *extra,
            pcre16_jit_callback callback, void *data);

       const unsigned char *pcre16_maketables(void);

       int pcre16_fullinfo(const pcre16 *code, const pcre16_extra *extra,
            int what, void *where);

       int pcre16_refcount(pcre16 *code, int adjust);

       int pcre16_config(int what, void *where);

       const char *pcre16_version(void);

       int pcre16_pattern_to_host_byte_order(pcre16 *code,
            pcre16_extra *extra, const unsigned char *tables);


PCRE 16-BIT API INDIRECTED FUNCTIONS

       void *(*pcre16_malloc)(size_t);

       void (*pcre16_free)(void *);

       void *(*pcre16_stack_malloc)(size_t);

       void (*pcre16_stack_free)(void *);

       int (*pcre16_callout)(pcre16_callout_block *);


PCRE 16-BIT API 16-BIT-ONLY FUNCTION

       int pcre16_utf16_to_host_byte_order(PCRE_UCHAR16 *output,
            PCRE_SPTR16 input, int length, int *byte_order,
            int keep_boms);


THE PCRE 16-BIT LIBRARY

       Starting  with  release  8.30, it is possible to compile a PCRE library
       that supports 16-bit character strings, including  UTF-16  strings,  as
       well  as  or instead of the original 8-bit library. The majority of the
       work to make  this  possible  was  done  by  Zoltan  Herczeg.  The  two
       libraries contain identical sets of functions, used in exactly the same
       way. Only the names of the functions and the data types of their  argu-
       ments  and results are different. To avoid over-complication and reduce
       the documentation maintenance load,  most  of  the  PCRE  documentation
       describes  the  8-bit  library,  with only occasional references to the
       16-bit library. This page describes what is different when you use  the
       16-bit library.

       WARNING:  A  single  application can be linked with both libraries, but
       you must take care when processing any particular pattern to use  func-
       tions  from  just one library. For example, if you want to study a pat-
       tern that was compiled with  pcre16_compile(),  you  must  do  so  with
       pcre16_study(), not pcre_study(), and you must free the study data with
       pcre16_free_study().


THE HEADER FILE

       There is only one header file, pcre.h. It contains prototypes  for  all
       the functions in all libraries, as well as definitions of flags, struc-
       tures, error codes, etc.


THE LIBRARY NAME

       In Unix-like systems, the 16-bit library is called libpcre16,  and  can
       normally  be  accesss  by adding -lpcre16 to the command for linking an
       application that uses PCRE.


STRING TYPES

       In the 8-bit library, strings are passed to PCRE library  functions  as
       vectors  of  bytes  with  the  C  type "char *". In the 16-bit library,
       strings are passed as vectors of unsigned 16-bit quantities. The  macro
       PCRE_UCHAR16  specifies  an  appropriate  data type, and PCRE_SPTR16 is
       defined as "const PCRE_UCHAR16 *". In very  many  environments,  "short
       int" is a 16-bit data type. When PCRE is built, it defines PCRE_UCHAR16
       as "unsigned short int", but checks that it really  is  a  16-bit  data
       type.  If  it is not, the build fails with an error message telling the
       maintainer to modify the definition appropriately.


STRUCTURE TYPES

       The types of the opaque structures that are used  for  compiled  16-bit
       patterns  and  JIT stacks are pcre16 and pcre16_jit_stack respectively.
       The  type  of  the  user-accessible  structure  that  is  returned   by
       pcre16_study()  is  pcre16_extra, and the type of the structure that is
       used for passing data to a callout  function  is  pcre16_callout_block.
       These structures contain the same fields, with the same names, as their
       8-bit counterparts. The only difference is that pointers  to  character
       strings are 16-bit instead of 8-bit types.


16-BIT FUNCTIONS

       For  every function in the 8-bit library there is a corresponding func-
       tion in the 16-bit library with a name that starts with pcre16_ instead
       of  pcre_.  The  prototypes are listed above. In addition, there is one
       extra function, pcre16_utf16_to_host_byte_order(). This  is  a  utility
       function  that converts a UTF-16 character string to host byte order if
       necessary. The other 16-bit  functions  expect  the  strings  they  are
       passed to be in host byte order.

       The input and output arguments of pcre16_utf16_to_host_byte_order() may
       point to the same address, that is, conversion in place  is  supported.
       The output buffer must be at least as long as the input.

       The  length  argument  specifies the number of 16-bit data units in the
       input string; a negative value specifies a zero-terminated string.

       If byte_order is NULL, it is assumed that the string starts off in host
       byte  order. This may be changed by byte-order marks (BOMs) anywhere in
       the string (commonly as the first character).

       If byte_order is not NULL, a non-zero value of the integer to which  it
       points  means  that  the input starts off in host byte order, otherwise
       the opposite order is assumed. Again, BOMs in  the  string  can  change
       this. The final byte order is passed back at the end of processing.

       If  keep_boms  is  not  zero,  byte-order  mark characters (0xfeff) are
       copied into the output string. Otherwise they are discarded.

       The result of the function is the number of 16-bit  units  placed  into
       the  output  buffer,  including  the  zero terminator if the string was
       zero-terminated.


SUBJECT STRING OFFSETS

       The offsets within subject strings that are returned  by  the  matching
       functions are in 16-bit units rather than bytes.


NAMED SUBPATTERNS

       The  name-to-number translation table that is maintained for named sub-
       patterns uses 16-bit characters.  The  pcre16_get_stringtable_entries()
       function returns the length of each entry in the table as the number of
       16-bit data units.


OPTION NAMES

       There   are   two   new   general   option   names,   PCRE_UTF16    and
       PCRE_NO_UTF16_CHECK,     which     correspond    to    PCRE_UTF8    and
       PCRE_NO_UTF8_CHECK in the 8-bit library. In  fact,  these  new  options
       define  the  same bits in the options word. There is a discussion about
       the validity of UTF-16 strings in the pcreunicode page.

       For the pcre16_config() function there is an  option  PCRE_CONFIG_UTF16
       that  returns  1  if UTF-16 support is configured, otherwise 0. If this
       option  is  given  to  pcre_config()  or  pcre32_config(),  or  if  the
       PCRE_CONFIG_UTF8  or  PCRE_CONFIG_UTF32  option is given to pcre16_con-
       fig(), the result is the PCRE_ERROR_BADOPTION error.


CHARACTER CODES

       In 16-bit mode, when  PCRE_UTF16  is  not  set,  character  values  are
       treated in the same way as in 8-bit, non UTF-8 mode, except, of course,
       that they can range from 0 to 0xffff instead of 0  to  0xff.  Character
       types  for characters less than 0xff can therefore be influenced by the
       locale in the same way as before.  Characters greater  than  0xff  have
       only one case, and no "type" (such as letter or digit).

       In  UTF-16  mode,  the  character  code  is  Unicode, in the range 0 to
       0x10ffff, with the exception of values in the range  0xd800  to  0xdfff
       because  those  are "surrogate" values that are used in pairs to encode
       values greater than 0xffff.

       A UTF-16 string can indicate its endianness by special code knows as  a
       byte-order mark (BOM). The PCRE functions do not handle this, expecting
       strings  to  be  in  host  byte  order.  A  utility   function   called
       pcre16_utf16_to_host_byte_order()  is  provided  to help with this (see
       above).


ERROR NAMES

       The errors PCRE_ERROR_BADUTF16_OFFSET and PCRE_ERROR_SHORTUTF16  corre-
       spond  to  their  8-bit  counterparts.  The error PCRE_ERROR_BADMODE is
       given when a compiled pattern is passed to a  function  that  processes
       patterns  in  the  other  mode, for example, if a pattern compiled with
       pcre_compile() is passed to pcre16_exec().

       There are new error codes whose names  begin  with  PCRE_UTF16_ERR  for
       invalid  UTF-16  strings,  corresponding to the PCRE_UTF8_ERR codes for
       UTF-8 strings that are described in the section entitled "Reason  codes
       for  invalid UTF-8 strings" in the main pcreapi page. The UTF-16 errors
       are:

         PCRE_UTF16_ERR1  Missing low surrogate at end of string
         PCRE_UTF16_ERR2  Invalid low surrogate follows high surrogate
         PCRE_UTF16_ERR3  Isolated low surrogate
         PCRE_UTF16_ERR4  Non-character


ERROR TEXTS

       If there is an error while compiling a pattern, the error text that  is
       passed  back by pcre16_compile() or pcre16_compile2() is still an 8-bit
       character string, zero-terminated.


CALLOUTS

       The subject and mark fields in the callout block that is  passed  to  a
       callout function point to 16-bit vectors.


TESTING

       The  pcretest  program continues to operate with 8-bit input and output
       files, but it can be used for testing the 16-bit library. If it is  run
       with the command line option -16, patterns and subject strings are con-
       verted from 8-bit to 16-bit before being passed to PCRE, and the 16-bit
       library  functions  are used instead of the 8-bit ones. Returned 16-bit
       strings are converted to 8-bit for output. If both the  8-bit  and  the
       32-bit libraries were not compiled, pcretest defaults to 16-bit and the
       -16 option is ignored.

       When PCRE is being built, the RunTest script that is  called  by  "make
       check"  uses  the  pcretest  -C  option to discover which of the 8-bit,
       16-bit and 32-bit libraries has been built, and runs the  tests  appro-
       priately.


NOT SUPPORTED IN 16-BIT MODE

       Not all the features of the 8-bit library are available with the 16-bit
       library. The C++ and POSIX wrapper functions  support  only  the  8-bit
       library, and the pcregrep program is at present 8-bit only.


AUTHOR

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


REVISION

       Last updated: 08 November 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRE(3)                                                                PCRE(3)


NAME
       PCRE - Perl-compatible regular expressions

       #include <pcre.h>


PCRE 32-BIT API BASIC FUNCTIONS

       pcre32 *pcre32_compile(PCRE_SPTR32 pattern, int options,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre32 *pcre32_compile2(PCRE_SPTR32 pattern, int options,
            int *errorcodeptr,
            const char **errptr, int *erroffset,
            const unsigned char *tableptr);

       pcre32_extra *pcre32_study(const pcre32 *code, int options,
            const char **errptr);

       void pcre32_free_study(pcre32_extra *extra);

       int pcre32_exec(const pcre32 *code, const pcre32_extra *extra,
            PCRE_SPTR32 subject, int length, int startoffset,
            int options, int *ovector, int ovecsize);

       int pcre32_dfa_exec(const pcre32 *code, const pcre32_extra *extra,
            PCRE_SPTR32 subject, int length, int startoffset,
            int options, int *ovector, int ovecsize,
            int *workspace, int wscount);


PCRE 32-BIT API STRING EXTRACTION FUNCTIONS

       int pcre32_copy_named_substring(const pcre32 *code,
            PCRE_SPTR32 subject, int *ovector,
            int stringcount, PCRE_SPTR32 stringname,
            PCRE_UCHAR32 *buffer, int buffersize);

       int pcre32_copy_substring(PCRE_SPTR32 subject, int *ovector,
            int stringcount, int stringnumber, PCRE_UCHAR32 *buffer,
            int buffersize);

       int pcre32_get_named_substring(const pcre32 *code,
            PCRE_SPTR32 subject, int *ovector,
            int stringcount, PCRE_SPTR32 stringname,
            PCRE_SPTR32 *stringptr);

       int pcre32_get_stringnumber(const pcre32 *code,
            PCRE_SPTR32 name);

       int pcre32_get_stringtable_entries(const pcre32 *code,
            PCRE_SPTR32 name, PCRE_UCHAR32 **first, PCRE_UCHAR32 **last);

       int pcre32_get_substring(PCRE_SPTR32 subject, int *ovector,
            int stringcount, int stringnumber,
            PCRE_SPTR32 *stringptr);

       int pcre32_get_substring_list(PCRE_SPTR32 subject,
            int *ovector, int stringcount, PCRE_SPTR32 **listptr);

       void pcre32_free_substring(PCRE_SPTR32 stringptr);

       void pcre32_free_substring_list(PCRE_SPTR32 *stringptr);


PCRE 32-BIT API AUXILIARY FUNCTIONS

       pcre32_jit_stack *pcre32_jit_stack_alloc(int startsize, int maxsize);

       void pcre32_jit_stack_free(pcre32_jit_stack *stack);

       void pcre32_assign_jit_stack(pcre32_extra *extra,
            pcre32_jit_callback callback, void *data);

       const unsigned char *pcre32_maketables(void);

       int pcre32_fullinfo(const pcre32 *code, const pcre32_extra *extra,
            int what, void *where);

       int pcre32_refcount(pcre32 *code, int adjust);

       int pcre32_config(int what, void *where);

       const char *pcre32_version(void);

       int pcre32_pattern_to_host_byte_order(pcre32 *code,
            pcre32_extra *extra, const unsigned char *tables);


PCRE 32-BIT API INDIRECTED FUNCTIONS

       void *(*pcre32_malloc)(size_t);

       void (*pcre32_free)(void *);

       void *(*pcre32_stack_malloc)(size_t);

       void (*pcre32_stack_free)(void *);

       int (*pcre32_callout)(pcre32_callout_block *);


PCRE 32-BIT API 32-BIT-ONLY FUNCTION

       int pcre32_utf32_to_host_byte_order(PCRE_UCHAR32 *output,
            PCRE_SPTR32 input, int length, int *byte_order,
            int keep_boms);


THE PCRE 32-BIT LIBRARY

       Starting  with  release  8.32, it is possible to compile a PCRE library
       that supports 32-bit character strings, including  UTF-32  strings,  as
       well as or instead of the original 8-bit library. This work was done by
       Christian Persch, based on the work done  by  Zoltan  Herczeg  for  the
       16-bit  library.  All  three  libraries contain identical sets of func-
       tions, used in exactly the same way.  Only the names of  the  functions
       and  the  data  types  of their arguments and results are different. To
       avoid over-complication and reduce the documentation maintenance  load,
       most  of  the PCRE documentation describes the 8-bit library, with only
       occasional references to the 16-bit and  32-bit  libraries.  This  page
       describes what is different when you use the 32-bit library.

       WARNING:  A  single  application  can  be linked with all or any of the
       three libraries, but you must take care when processing any  particular
       pattern  to  use  functions  from just one library. For example, if you
       want to study a pattern that was compiled  with  pcre32_compile(),  you
       must do so with pcre32_study(), not pcre_study(), and you must free the
       study data with pcre32_free_study().


THE HEADER FILE

       There is only one header file, pcre.h. It contains prototypes  for  all
       the functions in all libraries, as well as definitions of flags, struc-
       tures, error codes, etc.


THE LIBRARY NAME

       In Unix-like systems, the 32-bit library is called libpcre32,  and  can
       normally  be  accesss  by adding -lpcre32 to the command for linking an
       application that uses PCRE.


STRING TYPES

       In the 8-bit library, strings are passed to PCRE library  functions  as
       vectors  of  bytes  with  the  C  type "char *". In the 32-bit library,
       strings are passed as vectors of unsigned 32-bit quantities. The  macro
       PCRE_UCHAR32  specifies  an  appropriate  data type, and PCRE_SPTR32 is
       defined as "const PCRE_UCHAR32 *". In very many environments, "unsigned
       int" is a 32-bit data type. When PCRE is built, it defines PCRE_UCHAR32
       as "unsigned int", but checks that it really is a 32-bit data type.  If
       it is not, the build fails with an error message telling the maintainer
       to modify the definition appropriately.


STRUCTURE TYPES

       The types of the opaque structures that are used  for  compiled  32-bit
       patterns  and  JIT stacks are pcre32 and pcre32_jit_stack respectively.
       The  type  of  the  user-accessible  structure  that  is  returned   by
       pcre32_study()  is  pcre32_extra, and the type of the structure that is
       used for passing data to a callout  function  is  pcre32_callout_block.
       These structures contain the same fields, with the same names, as their
       8-bit counterparts. The only difference is that pointers  to  character
       strings are 32-bit instead of 8-bit types.


32-BIT FUNCTIONS

       For  every function in the 8-bit library there is a corresponding func-
       tion in the 32-bit library with a name that starts with pcre32_ instead
       of  pcre_.  The  prototypes are listed above. In addition, there is one
       extra function, pcre32_utf32_to_host_byte_order(). This  is  a  utility
       function  that converts a UTF-32 character string to host byte order if
       necessary. The other 32-bit  functions  expect  the  strings  they  are
       passed to be in host byte order.

       The input and output arguments of pcre32_utf32_to_host_byte_order() may
       point to the same address, that is, conversion in place  is  supported.
       The output buffer must be at least as long as the input.

       The  length  argument  specifies the number of 32-bit data units in the
       input string; a negative value specifies a zero-terminated string.

       If byte_order is NULL, it is assumed that the string starts off in host
       byte  order. This may be changed by byte-order marks (BOMs) anywhere in
       the string (commonly as the first character).

       If byte_order is not NULL, a non-zero value of the integer to which  it
       points  means  that  the input starts off in host byte order, otherwise
       the opposite order is assumed. Again, BOMs in  the  string  can  change
       this. The final byte order is passed back at the end of processing.

       If  keep_boms  is  not  zero,  byte-order  mark characters (0xfeff) are
       copied into the output string. Otherwise they are discarded.

       The result of the function is the number of 32-bit  units  placed  into
       the  output  buffer,  including  the  zero terminator if the string was
       zero-terminated.


SUBJECT STRING OFFSETS

       The offsets within subject strings that are returned  by  the  matching
       functions are in 32-bit units rather than bytes.


NAMED SUBPATTERNS

       The  name-to-number translation table that is maintained for named sub-
       patterns uses 32-bit characters.  The  pcre32_get_stringtable_entries()
       function returns the length of each entry in the table as the number of
       32-bit data units.


OPTION NAMES

       There   are   two   new   general   option   names,   PCRE_UTF32    and
       PCRE_NO_UTF32_CHECK,     which     correspond    to    PCRE_UTF8    and
       PCRE_NO_UTF8_CHECK in the 8-bit library. In  fact,  these  new  options
       define  the  same bits in the options word. There is a discussion about
       the validity of UTF-32 strings in the pcreunicode page.

       For the pcre32_config() function there is an  option  PCRE_CONFIG_UTF32
       that  returns  1  if UTF-32 support is configured, otherwise 0. If this
       option  is  given  to  pcre_config()  or  pcre16_config(),  or  if  the
       PCRE_CONFIG_UTF8  or  PCRE_CONFIG_UTF16  option is given to pcre32_con-
       fig(), the result is the PCRE_ERROR_BADOPTION error.


CHARACTER CODES

       In 32-bit mode, when  PCRE_UTF32  is  not  set,  character  values  are
       treated in the same way as in 8-bit, non UTF-8 mode, except, of course,
       that they can range from 0 to 0x7fffffff instead of 0 to 0xff.  Charac-
       ter  types for characters less than 0xff can therefore be influenced by
       the locale in the same way as before.   Characters  greater  than  0xff
       have only one case, and no "type" (such as letter or digit).

       In  UTF-32  mode,  the  character  code  is  Unicode, in the range 0 to
       0x10ffff, with the exception of values in the range  0xd800  to  0xdfff
       because those are "surrogate" values that are ill-formed in UTF-32.

       A  UTF-32 string can indicate its endianness by special code knows as a
       byte-order mark (BOM). The PCRE functions do not handle this, expecting
       strings   to   be  in  host  byte  order.  A  utility  function  called
       pcre32_utf32_to_host_byte_order() is provided to help  with  this  (see
       above).


ERROR NAMES

       The  error  PCRE_ERROR_BADUTF32  corresponds  to its 8-bit counterpart.
       The error PCRE_ERROR_BADMODE is given when a compiled pattern is passed
       to  a  function that processes patterns in the other mode, for example,
       if a pattern compiled with pcre_compile() is passed to pcre32_exec().

       There are new error codes whose names  begin  with  PCRE_UTF32_ERR  for
       invalid  UTF-32  strings,  corresponding to the PCRE_UTF8_ERR codes for
       UTF-8 strings that are described in the section entitled "Reason  codes
       for  invalid UTF-8 strings" in the main pcreapi page. The UTF-32 errors
       are:

         PCRE_UTF32_ERR1  Surrogate character (range from 0xd800 to 0xdfff)
         PCRE_UTF32_ERR2  Non-character
         PCRE_UTF32_ERR3  Character > 0x10ffff


ERROR TEXTS

       If there is an error while compiling a pattern, the error text that  is
       passed  back by pcre32_compile() or pcre32_compile2() is still an 8-bit
       character string, zero-terminated.


CALLOUTS

       The subject and mark fields in the callout block that is  passed  to  a
       callout function point to 32-bit vectors.


TESTING

       The  pcretest  program continues to operate with 8-bit input and output
       files, but it can be used for testing the 32-bit library. If it is  run
       with the command line option -32, patterns and subject strings are con-
       verted from 8-bit to 32-bit before being passed to PCRE, and the 32-bit
       library  functions  are used instead of the 8-bit ones. Returned 32-bit
       strings are converted to 8-bit for output. If both the  8-bit  and  the
       16-bit libraries were not compiled, pcretest defaults to 32-bit and the
       -32 option is ignored.

       When PCRE is being built, the RunTest script that is  called  by  "make
       check"  uses  the  pcretest  -C  option to discover which of the 8-bit,
       16-bit and 32-bit libraries has been built, and runs the  tests  appro-
       priately.


NOT SUPPORTED IN 32-BIT MODE

       Not all the features of the 8-bit library are available with the 32-bit
       library. The C++ and POSIX wrapper functions  support  only  the  8-bit
       library, and the pcregrep program is at present 8-bit only.


AUTHOR

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


REVISION

       Last updated: 08 November 2012
       Copyright (c) 1997-2012 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 without using con-
       figure (including information about using CMake or building "by  hand")
       in  the file called NON-AUTOTOOLS-BUILD, which is part of the PCRE dis-
       tribution. 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.


BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES

       By default, a library called libpcre  is  built,  containing  functions
       that  take  string  arguments  contained in vectors of bytes, either as
       single-byte characters, or interpreted as UTF-8 strings. You  can  also
       build  a  separate library, called libpcre16, in which strings are con-
       tained in vectors of 16-bit data units and interpreted either  as  sin-
       gle-unit characters or UTF-16 strings, by adding

         --enable-pcre16

       to the configure command. You can also build a separate library, called
       libpcre32, in which strings are contained in  vectors  of  32-bit  data
       units  and  interpreted  either  as  single-unit  characters  or UTF-32
       strings, by adding

         --enable-pcre32

       to the configure command. If you do not want the 8-bit library, add

         --disable-pcre8

       as well. At least one of the three libraries must be built.  Note  that
       the  C++  and  POSIX  wrappers are for the 8-bit library only, and that
       pcregrep is an 8-bit program. None of these are  built  if  you  select
       only the 16-bit or 32-bit libraries.


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.


C++ SUPPORT

       By  default,  if the 8-bit library is being built, the configure script
       will search for a C++ compiler and C++ header files. If it finds  them,
       it  automatically  builds  the C++ wrapper library (which supports only
       8-bit strings). You can disable this by adding

         --disable-cpp

       to the configure command.


UTF-8, UTF-16 AND UTF-32 SUPPORT

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

         --enable-utf

       to the configure command. This setting applies to all three  libraries,
       adding  support  for  UTF-8 to the 8-bit library, support for UTF-16 to
       the 16-bit library, and  support  for  UTF-32  to  the  to  the  32-bit
       library.  There  are no separate options for enabling UTF-8, UTF-16 and
       UTF-32 independently because that would allow ridiculous settings  such
       as  requesting UTF-16 support while building only the 8-bit library. It
       is not possible to build one library with UTF support and another with-
       out  in the same configuration. (For backwards compatibility, --enable-
       utf8 is a synonym of --enable-utf.)

       Of itself, this setting does not make  PCRE  treat  strings  as  UTF-8,
       UTF-16  or UTF-32. As well as compiling PCRE with this option, you also
       have have to set the PCRE_UTF8, PCRE_UTF16  or  PCRE_UTF32  option  (as
       appropriate) when you call one of the pattern compiling functions.

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


UNICODE CHARACTER PROPERTY SUPPORT

       UTF  support allows the libraries to process character codepoints up to
       0x10ffff in the strings that they handle. On its own, however, it  does
       not provide any facilities for accessing the properties of such charac-
       ters. 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 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.


JUST-IN-TIME COMPILER SUPPORT

       Just-in-time compiler support is included in the build by specifying

         --enable-jit

       This support is available only for certain hardware  architectures.  If
       this  option  is  set  for  an unsupported architecture, a compile time
       error occurs.  See the pcrejit documentation for a  discussion  of  JIT
       usage. When JIT support is enabled, pcregrep automatically makes use of
       it, unless you add

         --disable-pcregrep-jit

       to the "configure" command.


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.


POSIX MALLOC USAGE

       When the 8-bit library is called through the POSIX interface  (see  the
       pcreposix  documentation),  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  func-
       tion  uses  space  on the stack, because this is faster than using mal-
       loc() 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, in the 8-bit and 16-bit libraries,
       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  truly  enormous patterns, so it is possible to compile PCRE to
       use three-byte or four-byte offsets by adding a setting such as

         --with-link-size=3

       to the configure command. The value given must be 2, 3, or 4.  For  the
       16-bit  library,  a  value of 3 is rounded up to 4. In these libraries,
       using longer offsets slows down the operation of PCRE because it has to
       load  additional  data  when  handling them. For the 32-bit library the
       value is always 4 and cannot be overridden; the value  of  --with-link-
       size is ignored.


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 run-time 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-utf.

       The EBCDIC character that corresponds to an ASCII LF is assumed to have
       the value 0x15 by default. However, in some EBCDIC  environments,  0x25
       is used. In such an environment you should use

         --enable-ebcdic-nl25

       as well as, or instead of, --enable-ebcdic. The EBCDIC character for CR
       has the same value as in ASCII, namely, 0x0d.  Whichever  of  0x15  and
       0x25 is not chosen as LF is made to correspond to the Unicode NEL char-
       acter (which, in Unicode, is 0x85).

       The options that select newline behaviour, such as --enable-newline-is-
       cr, and equivalent run-time options, refer to these character values in
       an EBCDIC environment.


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.


PCREGREP BUFFER SIZE

       pcregrep uses an internal buffer to hold a "window" on the file  it  is
       scanning, in order to be able to output "before" and "after" lines when
       it finds a match. The size of the buffer is controlled by  a  parameter
       whose default value is 20K. The buffer itself is three times this size,
       but because of the way it is used for holding "before" lines, the long-
       est  line  that  is guaranteed to be processable is the parameter size.
       You can change the default parameter value by adding, for example,

         --with-pcregrep-bufsize=50K

       to the configure command. The caller of pcregrep can, however, override
       this value by specifying a run-time option.


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.


DEBUGGING WITH VALGRIND SUPPORT

       By adding the

         --enable-valgrind

       option  to to the configure command, PCRE will use valgrind annotations
       to mark certain memory regions as  unaddressable.  This  allows  it  to
       detect invalid memory accesses, and is mostly useful for debugging PCRE
       itself.


CODE COVERAGE REPORTING

       If your C compiler is gcc, you can build a version  of  PCRE  that  can
       generate a code coverage report for its test suite. To enable this, you
       must install lcov version 1.6 or above. Then specify

         --enable-coverage

       to the configure command and build PCRE in the usual way.

       Note that using ccache (a caching C compiler) is incompatible with code
       coverage  reporting. If you have configured ccache to run automatically
       on your system, you must set the environment variable

         CCACHE_DISABLE=1

       before running make to build PCRE, so that ccache is not used.

       When --enable-coverage is used,  the  following  addition  targets  are
       added to the Makefile:

         make coverage

       This  creates  a  fresh  coverage report for the PCRE test suite. It is
       equivalent to running "make coverage-reset", "make  coverage-baseline",
       "make check", and then "make coverage-report".

         make coverage-reset

       This zeroes the coverage counters, but does nothing else.

         make coverage-baseline

       This captures baseline coverage information.

         make coverage-report

       This creates the coverage report.

         make coverage-clean-report

       This  removes the generated coverage report without cleaning the cover-
       age data itself.

         make coverage-clean-data

       This removes the captured coverage data without removing  the  coverage
       files created at compile time (*.gcno).

         make coverage-clean

       This  cleans all coverage data including the generated coverage report.
       For more information about code coverage, see the gcov and  lcov  docu-
       mentation.


SEE ALSO

       pcreapi(3), pcre16, pcre32, pcre_config(3).


AUTHOR

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


REVISION

       Last updated: 30 October 2012
       Copyright (c) 1997-2012 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(), pcre16_exec() and pcre32_exec() functions. These  work  in
       the  same as as Perl's matching function, and provide a Perl-compatible
       matching  operation.   The  just-in-time  (JIT)  optimization  that  is
       described  in  the pcrejit documentation is compatible with these func-
       tions.

       An  alternative  algorithm  is   provided   by   the   pcre_dfa_exec(),
       pcre16_dfa_exec()  and  pcre32_dfa_exec()  functions; they operate in a
       different way, and are not Perl-compatible. This alternative has advan-
       tages 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) always
       matches a single data unit, even in UTF-8, UTF-16 or UTF-32  modes,  is
       not  supported  in these modes, because the alternative algorithm moves
       through the subject string one character (not data unit) 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 (except for lookbehinds), it is pos-
       sible  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 by
       retaining partially matched substrings, it  is  more  complicated.  The
       pcrepartial  documentation  gives  details of partial matching and dis-
       cusses 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: 08 January 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCREAPI(3)                                                          PCREAPI(3)


NAME
       PCRE - Perl-compatible regular expressions

       #include <pcre.h>


PCRE NATIVE API BASIC FUNCTIONS

       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);

       void pcre_free_study(pcre_extra *extra);

       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);


PCRE NATIVE API STRING EXTRACTION FUNCTIONS

       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);


PCRE NATIVE API AUXILIARY FUNCTIONS

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

       pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize);

       void pcre_jit_stack_free(pcre_jit_stack *stack);

       void pcre_assign_jit_stack(pcre_extra *extra,
            pcre_jit_callback callback, void *data);

       const unsigned char *pcre_maketables(void);

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

       int pcre_refcount(pcre *code, int adjust);

       int pcre_config(int what, void *where);

       const char *pcre_version(void);

       int pcre_pattern_to_host_byte_order(pcre *code,
            pcre_extra *extra, const unsigned char *tables);


PCRE NATIVE API INDIRECTED FUNCTIONS

       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 8-BIT, 16-BIT, AND 32-BIT LIBRARIES

       As  well  as  support  for  8-bit character strings, PCRE also supports
       16-bit strings (from release 8.30) and  32-bit  strings  (from  release
       8.32),  by means of two additional libraries. They can be built as well
       as, or instead of, the 8-bit library. To avoid too  much  complication,
       this  document describes the 8-bit versions of the functions, with only
       occasional references to the 16-bit and 32-bit libraries.

       The 16-bit and 32-bit functions operate in the same way as their  8-bit
       counterparts;  they  just  use different data types for their arguments
       and results, and their names start with pcre16_ or pcre32_  instead  of
       pcre_.  For  every  option  that  has  UTF8  in  its name (for example,
       PCRE_UTF8), there are corresponding 16-bit and 32-bit names  with  UTF8
       replaced by UTF16 or UTF32, respectively. This facility is in fact just
       cosmetic; the 16-bit and 32-bit option names define the same  bit  val-
       ues.

       References to bytes and UTF-8 in this document should be read as refer-
       ences to 16-bit data  quantities  and  UTF-16  when  using  the  16-bit
       library,  or  32-bit  data  quantities and UTF-32 when using the 32-bit
       library, unless specified otherwise. More details of the specific  dif-
       ferences  for  the  16-bit and 32-bit libraries are given in the pcre16
       and pcre32 pages.


PCRE API OVERVIEW

       PCRE has its own native API, which is described in this document. There
       are  also some wrapper functions (for the 8-bit library only) that cor-
       respond to the POSIX regular expression  API,  but  they  do  not  give
       access  to  all  the functionality. They are described in the pcreposix
       documentation. Both of these APIs define a set of C function  calls.  A
       C++ wrapper (again for the 8-bit library only) is also 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-like systems the (8-bit) 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
       numbers  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.

       Just-in-time compiler support is an optional feature of PCRE  that  can
       be built in appropriate hardware environments. It greatly speeds up the
       matching performance of  many  patterns.  Simple  programs  can  easily
       request  that  it  be  used  if available, by setting an option that is
       ignored when it is not relevant. More complicated programs  might  need
       to     make    use    of    the    functions    pcre_jit_stack_alloc(),
       pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to  control
       the JIT code's memory usage.

       From  release  8.32 there is also a direct interface for JIT execution,
       which gives improved performance. The JIT-specific functions  are  dis-
       cussed in the pcrejit documentation.

       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.  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 (form feed, 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.

       If  the just-in-time optimization feature is being used, it needs sepa-
       rate memory stack areas for each thread. See the pcrejit  documentation
       for more details.


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,  which  includes  a  description  of  the
       pcre_pattern_to_host_byte_order()  function. However, compiling a regu-
       lar expression with one version of PCRE for use with a  different  ver-
       sion is not guaranteed 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 returned  value  is  zero  on
       success,  or  the negative error code PCRE_ERROR_BADOPTION if the value
       in the first argument is not recognized. 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. This value should normally be  given
       to the 8-bit version of this function, pcre_config(). If it is given to
       the  16-bit  or  32-bit  version  of  this  function,  the  result   is
       PCRE_ERROR_BADOPTION.

         PCRE_CONFIG_UTF16

       The output is an integer that is set to one if UTF-16 support is avail-
       able; otherwise it is set to zero. This value should normally be  given
       to the 16-bit version of this function, pcre16_config(). If it is given
       to the 8-bit  or  32-bit  version  of  this  function,  the  result  is
       PCRE_ERROR_BADOPTION.

         PCRE_CONFIG_UTF32

       The output is an integer that is set to one if UTF-32 support is avail-
       able; otherwise it is set to zero. This value should normally be  given
       to the 32-bit version of this function, pcre32_config(). If it is given
       to the 8-bit  or  16-bit  version  of  this  function,  the  result  is
       PCRE_ERROR_BADOPTION.

         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_JIT

       The output is an integer that is set to one if support for just-in-time
       compiling is available; otherwise it is set to zero.

         PCRE_CONFIG_JITTARGET

       The  output is a pointer to a zero-terminated "const char *" string. If
       JIT support is available, the string contains the name of the architec-
       ture  for  which the JIT compiler is configured, for example "x86 32bit
       (little endian + unaligned)". If JIT  support  is  not  available,  the
       result is NULL.

         PCRE_CONFIG_NEWLINE

       The  output  is  an integer whose value specifies the default character
       sequence that is recognized as meaning "newline". The values  that  are
       supported in ASCII/Unicode environments are: 10 for LF, 13 for CR, 3338
       for CRLF, -2 for ANYCRLF, and -1 for ANY. In EBCDIC  environments,  CR,
       ANYCRLF,  and  ANY  yield the same values. However, the value for LF is
       normally 21, though some EBCDIC environments use 37. The  corresponding
       values  for  CRLF are 3349 and 3365. 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.  For  the  8-bit
       library, the value can be 2, 3, or 4. For the 16-bit library, the value
       is  either  2  or  4  and  is  still  a number of bytes. For the 32-bit
       library, the value is either 2 or 4 and is still a number of bytes. 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.  Larger
       values  allow larger regular expressions to be compiled, at the expense
       of slower matching.

         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_OPTIMIZE 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. Normally, 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). However, for an invalid  UTF-8
       string, the offset is that of the first byte of the failing character.

       Some  errors are not detected until the whole pattern has been scanned;
       in these cases, the offset passed back is the length  of  the  pattern.
       Note  that  the offset is in bytes, not characters, even in UTF-8 mode.
       It may sometimes point into the middle of a UTF-8 character.

       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, white space 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.
       White  space  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.

       (3) \U matches an upper case "U" character; by default \U causes a com-
       pile time error (Perl uses \U to upper case subsequent characters).

       (4) \u matches a lower case "u" character unless it is followed by four
       hexadecimal  digits,  in  which case the hexadecimal number defines the
       code point to match. By default, \u causes a compile time  error  (Perl
       uses it to upper case the following character).

       (5)  \x matches a lower case "x" character unless it is followed by two
       hexadecimal digits, in which case the hexadecimal  number  defines  the
       code  point  to  match. By default, as in Perl, a hexadecimal number is
       always expected after \x, but it may have zero, one, or two digits (so,
       for example, \xz matches a binary zero character followed by z).

         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.

       In  an ASCII/Unicode environment, the Unicode newline sequences are the
       three just mentioned, plus the  single  characters  VT  (vertical  tab,
       U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line sep-
       arator, U+2028), and PS (paragraph separator, U+2029).  For  the  8-bit
       library, the last two are recognized only in UTF-8 mode.

       When  PCRE is compiled to run in an EBCDIC (mainframe) environment, the
       code for CR is 0x0d, the same as ASCII. However, the character code for
       LF  is  normally 0x15, though in some EBCDIC environments 0x25 is used.
       Whichever of these is not LF is made to  correspond  to  Unicode's  NEL
       character.  EBCDIC  codes  are all less than 256. For more details, see
       the pcrebuild documentation.

       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 white space
       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 strings. However, it
       is available only when PCRE is built to include UTF  support.  If  not,
       the  use  of  this option provokes an error. Details of how this option
       changes the behaviour of PCRE are given in the pcreunicode 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 pcreunicode page. If an invalid UTF-8 sequence  is
       found,  pcre_compile()  returns an error. If you already know that your
       pattern is valid, and you want to skip this check for performance  rea-
       sons,  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  validity
       checking  of  subject strings only. If the same string is being matched
       many times, the option can be safely set for the second and  subsequent
       matchings to improve performance.


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.  Note  that error messages are always 8-bit
       ASCII strings, even in 16-bit or 32-bit mode. 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 compiled without UTF 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{name}, \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 (specifically UTF-8)
         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 in 8-bit non-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 Unicode property
               support
         68  \c must be followed by an ASCII character
         69  \k is not followed by a braced, angle-bracketed, or quoted name
         70  internal error: unknown opcode in find_fixedlength()
         71  \N is not supported in a class
         72  too many forward references
         73  disallowed Unicode code point (>= 0xd800 && <= 0xdfff)
         74  invalid UTF-16 string (specifically UTF-16)
         75  name is too long in (*MARK), (*PRUNE), (*SKIP), or (*THEN)
         76  character value in \u.... sequence is too large
         77  invalid UTF-32 string (specifically UTF-32)

       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  by  default.  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. However, if
       pcre_study() is called  with  the  PCRE_STUDY_EXTRA_NEEDED  option,  it
       returns a pcre_extra block even if studying did not find any additional
       information. It may still return NULL, however, if an error  occurs  in
       pcre_study().

       The  second  argument  of  pcre_study() contains option bits. There are
       three further options in addition to PCRE_STUDY_EXTRA_NEEDED:

         PCRE_STUDY_JIT_COMPILE
         PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
         PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE

       If any of these are set, and the just-in-time  compiler  is  available,
       the  pattern  is  further compiled into machine code that executes much
       faster than the pcre_exec()  interpretive  matching  function.  If  the
       just-in-time  compiler is not available, these options are ignored. All
       undefined bits in the options argument must be zero.

       JIT compilation is a heavyweight optimization. It can  take  some  time
       for  patterns  to  be analyzed, and for one-off matches and simple pat-
       terns the benefit of faster execution might be offset by a much  slower
       study time.  Not all patterns can be optimized by the JIT compiler. For
       those that cannot be handled, matching automatically falls back to  the
       pcre_exec()  interpreter.  For more details, see the pcrejit documenta-
       tion.

       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.

       When  you are finished with a pattern, you can free the memory used for
       the study data by calling pcre_free_study(). This function was added to
       the  API  for  release  8.20. For earlier versions, the memory could be
       freed with pcre_free(), just like the pattern itself. This  will  still
       work  in  cases where JIT optimization is not used, but it is advisable
       to change to the new function when convenient.

       This is a typical way in which pcre_study() is used (except that  in  a
       real application there should be tests for errors):

         int rc;
         pcre *re;
         pcre_extra *sd;
         re = pcre_compile("pattern", 0, &error, &erroroffset, NULL);
         sd = pcre_study(
           re,             /* result of pcre_compile() */
           0,              /* no options */
           &error);        /* set to NULL or points to a message */
         rc = pcre_exec(   /* see below for details of pcre_exec() options */
           re, sd, "subject", 7, 0, 0, ovector, 30);
         ...
         pcre_free_study(sd);
         pcre_free(re);

       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  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. (In 16-bit mode, the bitmap is used for 16-bit
       values  less  than  256.  In 32-bit mode, the bitmap is used for 32-bit
       values less than 256.)

       These two optimizations apply to both pcre_exec() and  pcre_dfa_exec(),
       and  the  information  is also used by the JIT compiler.  The optimiza-
       tions can be disabled by setting the PCRE_NO_START_OPTIMIZE option when
       calling pcre_exec() or pcre_dfa_exec(), but if this is done, JIT execu-
       tion is also disabled. You might want to do this if your  pattern  con-
       tains  callouts or (*MARK) and you want to make use of these facilities
       in   cases   where   matching   fails.   See    the    discussion    of
       PCRE_NO_START_OPTIMIZE 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 pcre_info() function, which was removed from the
       library at version 8.30, after more than 10 years of obsolescence.

       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_BADENDIANNESS  the pattern was compiled with different
                                   endianness
         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. The  endi-
       anness error can occur if a compiled pattern is saved and reloaded on a
       different host. 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() */
           sd,               /* 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 data unit of any matched string, for
       a non-anchored pattern. (The name of this option refers  to  the  8-bit
       library,  where data units are bytes.) The fourth argument should point
       to an int variable.

       If there is a fixed first value, for example, the  letter  "c"  from  a
       pattern  such  as (cat|cow|coyote), its value is returned. In the 8-bit
       library, the value is always less than 256. In the 16-bit  library  the
       value can be up to 0xffff. In the 32-bit library the value can be up to
       0x10ffff.

       If there is no fixed first value, and 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.

       Since for the 32-bit library using the non-UTF-32 mode,  this  function
       is  unable to return the full 32-bit range of the character, this value
       is   deprecated;   instead   the   PCRE_INFO_FIRSTCHARACTERFLAGS    and
       PCRE_INFO_FIRSTCHARACTER values should be used.

         PCRE_INFO_FIRSTTABLE

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

         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_JIT

       Return 1 if the pattern was studied with one of the  JIT  options,  and
       just-in-time compiling was successful. The fourth argument should point
       to an int variable. A return value of 0 means that JIT support  is  not
       available  in this version of PCRE, or that the pattern was not studied
       with a JIT option, or that the JIT compiler could not handle this  par-
       ticular  pattern. See the pcrejit documentation for details of what can
       and cannot be handled.

         PCRE_INFO_JITSIZE

       If the pattern was successfully studied with a JIT option,  return  the
       size  of the JIT compiled code, otherwise return zero. The fourth argu-
       ment should point to a size_t variable.

         PCRE_INFO_LASTLITERAL

       Return the value of the rightmost literal data unit that must exist  in
       any  matched  string, other than at its start, if such a value has been
       recorded. The fourth argument should point to an int variable. If there
       is no such value, -1 is returned. For anchored patterns, a last literal
       value 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.

       Since for the 32-bit library using the non-UTF-32 mode,  this  function
       is  unable to return the full 32-bit range of the character, this value
       is   deprecated;   instead    the    PCRE_INFO_REQUIREDCHARFLAGS    and
       PCRE_INFO_REQUIREDCHAR values should be used.

         PCRE_INFO_MAXLOOKBEHIND

       Return  the  number of characters (NB not bytes) in the longest lookbe-
       hind assertion in the pattern. Note that the simple assertions  \b  and
       \B  require a one-character lookbehind. This information is useful when
       doing multi-segment matching using the partial matching facilities.

         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, which in  UTF-8  mode
       may  be  different from the number of bytes. 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. This is a pointer to char in
       the 8-bit library, where the first two bytes of each entry are the num-
       ber  of  the capturing parenthesis, most significant byte first. In the
       16-bit library, the pointer points to 16-bit data units, the  first  of
       which  contains  the  parenthesis  number.   In the 32-bit library, the
       pointer points to 32-bit data units, the first of  which  contains  the
       parenthesis  number.  The  rest of the entry is the corresponding 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 after compilation by the 8-bit library (assume PCRE_EXTENDED is
       set, so white space - including newlines - 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 in bytes (for both libraries).
       The fourth argument should point to a size_t variable. This value  does
       not  include  the  size  of  the  pcre  structure  that  is returned by
       pcre_compile(). The value that is passed as the argument  to  pcre_mal-
       loc()  when pcre_compile() is getting memory in which to place the com-
       piled data is the value returned by this option plus the  size  of  the
       pcre  structure. Studying a compiled pattern, with or without JIT, does
       not alter the value returned by this option.

         PCRE_INFO_STUDYSIZE

       Return the size in bytes of the data block pointed to by the study_data
       field  in  a  pcre_extra  block.  If pcre_extra is NULL, or there is no
       study data, zero is returned. The fourth argument  should  point  to  a
       size_t  variable. The study_data field is set by pcre_study() to record
       information that will speed  up  matching  (see  the  section  entitled
       "Studying a pattern" above). The format of the study_data block is pri-
       vate, but its length is made available via this option so that  it  can
       be  saved  and  restored  (see  the  pcreprecompile  documentation  for
       details).

         PCRE_INFO_FIRSTCHARACTERFLAGS

       Return information about the first data unit of any matched string, for
       a  non-anchored  pattern.  The  fourth  argument should point to an int
       variable.

       If there is a fixed first value, for example, the  letter  "c"  from  a
       pattern  such  as  (cat|cow|coyote),  1  is returned, and the character
       value can be retrieved using PCRE_INFO_FIRSTCHARACTER.

       If there is no fixed first value, and 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),

       2 is returned, indicating that the pattern matches only at the start of
       a subject string or after any newline within the string. Otherwise 0 is
       returned. For anchored patterns, 0 is returned.

         PCRE_INFO_FIRSTCHARACTER

       Return the fixed first character  value,  if  PCRE_INFO_FIRSTCHARACTER-
       FLAGS returned 1; otherwise returns 0. The fourth argument should point
       to an uint_t variable.

       In the 8-bit library, the value is always less than 256. In the  16-bit
       library  the value can be up to 0xffff. In the 32-bit library in UTF-32
       mode the value can be up to 0x10ffff, and up  to  0xffffffff  when  not
       using UTF-32 mode.

       If there is no fixed first value, and 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_REQUIREDCHARFLAGS

       Returns  1 if there is a rightmost literal data unit that must exist in
       any matched string, other than at its start. The fourth argument should
       point  to an int variable. If there is no such value, 0 is returned. If
       returning  1,  the  character  value  itself  can  be  retrieved  using
       PCRE_INFO_REQUIREDCHAR.

       For anchored patterns, a last literal value is recorded only if it fol-
       lows something  of  variable  length.  For  example,  for  the  pattern
       /^a\d+z\d+/   the   returned   value   1   (with   "z"   returned  from
       PCRE_INFO_REQUIREDCHAR), but for /^a\dz\d/ the returned value is 0.

         PCRE_INFO_REQUIREDCHAR

       Return the value of the rightmost literal data unit that must exist  in
       any  matched  string, other than at its start, if such a value has been
       recorded. The fourth argument should point to an uint32_t variable.  If
       there is no such value, 0 is returned.


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.  You  can call pcre_exec() with the same code and extra argu-
       ments as many times as you like, in order to  match  different  subject
       strings with the same pattern.

       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 section
       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;
         void *executable_jit;
         unsigned long int match_limit;
         unsigned long int match_limit_recursion;
         void *callout_data;
         const unsigned char *tables;
         unsigned char **mark;

       In the 16-bit version of  this  structure,  the  mark  field  has  type
       "PCRE_UCHAR16 **".

       In  the  32-bit  version  of  this  structure,  the mark field has type
       "PCRE_UCHAR32 **".

       The flags field is used to specify which of the other fields  are  set.
       The flag bits are:

         PCRE_EXTRA_CALLOUT_DATA
         PCRE_EXTRA_EXECUTABLE_JIT
         PCRE_EXTRA_MARK
         PCRE_EXTRA_MATCH_LIMIT
         PCRE_EXTRA_MATCH_LIMIT_RECURSION
         PCRE_EXTRA_STUDY_DATA
         PCRE_EXTRA_TABLES

       Other  flag  bits should be set to zero. The study_data field and some-
       times the executable_jit field are set in the pcre_extra block that  is
       returned  by pcre_study(), together with the appropriate flag bits. You
       should not set these yourself, but you may add to the block by  setting
       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_exec() uses a function called match(), which it calls
       repeatedly (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.

       When pcre_exec() is called with a pattern that was successfully studied
       with  a  JIT  option, the way that the matching is executed is entirely
       different.  However, there is still the possibility of runaway matching
       that goes on for a very long time, and so the match_limit value is also
       used in this case (but in a different way) to limit how long the match-
       ing can continue.

       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  machine  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.  This
       limit  is not relevant, and is ignored, when matching is done using JIT
       compiled code.

       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 suitable 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  is  set  to NULL. For details of the backtracking control verbs,
       see the section entitled "Backtracking control" in the pcrepattern doc-
       umentation.

   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_HARD,   and
       PCRE_PARTIAL_SOFT.

       If  the  pattern  was successfully studied with one of the just-in-time
       (JIT) compile options, the only supported options for JIT execution are
       PCRE_NO_UTF8_CHECK,     PCRE_NOTBOL,     PCRE_NOTEOL,    PCRE_NOTEMPTY,
       PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT. If  an
       unsupported  option  is  used, JIT execution is disabled and the normal
       interpretive code in pcre_exec() is run.

         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.  The  use  of
       PCRE_NO_START_OPTIMIZE disables JIT execution; when it is set, matching
       is always done using interpretively.

       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 entire string is checked before any other processing takes
       place.  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 pcreunicode page. If an invalid
       sequence  of  bytes   is   found,   pcre_exec()   returns   the   error
       PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and the problem is a
       truncated character at the end of the subject, PCRE_ERROR_SHORTUTF8. In
       both  cases, information about the precise nature of the error may also
       be returned (see the descriptions of these errors in the section  enti-
       tled  Error return values from pcre_exec() below).  If startoffset con-
       tains 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 character (or the end of the subject). When
       PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid 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 neither the actual string matched
       nor any captured substrings are of interest, pcre_exec() may be  called
       with  ovector passed as NULL and ovecsize as zero. However, if the pat-
       tern 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 usually advisable to supply an  ovector
       of reasonable size.

       There  are  some  cases where zero is returned (indicating vector over-
       flow) when in fact the vector is exactly the right size for  the  final
       match. For example, consider the pattern

         (a)(?:(b)c|bd)

       If  a  vector of 6 elements (allowing for only 1 captured substring) is
       given with subject string "abd", pcre_exec() will try to set the second
       captured string, thereby recording a vector overflow, before failing to
       match "c" and backing up  to  try  the  second  alternative.  The  zero
       return,  however,  does  correctly  indicate that the maximum number of
       slots (namely 2) have been filled. In similar cases where there is tem-
       porary  overflow,  but  the final number of used slots is actually less
       than the maximum, a non-zero value is returned.

       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 in the first two-thirds of ovector that  do  not  corre-
       spond  to  capturing parentheses in the pattern are never changed. That
       is, if a pattern contains n capturing parentheses, no more  than  ovec-
       tor[0]  to ovector[2n+1] are set by pcre_exec(). The other elements (in
       the first two-thirds) retain whatever values they previously 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, and the PCRE_NO_UTF8_CHECK option was not set. If the size  of
       the  output  vector  (ovecsize)  is  at least 2, the byte offset to the
       start of the the invalid UTF-8 character is placed in  the  first  ele-
       ment,  and  a  reason  code is placed in the second element. The reason
       codes are listed in the following section.  For backward compatibility,
       if  PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char-
       acter  at  the  end  of  the   subject   (reason   codes   1   to   5),
       PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8.

         PCRE_ERROR_BADUTF8_OFFSET (-11)

       The  UTF-8  byte  sequence that was passed as a subject was checked and
       found to be valid (the PCRE_NO_UTF8_CHECK option was not set), 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)

       This error is returned instead of PCRE_ERROR_BADUTF8 when  the  subject
       string  ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD
       option is set.  Information  about  the  failure  is  returned  as  for
       PCRE_ERROR_BADUTF8.  It  is in fact sufficient to detect this case, but
       this special error code for PCRE_PARTIAL_HARD precedes the  implementa-
       tion  of returned information; it is retained for backwards compatibil-
       ity.

         PCRE_ERROR_RECURSELOOP    (-26)

       This error is returned when pcre_exec() detects a recursion loop within
       the  pattern. Specifically, it means that either the whole pattern or a
       subpattern has been called recursively for the second time at the  same
       position in the subject string. Some simple patterns that might do this
       are detected and faulted at compile time, but more  complicated  cases,
       in particular mutual recursions between two different subpatterns, can-
       not be detected until run time.

         PCRE_ERROR_JIT_STACKLIMIT (-27)

       This error is returned when a pattern  that  was  successfully  studied
       using  a  JIT compile option is being matched, but the memory available
       for the just-in-time processing stack is  not  large  enough.  See  the
       pcrejit documentation for more details.

         PCRE_ERROR_BADMODE        (-28)

       This error is given if a pattern that was compiled by the 8-bit library
       is passed to a 16-bit or 32-bit library function, or vice versa.

         PCRE_ERROR_BADENDIANNESS  (-29)

       This error is given if  a  pattern  that  was  compiled  and  saved  is
       reloaded  on  a  host  with  different endianness. The utility function
       pcre_pattern_to_host_byte_order() can be used to convert such a pattern
       so that it runs on the new host.

         PCRE_ERROR_JIT_BADOPTION

       This  error  is  returned  when a pattern that was successfully studied
       using a JIT compile option is being  matched,  but  the  matching  mode
       (partial  or complete match) does not correspond to any JIT compilation
       mode. When the JIT fast path function is used, this error may  be  also
       given  for  invalid  options.  See  the  pcrejit documentation for more
       details.

         PCRE_ERROR_BADLENGTH      (-32)

       This error is given if pcre_exec() is called with a negative value  for
       the length argument.

       Error numbers -16 to -20, -22, and 30 are not used by pcre_exec().

   Reason codes for invalid UTF-8 strings

       This  section  applies  only  to  the  8-bit library. The corresponding
       information for the 16-bit and 32-bit libraries is given in the  pcre16
       and pcre32 pages.

       When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT-
       UTF8, and the size of the output vector (ovecsize) is at least  2,  the
       offset  of  the  start  of the invalid UTF-8 character is placed in the
       first output vector element (ovector[0]) and a reason code is placed in
       the  second  element  (ovector[1]). The reason codes are given names in
       the pcre.h header file:

         PCRE_UTF8_ERR1
         PCRE_UTF8_ERR2
         PCRE_UTF8_ERR3
         PCRE_UTF8_ERR4
         PCRE_UTF8_ERR5

       The string ends with a truncated UTF-8 character;  the  code  specifies
       how  many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8
       characters to be no longer than 4 bytes, the  encoding  scheme  (origi-
       nally  defined  by  RFC  2279)  allows  for  up to 6 bytes, and this is
       checked first; hence the possibility of 4 or 5 missing bytes.

         PCRE_UTF8_ERR6
         PCRE_UTF8_ERR7
         PCRE_UTF8_ERR8
         PCRE_UTF8_ERR9
         PCRE_UTF8_ERR10

       The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
       the  character  do  not have the binary value 0b10 (that is, either the
       most significant bit is 0, or the next bit is 1).

         PCRE_UTF8_ERR11
         PCRE_UTF8_ERR12

       A character that is valid by the RFC 2279 rules is either 5 or 6  bytes
       long; these code points are excluded by RFC 3629.

         PCRE_UTF8_ERR13

       A  4-byte character has a value greater than 0x10fff; these code points
       are excluded by RFC 3629.

         PCRE_UTF8_ERR14

       A 3-byte character has a value in the  range  0xd800  to  0xdfff;  this
       range  of code points are reserved by RFC 3629 for use with UTF-16, and
       so are excluded from UTF-8.

         PCRE_UTF8_ERR15
         PCRE_UTF8_ERR16
         PCRE_UTF8_ERR17
         PCRE_UTF8_ERR18
         PCRE_UTF8_ERR19

       A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it  codes
       for  a  value that can be represented by fewer bytes, which is invalid.
       For example, the two bytes 0xc0, 0xae give the value 0x2e,  whose  cor-
       rect coding uses just one byte.

         PCRE_UTF8_ERR20

       The two most significant bits of the first byte of a character have the
       binary value 0b10 (that is, the most significant bit is 1 and the  sec-
       ond  is  0). Such a byte can only validly occur as the second or subse-
       quent byte of a multi-byte character.

         PCRE_UTF8_ERR21

       The first byte of a character has the value 0xfe or 0xff. These  values
       can never occur in a valid UTF-8 string.

         PCRE_UTF8_ERR2

       Non-character. These are the last two characters in each plane (0xfffe,
       0xffff, 0x1fffe, 0x1ffff .. 0x10fffe,  0x10ffff),  and  the  characters
       0xfdd0..0xfdef.


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 above.  Given all  the  rele-
       vant  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.


OBTAINING AN ESTIMATE OF STACK USAGE

       Matching  certain  patterns  using pcre_exec() can use a lot of process
       stack, which in certain environments can be  rather  limited  in  size.
       Some  users  find it helpful to have an estimate of the amount of stack
       that is used by pcre_exec(), to help  them  set  recursion  limits,  as
       described  in  the pcrestack documentation. The estimate that is output
       by pcretest when called with the -m and -C options is obtained by call-
       ing  pcre_exec with the values NULL, NULL, NULL, -999, and -999 for its
       first five arguments.

       Normally, if  its  first  argument  is  NULL,  pcre_exec()  immediately
       returns  the negative error code PCRE_ERROR_NULL, but with this special
       combination of arguments, it returns instead a  negative  number  whose
       absolute  value  is the approximate stack frame size in bytes. (A nega-
       tive number is used so that it is clear that no  match  has  happened.)
       The  value  is  approximate  because  in some cases, recursive calls to
       pcre_exec() occur when there are one or two additional variables on the
       stack.

       If  PCRE  has  been  compiled  to use the heap instead of the stack for
       recursion, the value returned  is  the  size  of  each  block  that  is
       obtained from the heap.


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. Unlike pcre_exec(), pcre_dfa_exec()
       can use the entire ovector for returning matched strings.

   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  or  match_limit_recursion
       fields.  This  is  not  supported (these fields are meaningless for DFA
       matching).

         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.

         PCRE_ERROR_DFA_BADRESTART (-30)

       When pcre_dfa_exec() is called with the PCRE_DFA_RESTART  option,  some
       plausibility  checks  are  made on the contents of the workspace, which
       should contain data about the previous partial match. If any  of  these
       checks fail, this error is given.


SEE ALSO

       pcre16(3),   pcre32(3),  pcrebuild(3),  pcrecallout(3),  pcrecpp(3)(3),
       pcrematching(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcre-
       sample(3), pcrestack(3).


AUTHOR

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


REVISION

       Last updated: 08 November 2012
       Copyright (c) 1997-2012 University of Cambridge.
------------------------------------------------------------------------------


PCRECALLOUT(3)                                                  PCRECALLOUT(3)


NAME
       PCRE - Perl-compatible regular expressions


SYNOPSIS

       #include <pcre.h>

       int (*pcre_callout)(pcre_callout_block *);

       int (*pcre16_callout)(pcre16_callout_block *);

       int (*pcre32_callout)(pcre32_callout_block *);


DESCRIPTION

       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 (pcre16_callout for
       the 16-bit library, pcre32_callout for the 32-bit library). 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 a pattern is  compiled,
       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.

       The use of callouts in a pattern makes it ineligible  for  optimization
       by  the  just-in-time  compiler.  Studying  such  a  pattern  with  the
       PCRE_STUDY_JIT_COMPILE option always fails.


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 the matching function, 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 or pcre[16|32]_callout is called (if it is
       set).  This  applies to both normal and DFA matching. The only argument
       to  the  callout  function  is  a  pointer   to   a   pcre_callout   or
       pcre[16|32]_callout  block.   These  structures  contains the following
       fields:

         int           version;
         int           callout_number;
         int          *offset_vector;
         const char   *subject;           (8-bit version)
         PCRE_SPTR16   subject;           (16-bit version)
         PCRE_SPTR32   subject;           (32-bit version)
         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;
         const unsigned char *mark;       (8-bit version)
         const PCRE_UCHAR16  *mark;       (16-bit version)
         const PCRE_UCHAR32  *mark;       (32-bit version)

       The version field is an integer containing the version  number  of  the
       block  format. The initial version was 0; the current version is 2. 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 the  matching  function.  When  pcre_exec()  or
       pcre[16|32]_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 the DFA
       matching functions, this field is not useful.

       The subject and subject_length fields contain copies of the values that
       were passed to the matching function.

       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()  or  pcre[16|32]_exec() is used, the capture_top
       field contains one more than the number of the  highest  numbered  cap-
       tured  substring so far. If no substrings have been captured, the value
       of capture_top is one. This is always the case when the  DFA  functions
       are used, because they do 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 for the DFA matching functions.

       The  callout_data  field  contains a value that is passed to a matching
       function specifically so that it can be passed back in callouts. It  is
       passed  in  the callout_data field of a pcre_extra or pcre[16|32]_extra
       data structure. If no such data was passed, the value  of  callout_data
       in  a  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  callout
       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  callout
       structure. It contains the length of the next item to be matched in the
       pattern string. When the callout immediately  precedes  an  alternation
       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.

       The  mark  field is present from version 2 of the callout structure. In
       callouts from pcre_exec() or pcre[16|32]_exec() it contains  a  pointer
       to  the  zero-terminated  name  of  the  most  recently passed (*MARK),
       (*PRUNE), or (*THEN) item in the match, or NULL if no such  items  have
       been  passed.  Instances  of  (*PRUNE) or (*THEN) without a name do not
       obliterate a previous (*MARK). In callouts from the DFA matching  func-
       tions this field always contains NULL.


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, the
       matching function 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: 24 June 2012
       Copyright (c) 1997-2012 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 Unicode support. Details of what it
       does have are given in the pcreunicode page.

       2. PCRE allows repeat quantifiers only on parenthesized assertions, 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 (in principle: PCRE optimizes
       this to run the assertion just once). Perl allows repeat quantifiers on
       other assertions such as \b, but these do not seem to have any use.

       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 when followed by a character name or Unicode value.  (\N  on
       its own, matching a non-newline character, is supported.) In fact these
       are implemented by Perl's general string-handling and are not  part  of
       its  pattern  matching engine. If any of these are encountered by PCRE,
       an error is generated by default. However, if the  PCRE_JAVASCRIPT_COM-
       PAT  option  is set, \U and \u are interpreted as JavaScript interprets
       them.

       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 as subroutines (whether or not recur-
       sively) are always treated as atomic  groups  in  PCRE.  This  is  like
       Python,  but  unlike Perl.  Captured values that are set outside a sub-
       routine call can be reference from inside in PCRE,  but  not  in  Perl.
       There is a discussion that explains these differences in more detail in
       the section on recursion differences from Perl in the pcrepattern page.

       10. If any of the backtracking control verbs are used in  an  assertion
       or  in  a  subpattern  that  is  called as a subroutine (whether or not
       recursively), their effect is confined to that subpattern; it does  not
       extend to the surrounding pattern. This is not always the case in Perl.
       In particular, if (*THEN) is present in a group that  is  called  as  a
       subroutine, its action is limited to that group, even if the group does
       not contain any | characters. There is one exception to this: the  name
       from  a *(MARK), (*PRUNE), or (*THEN) that is encountered in a success-
       ful positive assertion is passed back when a  match  succeeds  (compare
       capturing  parentheses  in  assertions). Note that such subpatterns are
       processed as anchored at the point where they are tested.

       11. 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".

       12.  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.

       13. Perl recognizes comments in some places that  PCRE  does  not,  for
       example,  between  the  ( and ? at the start of a subpattern. If the /x
       modifier is set, Perl allows white space between ( and ? but PCRE never
       does, even if the PCRE_EXTENDED option is set.

       14. 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. However, this
       does not apply to optimized data created by the just-in-time compiler.

       (k)    The    alternative    matching    functions    (pcre_dfa_exec(),
       pcre16_dfa_exec()  and pcre32_dfa_exec(),) match in a different way and
       are 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: 25 August 2012
       Copyright (c) 1997-2012 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 strings in the original
       library, an extra library that supports  16-bit  and  UTF-16  character
       strings,  and a third library that supports 32-bit and UTF-32 character
       strings. To use these features, PCRE must be built to include appropri-
       ate  support. When using UTF strings you must either call the compiling
       function with the PCRE_UTF8, PCRE_UTF16, or PCRE_UTF32 option,  or  the
       pattern must start with one of these special sequences:

         (*UTF8)
         (*UTF16)
         (*UTF32)
         (*UTF)

       (*UTF)  is  a  generic  sequence  that  can  be  used  with  any of the
       libraries.  Starting a pattern with such a sequence  is  equivalent  to
       setting  the  relevant option. This feature is not Perl-compatible. How
       setting a UTF mode affects pattern matching  is  mentioned  in  several
       places  below.  There  is also a summary of features in the pcreunicode
       page.

       Another special sequence that may appear at the start of a  pattern  or
       in combination with (*UTF8), (*UTF16), (*UTF32) or (*UTF) 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  one  its  main  matching  functions,  pcre_exec()
       (8-bit)  or  pcre[16|32]_exec() (16- or 32-bit), is used. PCRE also has
       alternative      matching      functions,      pcre_dfa_exec()      and
       pcre[16|32_dfa_exec(),  which match using a different algorithm that is
       not Perl-compatible. Some of  the  features  discussed  below  are  not
       available  when  DFA matching is used. The advantages and disadvantages
       of the alternative functions, and how they differ from the normal func-
       tions, are discussed in the pcrematching page.


EBCDIC CHARACTER CODES

       PCRE  can  be compiled to run in an environment that uses EBCDIC as its
       character code rather than ASCII or Unicode (typically a mainframe sys-
       tem).  In  the  sections below, character code values are ASCII or Uni-
       code; in an EBCDIC environment these characters may have different code
       values, and there are no code points greater than 255.


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 the compiling func-
       tion. 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 where the circumflex and dollar asser-
       tions are true. It also affects the interpretation of the dot metachar-
       acter when PCRE_DOTALL is not set, and the behaviour of \N. However, 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 "New-
       line  sequences"  below.  A change of \R setting can be combined 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 a UTF 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 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 a UTF 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, white space  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 white space 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. If \Q
       is not followed by \E later in the pattern, the literal  interpretation
       continues  to  the  end  of  the pattern (that is, \E is assumed at the
       end). If the isolated \Q is inside a character class,  this  causes  an
       error, because the character class is not terminated.

   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        form feed (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.. (non-JavaScript mode)
         \uhhhh    character with hex code hhhh (JavaScript mode only)

       The precise effect of \cx on ASCII characters 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 \cA to \cZ become hex 01 to hex 1A
       (A  is  41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes
       hex 7B (; is 3B). If the data item (byte or 16-bit value) following  \c
       has  a  value greater than 127, a compile-time error occurs. This locks
       out non-ASCII characters in all modes.

       The \c facility was designed for use with ASCII  characters,  but  with
       the  extension  to  Unicode it is even less useful than it once was. It
       is, however, recognized when PCRE is compiled  in  EBCDIC  mode,  where
       data  items  are always bytes. In this mode, all values are valid after
       \c. If the next character is a lower case letter, it  is  converted  to
       upper  case.  Then  the  0xc0  bits  of the byte are inverted. Thus \cA
       becomes hex 01, as in ASCII (A is C1), but because the  EBCDIC  letters
       are  disjoint,  \cZ becomes hex 29 (Z is E9), and other characters also
       generate different values.

       By default, after \x, from zero to  two  hexadecimal  digits  are  read
       (letters can be in upper or lower case). Any number of hexadecimal dig-
       its may appear between \x{ and }, but the character code is constrained
       as follows:

         8-bit non-UTF mode    less than 0x100
         8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
         16-bit non-UTF mode   less than 0x10000
         16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
         32-bit non-UTF mode   less than 0x80000000
         32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint

       Invalid  Unicode  codepoints  are  the  range 0xd800 to 0xdfff (the so-
       called "surrogate" codepoints), and 0xffef.

       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.

       If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation  of  \x
       is  as  just described only when it is followed by two hexadecimal dig-
       its.  Otherwise, it matches a  literal  "x"  character.  In  JavaScript
       mode, support for code points greater than 256 is provided by \u, which
       must be followed by four hexadecimal digits;  otherwise  it  matches  a
       literal  "u"  character.  Character codes specified by \u in JavaScript
       mode are constrained in the same was as those specified by \x  in  non-
       JavaScript mode.

       Characters whose value is less than 256 can be defined by either of the
       two syntaxes for \x (or by \u in JavaScript mode). There is no  differ-
       ence in the way they are handled. For example, \xdc is exactly the same
       as \x{dc} (or \u00dc in JavaScript mode).

       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. The
       value of the character is constrained in the  same  way  as  characters
       specified in hexadecimal.  For example:

         \040   is another way of writing an ASCII 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 value 255 (decimal)
         \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, \b is interpreted as the backspace character (hex 08).

       \N is not allowed in a character class. \B, \R, and \X are not  special
       inside  a  character  class.  Like other unrecognized escape sequences,
       they are treated as  the  literal  characters  "B",  "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.

   Unsupported escape sequences

       In Perl, the sequences \l, \L, \u, and \U are recognized by its  string
       handler  and  used  to  modify  the  case  of  following characters. By
       default, PCRE does not support these escape sequences. However, if  the
       PCRE_JAVASCRIPT_COMPAT  option  is set, \U matches a "U" character, and
       \u can be used to define a character by code point, as described in the
       previous section.

   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 white space character
         \H     any character that is not a horizontal white space character
         \s     any white space character
         \S     any character that is not a white space character
         \v     any vertical white space character
         \V     any character that is not a vertical white space 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.  Perl also uses \N to match characters by name; PCRE does not
       support this.

       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  a  UTF  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 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, whether or not PCRE_UCP is set. The horizontal space  char-
       acters are:

         U+0009     Horizontal tab (HT)
         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 (LF)
         U+000B     Vertical tab (VT)
         U+000C     Form feed (FF)
         U+000D     Carriage return (CR)
         U+0085     Next line (NEL)
         U+2028     Line separator
         U+2029     Paragraph separator

       In 8-bit, non-UTF-8 mode, only the characters with codepoints less than
       256 are relevant.

   Newline sequences

       Outside a character class, by default, the escape sequence  \R  matches
       any  Unicode newline sequence. In 8-bit 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 (form feed, U+000C), CR (car-
       riage return, U+000D), or NEL (next line,  U+0085).  The  two-character
       sequence is treated as a single unit that cannot be split.

       In  other modes, 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 the compiling func-
       tion, but they can themselves be  overridden  by  options  given  to  a
       matching  function.  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), (*UTF16), (*UTF32), (*UTF)
       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 in 8-bit non-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       a Unicode extended grapheme cluster

       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, Batak,  Bengali,  Bopomofo,
       Brahmi,  Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Chakma,
       Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic,  Deseret,
       Devanagari,   Egyptian_Hieroglyphs,   Ethiopic,  Georgian,  Glagolitic,
       Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew,  Hira-
       gana,   Imperial_Aramaic,  Inherited,  Inscriptional_Pahlavi,  Inscrip-
       tional_Parthian,  Javanese,  Kaithi,   Kannada,   Katakana,   Kayah_Li,
       Kharoshthi,  Khmer,  Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian,
       Lydian,    Malayalam,    Mandaic,    Meetei_Mayek,    Meroitic_Cursive,
       Meroitic_Hieroglyphs,   Miao,  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, Samari-
       tan, Saurashtra, Sharada, Shavian,  Sinhala,  Sora_Sompeng,  Sundanese,
       Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet,
       Takri, 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 Unicode strings and
       so cannot be tested by PCRE, unless  UTF  validity  checking  has  been
       turned    off    (see    the    discussion    of    PCRE_NO_UTF8_CHECK,
       PCRE_NO_UTF16_CHECK and PCRE_NO_UTF32_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.

       Matching characters by Unicode property is not fast, because  PCRE  has
       to  do  a  multistage table lookup in order to find a character's prop-
       erty. 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 or by starting  the  pattern  with
       (*UCP).

   Extended grapheme clusters

       The  \X  escape  matches  any number of Unicode characters that form an
       "extended grapheme cluster", and treats the sequence as an atomic group
       (see  below).   Up  to and including release 8.31, PCRE matched an ear-
       lier, simpler definition that was equivalent to

         (?>\PM\pM*)

       That is, it matched a character without the "mark"  property,  followed
       by  zero  or  more characters with the "mark" property. Characters with
       the "mark" property are typically non-spacing accents that  affect  the
       preceding character.

       This  simple definition was extended in Unicode to include more compli-
       cated kinds of composite character by giving each character a  grapheme
       breaking  property,  and  creating  rules  that use these properties to
       define the boundaries of extended grapheme  clusters.  In  releases  of
       PCRE later than 8.31, \X matches one of these clusters.

       \X  always  matches  at least one character. Then it decides whether to
       add additional characters according to the following rules for ending a
       cluster:

       1. End at the end of the subject string.

       2.  Do not end between CR and LF; otherwise end after any control char-
       acter.

       3. Do not break Hangul (a Korean  script)  syllable  sequences.  Hangul
       characters  are of five types: L, V, T, LV, and LVT. An L character may
       be followed by an L, V, LV, or LVT character; an LV or V character  may
       be followed by a V or T character; an LVT or T character may be follwed
       only by a T character.

       4. Do not end before extending characters or spacing marks.  Characters
       with  the  "mark"  property  always have the "extend" grapheme breaking
       property.

       5. Do not end after prepend characters.

       6. Otherwise, end the cluster.

   PCRE's additional properties

       As well as the standard Unicode properties described above,  PCRE  sup-
       ports  four  more  that  make it possible to convert traditional escape
       sequences such as \w and \s and POSIX character classes to use  Unicode
       properties.  PCRE  uses  these non-standard, non-Perl properties inter-
       nally 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,
       form feed, 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  a
       UTF  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

       The circumflex and dollar  metacharacters  are  zero-width  assertions.
       That  is,  they test for a particular condition being true without con-
       suming any characters from the subject string.

       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.)

       The  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). Note, however,
       that it does not actually match the newline. 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.  Dol-
       lar 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.

       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. Perl also uses
       \N to match characters by name; PCRE does not support this.


MATCHING A SINGLE DATA UNIT

       Outside a character class, the escape sequence \C matches any one  data
       unit,  whether or not a UTF mode is set. In the 8-bit library, one data
       unit is one byte; in the 16-bit library it is a  16-bit  unit;  in  the
       32-bit  library  it  is  a 32-bit unit. Unlike a dot, \C always matches
       line-ending characters. The feature is provided in  Perl  in  order  to
       match individual bytes in UTF-8 mode, but it is unclear how it can use-
       fully be used. Because \C breaks up  characters  into  individual  data
       units,  matching  one unit with \C in a UTF mode means that the rest of
       the string may start with a malformed UTF character. This has undefined
       results, because PCRE assumes that it is dealing with valid UTF strings
       (and by default it checks this at the start of  processing  unless  the
       PCRE_NO_UTF8_CHECK,  PCRE_NO_UTF16_CHECK  or PCRE_NO_UTF32_CHECK option
       is used).

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

       In general, the \C escape sequence is best avoided. However, one way of
       using  it that avoids the problem of malformed UTF characters is to use
       a lookahead to check the length of the next character, as in this  pat-
       tern,  which  could be used with a UTF-8 string (ignore white space and
       line breaks):

         (?| (?=[\x00-\x7f])(\C) |
             (?=[\x80-\x{7ff}])(\C)(\C) |
             (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
             (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

       A group that starts with (?| resets the capturing  parentheses  numbers
       in  each  alternative  (see  "Duplicate Subpattern Numbers" below). The
       assertions at the start of each branch check the next  UTF-8  character
       for  values  whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
       character's individual bytes are then captured by the appropriate  num-
       ber of groups.


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 a UTF
       mode, the character may be more than one  data  unit  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 (UTF-16, UTF-32) mode, characters with values greater than 255
       (0xffff) can be included in a class as a literal string of data  units,
       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 a UTF 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 a UTF mode for characters 128 and above, you must
       ensure that PCRE is compiled with Unicode property support as  well  as
       with UTF 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].  Ranges  can include any characters that are valid for the
       current mode.

       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  a  non-UTF  mode,  if
       character  tables  for  a French locale are in use, [\xc8-\xcb] matches
       accented E characters in both cases. In UTF modes,  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 modes, 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 modes, 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  compiling  or matching 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), (*UTF16),(*UTF32), and
       (*UCP) leading sequences that can be used to set UTF and Unicode  prop-
       erty  modes;  they are equivalent to setting the PCRE_UTF8, PCRE_UTF16,
       PCRE_UTF32 and the PCRE_UCP options, respectively. The (*UTF)  sequence
       is a generic version that can be used with any of the libraries.


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 the matching function. (This applies  only  to  the
       traditional  matching functions; the DFA matching functions do not sup-
       port capturing.)

       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 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
         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 (including assertions)
         a subroutine call to a subpattern (recursive or otherwise)

       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 modes, quantifiers apply to characters rather than to individual
       data units. Thus, for example, \x{100}{2} matches two characters,  each
       of which is represented by a two-byte sequence in a UTF-8 string. Simi-
       larly, \X{3} matches three Unicode extended grapheme clusters, each  of
       which  may  be  several  data  units 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 are some cases 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.

       Another case where implicit anchoring is not applied is when the  lead-
       ing  .* is inside an atomic group. Once again, a match at the start may
       fail where a later one succeeds. Consider this pattern:

         (?>.*?a)b

       It matches "ab" in the subject "aab". The use of the backtracking  con-
       trol verbs (*PRUNE) and (*SKIP) also disable this optimization.

       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