/[pcre]/code/trunk/doc/pcre.txt
ViewVC logotype

Diff of /code/trunk/doc/pcre.txt

Parent Directory Parent Directory | Revision Log Revision Log | View Patch Patch

revision 53 by nigel, Sat Feb 24 21:39:42 2007 UTC revision 83 by nigel, Sat Feb 24 21:41:06 2007 UTC
# Line 1  Line 1 
1    -----------------------------------------------------------------------------
2    This file contains a concatenation of the PCRE man pages, converted to plain
3    text format for ease of searching with a text editor, or for use on systems
4    that do not have a man page processor. The small individual files that give
5    synopses of each function in the library have not been included. There are
6    separate text files for the pcregrep and pcretest commands.
7    -----------------------------------------------------------------------------
8    
9    
10    PCRE(3)                                                                PCRE(3)
11    
12    
13  NAME  NAME
14       pcre - Perl-compatible regular expressions.         PCRE - Perl-compatible regular expressions
15    
16    
17    INTRODUCTION
18    
19  SYNOPSIS         The  PCRE  library is a set of functions that implement regular expres-
20       #include <pcre.h>         sion pattern matching using the same syntax and semantics as Perl, with
21           just  a  few  differences.  The current implementation of PCRE (release
22           6.x) corresponds approximately with Perl  5.8,  including  support  for
23           UTF-8 encoded strings and Unicode general category properties. However,
24           this support has to be explicitly enabled; it is not the default.
25    
26           In addition to the Perl-compatible matching function,  PCRE  also  con-
27           tains  an  alternative matching function that matches the same compiled
28           patterns in a different way. In certain circumstances, the  alternative
29           function  has  some  advantages.  For  a discussion of the two matching
30           algorithms, see the pcrematching page.
31    
32           PCRE is written in C and released as a C library. A  number  of  people
33           have  written  wrappers and interfaces of various kinds. In particular,
34           Google Inc.  have provided a comprehensive C++  wrapper.  This  is  now
35           included as part of the PCRE distribution. The pcrecpp page has details
36           of this interface. Other people's contributions can  be  found  in  the
37           Contrib directory at the primary FTP site, which is:
38    
39           ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
40    
41           Details  of  exactly which Perl regular expression features are and are
42           not supported by PCRE are given in separate documents. See the pcrepat-
43           tern and pcrecompat pages.
44    
45           Some  features  of  PCRE can be included, excluded, or changed when the
46           library is built. The pcre_config() function makes it  possible  for  a
47           client  to  discover  which  features are available. The features them-
48           selves are described in the pcrebuild page. Documentation about  build-
49           ing  PCRE for various operating systems can be found in the README file
50           in the source distribution.
51    
52           The library contains a number of undocumented  internal  functions  and
53           data  tables  that  are  used by more than one of the exported external
54           functions, but which are not intended  for  use  by  external  callers.
55           Their  names  all begin with "_pcre_", which hopefully will not provoke
56           any name clashes. In some environments, it is possible to control which
57           external  symbols  are  exported when a shared library is built, and in
58           these cases the undocumented symbols are not exported.
59    
60    
61    USER DOCUMENTATION
62    
63           The user documentation for PCRE comprises a number  of  different  sec-
64           tions.  In the "man" format, each of these is a separate "man page". In
65           the HTML format, each is a separate page, linked from the  index  page.
66           In  the  plain text format, all the sections are concatenated, for ease
67           of searching. The sections are as follows:
68    
69             pcre              this document
70             pcreapi           details of PCRE's native C API
71             pcrebuild         options for building PCRE
72             pcrecallout       details of the callout feature
73             pcrecompat        discussion of Perl compatibility
74             pcrecpp           details of the C++ wrapper
75             pcregrep          description of the pcregrep command
76             pcrematching      discussion of the two matching algorithms
77             pcrepartial       details of the partial matching facility
78             pcrepattern       syntax and semantics of supported
79                                 regular expressions
80             pcreperform       discussion of performance issues
81             pcreposix         the POSIX-compatible C API
82             pcreprecompile    details of saving and re-using precompiled patterns
83             pcresample        discussion of the sample program
84             pcretest          description of the pcretest testing command
85    
86       pcre *pcre_compile(const char *pattern, int options,         In  addition,  in the "man" and HTML formats, there is a short page for
87            const char **errptr, int *erroffset,         each C library function, listing its arguments and results.
           const unsigned char *tableptr);  
88    
      pcre_extra *pcre_study(const pcre *code, int options,  
           const char **errptr);  
89    
90       int pcre_exec(const pcre *code, const pcre_extra *extra,  LIMITATIONS
           const char *subject, int length, int startoffset,  
           int options, int *ovector, int ovecsize);  
91    
92       int pcre_copy_substring(const char *subject, int *ovector,         There are some size limitations in PCRE but it is hoped that they  will
93            int stringcount, int stringnumber, char *buffer,         never in practice be relevant.
           int buffersize);  
94    
95       int pcre_get_substring(const char *subject, int *ovector,         The  maximum  length of a compiled pattern is 65539 (sic) bytes if PCRE
96            int stringcount, int stringnumber,         is compiled with the default internal linkage size of 2. If you want to
97            const char **stringptr);         process  regular  expressions  that are truly enormous, you can compile
98           PCRE with an internal linkage size of 3 or 4 (see the  README  file  in
99           the  source  distribution and the pcrebuild documentation for details).
100           In these cases the limit is substantially larger.  However,  the  speed
101           of execution will be slower.
102    
103           All values in repeating quantifiers must be less than 65536.  The maxi-
104           mum number of capturing subpatterns is 65535.
105    
106           There is no limit to the number of non-capturing subpatterns,  but  the
107           maximum  depth  of  nesting  of  all kinds of parenthesized subpattern,
108           including capturing subpatterns, assertions, and other types of subpat-
109           tern, is 200.
110    
111           The  maximum  length of a subject string is the largest positive number
112           that an integer variable can hold. However, when using the  traditional
113           matching function, PCRE uses recursion to handle subpatterns and indef-
114           inite repetition.  This means that the available stack space may  limit
115           the size of a subject string that can be processed by certain patterns.
116    
117    
118    UTF-8 AND UNICODE PROPERTY SUPPORT
119    
120           From release 3.3, PCRE has  had  some  support  for  character  strings
121           encoded  in the UTF-8 format. For release 4.0 this was greatly extended
122           to cover most common requirements, and in release 5.0  additional  sup-
123           port for Unicode general category properties was added.
124    
125           In  order  process  UTF-8 strings, you must build PCRE to include UTF-8
126           support in the code, and, in addition,  you  must  call  pcre_compile()
127           with  the PCRE_UTF8 option flag. When you do this, both the pattern and
128           any subject strings that are matched against it are  treated  as  UTF-8
129           strings instead of just strings of bytes.
130    
131           If  you compile PCRE with UTF-8 support, but do not use it at run time,
132           the library will be a bit bigger, but the additional run time  overhead
133           is  limited  to testing the PCRE_UTF8 flag in several places, so should
134           not be very large.
135    
136           If PCRE is built with Unicode character property support (which implies
137           UTF-8  support),  the  escape sequences \p{..}, \P{..}, and \X are sup-
138           ported.  The available properties that can be tested are limited to the
139           general  category  properties such as Lu for an upper case letter or Nd
140           for a decimal number. A full list is given in the pcrepattern  documen-
141           tation. The PCRE library is increased in size by about 90K when Unicode
142           property support is included.
143    
144           The following comments apply when PCRE is running in UTF-8 mode:
145    
146           1. When you set the PCRE_UTF8 flag, the strings passed as patterns  and
147           subjects  are  checked for validity on entry to the relevant functions.
148           If an invalid UTF-8 string is passed, an error return is given. In some
149           situations,  you  may  already  know  that  your strings are valid, and
150           therefore want to skip these checks in order to improve performance. If
151           you  set  the  PCRE_NO_UTF8_CHECK  flag at compile time or at run time,
152           PCRE assumes that the pattern or subject  it  is  given  (respectively)
153           contains  only valid UTF-8 codes. In this case, it does not diagnose an
154           invalid UTF-8 string. If you pass an invalid UTF-8 string to PCRE  when
155           PCRE_NO_UTF8_CHECK  is set, the results are undefined. Your program may
156           crash.
157    
158           2. In a pattern, the escape sequence \x{...}, where the contents of the
159           braces  is  a  string  of hexadecimal digits, is interpreted as a UTF-8
160           character whose code number is the given hexadecimal number, for  exam-
161           ple:  \x{1234}.  If a non-hexadecimal digit appears between the braces,
162           the item is not recognized.  This escape sequence can be used either as
163           a literal, or within a character class.
164    
165           3.  The  original hexadecimal escape sequence, \xhh, matches a two-byte
166           UTF-8 character if the value is greater than 127.
167    
168           4. Repeat quantifiers apply to complete UTF-8 characters, not to  indi-
169           vidual bytes, for example: \x{100}{3}.
170    
171           5.  The dot metacharacter matches one UTF-8 character instead of a sin-
172           gle byte.
173    
174           6. The escape sequence \C can be used to match a single byte  in  UTF-8
175           mode,  but  its  use can lead to some strange effects. This facility is
176           not available in the alternative matching function, pcre_dfa_exec().
177    
178           7. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly
179           test  characters of any code value, but the characters that PCRE recog-
180           nizes as digits, spaces, or word characters  remain  the  same  set  as
181           before, all with values less than 256. This remains true even when PCRE
182           includes Unicode property support, because to do otherwise  would  slow
183           down  PCRE in many common cases. If you really want to test for a wider
184           sense of, say, "digit", you must use Unicode  property  tests  such  as
185           \p{Nd}.
186    
187           8.  Similarly,  characters that match the POSIX named character classes
188           are all low-valued characters.
189    
190           9. Case-insensitive matching applies only to  characters  whose  values
191           are  less than 128, unless PCRE is built with Unicode property support.
192           Even when Unicode property support is available, PCRE  still  uses  its
193           own  character  tables when checking the case of low-valued characters,
194           so as not to degrade performance.  The Unicode property information  is
195           used only for characters with higher values.
196    
      int pcre_get_substring_list(const char *subject,  
           int *ovector, int stringcount, const char ***listptr);  
197    
198       void pcre_free_substring(const char *stringptr);  AUTHOR
199    
200       void pcre_free_substring_list(const char **stringptr);         Philip Hazel
201           University Computing Service,
202           Cambridge CB2 3QG, England.
203    
204       const unsigned char *pcre_maketables(void);         Putting  an actual email address here seems to have been a spam magnet,
205           so I've taken it away. If you want to email me, use my initial and sur-
206           name, separated by a dot, at the domain ucs.cam.ac.uk.
207    
208       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,  Last updated: 07 March 2005
209            int what, void *where);  Copyright (c) 1997-2005 University of Cambridge.
210    ------------------------------------------------------------------------------
211    
      int pcre_info(const pcre *code, int *optptr, *firstcharptr);  
212    
213       char *pcre_version(void);  PCREBUILD(3)                                                      PCREBUILD(3)
214    
      void *(*pcre_malloc)(size_t);  
215    
216       void (*pcre_free)(void *);  NAME
217           PCRE - Perl-compatible regular expressions
218    
219    
220    PCRE BUILD-TIME OPTIONS
221    
222           This  document  describes  the  optional  features  of PCRE that can be
223           selected when the library is compiled. They are all selected, or  dese-
224           lected, by providing options to the configure script that is run before
225           the make command. The complete list of  options  for  configure  (which
226           includes  the  standard  ones such as the selection of the installation
227           directory) can be obtained by running
228    
229  DESCRIPTION           ./configure --help
230       The PCRE library is a set of functions that implement  regu-  
231       lar  expression  pattern  matching using the same syntax and         The following sections describe certain options whose names begin  with
232       semantics as Perl  5,  with  just  a  few  differences  (see         --enable  or  --disable. These settings specify changes to the defaults
233           for the configure command. Because of the  way  that  configure  works,
234       below).  The  current  implementation  corresponds  to  Perl         --enable  and  --disable  always  come  in  pairs, so the complementary
235       5.005, with some additional features  from  later  versions.         option always exists as well, but as it specifies the  default,  it  is
236       This  includes  some  experimental,  incomplete  support for         not described.
237       UTF-8 encoded strings. Details of exactly what is  and  what  
238       is not supported are given below.  
239    C++ SUPPORT
240       PCRE has its own native API,  which  is  described  in  this  
241       document.  There  is  also  a  set of wrapper functions that         By default, the configure script will search for a C++ compiler and C++
242       correspond to the POSIX regular expression API.   These  are         header files. If it finds them, it automatically builds the C++ wrapper
243       described in the pcreposix documentation.         library for PCRE. You can disable this by adding
244    
245       The native API function prototypes are defined in the header           --disable-cpp
246       file  pcre.h,  and  on  Unix  systems  the library itself is  
247       called libpcre.a, so can be accessed by adding -lpcre to the         to the configure command.
248       command  for  linking  an  application  which  calls it. The  
249       header file defines the macros PCRE_MAJOR and PCRE_MINOR  to  
250       contain the major and minor release numbers for the library.  UTF-8 SUPPORT
251       Applications can use these to include support for  different  
252       releases.         To build PCRE with support for UTF-8 character strings, add
253    
254       The functions pcre_compile(), pcre_study(), and  pcre_exec()           --enable-utf8
255       are  used  for compiling and matching regular expressions. A  
256       sample program that demonstrates the simplest way  of  using         to  the  configure  command.  Of  itself, this does not make PCRE treat
257       them  is  given  in the file pcredemo.c. The last section of         strings as UTF-8. As well as compiling PCRE with this option, you  also
258       this man page describes how to run it.         have  have to set the PCRE_UTF8 option when you call the pcre_compile()
259           function.
260       The functions  pcre_copy_substring(),  pcre_get_substring(),  
261       and  pcre_get_substring_list() are convenience functions for  
262       extracting  captured  substrings  from  a  matched   subject  UNICODE CHARACTER PROPERTY SUPPORT
263       string; pcre_free_substring() and pcre_free_substring_list()  
264       are also provided, to free the  memory  used  for  extracted         UTF-8 support allows PCRE to process character values greater than  255
265       strings.         in  the  strings that it handles. On its own, however, it does not pro-
266           vide any facilities for accessing the properties of such characters. If
267       The function pcre_maketables() is used (optionally) to build         you  want  to  be able to use the pattern escapes \P, \p, and \X, which
268       a  set of character tables in the current locale for passing         refer to Unicode character properties, you must add
269       to pcre_compile().  
270             --enable-unicode-properties
271       The function pcre_fullinfo() is used to find out information  
272       about a compiled pattern; pcre_info() is an obsolete version         to the configure command. This implies UTF-8 support, even if you  have
273       which returns only some of the available information, but is         not explicitly requested it.
274       retained   for   backwards   compatibility.    The  function  
275       pcre_version() returns a pointer to a string containing  the         Including  Unicode  property  support  adds around 90K of tables to the
276       version of PCRE and its date of release.         PCRE library, approximately doubling its size. Only the  general  cate-
277           gory  properties  such as Lu and Nd are supported. Details are given in
278       The global variables  pcre_malloc  and  pcre_free  initially         the pcrepattern documentation.
279       contain the entry points of the standard malloc() and free()  
280       functions respectively. PCRE  calls  the  memory  management  
281       functions  via  these  variables,  so  a calling program can  CODE VALUE OF NEWLINE
282       replace them if it  wishes  to  intercept  the  calls.  This  
283       should be done before calling any PCRE functions.         By default, PCRE treats character 10 (linefeed) as the newline  charac-
284           ter. This is the normal newline character on Unix-like systems. You can
285           compile PCRE to use character 13 (carriage return) instead by adding
286    
287  MULTI-THREADING           --enable-newline-is-cr
288       The PCRE functions can be used in  multi-threading  applica-  
289       tions, with the proviso that the memory management functions         to the configure command. For completeness there is  also  a  --enable-
290       pointed to by pcre_malloc and pcre_free are  shared  by  all         newline-is-lf  option,  which explicitly specifies linefeed as the new-
291       threads.         line character.
292    
293       The compiled form of a regular  expression  is  not  altered  
294       during  matching, so the same compiled pattern can safely be  BUILDING SHARED AND STATIC LIBRARIES
295       used by several threads at once.  
296           The PCRE building process uses libtool to build both shared and  static
297           Unix  libraries by default. You can suppress one of these by adding one
298           of
299    
300             --disable-shared
301             --disable-static
302    
303           to the configure command, as required.
304    
305    
306    POSIX MALLOC USAGE
307    
308           When PCRE is called through the POSIX interface (see the pcreposix doc-
309           umentation),  additional  working  storage  is required for holding the
310           pointers to capturing substrings, because PCRE requires three  integers
311           per  substring,  whereas  the POSIX interface provides only two. If the
312           number of expected substrings is small, the wrapper function uses space
313           on the stack, because this is faster than using malloc() for each call.
314           The default threshold above which the stack is no longer used is 10; it
315           can be changed by adding a setting such as
316    
317             --with-posix-malloc-threshold=20
318    
319           to the configure command.
320    
321    
322    LIMITING PCRE RESOURCE USAGE
323    
324           Internally,  PCRE has a function called match(), which it calls repeat-
325           edly  (possibly  recursively)  when  matching  a   pattern   with   the
326           pcre_exec()  function.  By controlling the maximum number of times this
327           function may be called during a single matching operation, a limit  can
328           be  placed  on  the resources used by a single call to pcre_exec(). The
329           limit can be changed at run time, as described in the pcreapi  documen-
330           tation.  The default is 10 million, but this can be changed by adding a
331           setting such as
332    
333             --with-match-limit=500000
334    
335           to  the  configure  command.  This  setting  has  no  effect   on   the
336           pcre_dfa_exec() matching function.
337    
338    
339    HANDLING VERY LARGE PATTERNS
340    
341           Within  a  compiled  pattern,  offset values are used to point from one
342           part to another (for example, from an opening parenthesis to an  alter-
343           nation  metacharacter).  By default, two-byte values are used for these
344           offsets, leading to a maximum size for a  compiled  pattern  of  around
345           64K.  This  is sufficient to handle all but the most gigantic patterns.
346           Nevertheless, some people do want to process enormous patterns,  so  it
347           is  possible  to compile PCRE to use three-byte or four-byte offsets by
348           adding a setting such as
349    
350             --with-link-size=3
351    
352           to the configure command. The value given must be 2,  3,  or  4.  Using
353           longer  offsets slows down the operation of PCRE because it has to load
354           additional bytes when handling them.
355    
356           If you build PCRE with an increased link size, test 2 (and  test  5  if
357           you  are using UTF-8) will fail. Part of the output of these tests is a
358           representation of the compiled pattern, and this changes with the  link
359           size.
360    
361    
362    AVOIDING EXCESSIVE STACK USAGE
363    
364           When matching with the pcre_exec() function, PCRE implements backtrack-
365           ing by making recursive calls to an internal function  called  match().
366           In  environments  where  the size of the stack is limited, this can se-
367           verely limit PCRE's operation. (The Unix environment does  not  usually
368           suffer  from  this  problem.)  An alternative approach that uses memory
369           from the heap to remember data, instead  of  using  recursive  function
370           calls,  has been implemented to work round this problem. If you want to
371           build a version of PCRE that works this way, add
372    
373             --disable-stack-for-recursion
374    
375           to the configure command. With this configuration, PCRE  will  use  the
376           pcre_stack_malloc  and pcre_stack_free variables to call memory manage-
377           ment functions. Separate functions are provided because  the  usage  is
378           very  predictable:  the  block sizes requested are always the same, and
379           the blocks are always freed in reverse order. A calling  program  might
380           be  able  to implement optimized functions that perform better than the
381           standard malloc() and  free()  functions.  PCRE  runs  noticeably  more
382           slowly when built in this way. This option affects only the pcre_exec()
383           function; it is not relevant for the the pcre_dfa_exec() function.
384    
385    
386    USING EBCDIC CODE
387    
388           PCRE assumes by default that it will run in an  environment  where  the
389           character  code  is  ASCII  (or Unicode, which is a superset of ASCII).
390           PCRE can, however, be compiled to  run  in  an  EBCDIC  environment  by
391           adding
392    
393             --enable-ebcdic
394    
395           to the configure command.
396    
397    Last updated: 15 August 2005
398    Copyright (c) 1997-2005 University of Cambridge.
399    ------------------------------------------------------------------------------
400    
401    
402    PCREMATCHING(3)                                                PCREMATCHING(3)
403    
404    
405    NAME
406           PCRE - Perl-compatible regular expressions
407    
408    
409    PCRE MATCHING ALGORITHMS
410    
411           This document describes the two different algorithms that are available
412           in PCRE for matching a compiled regular expression against a given sub-
413           ject  string.  The  "standard"  algorithm  is  the  one provided by the
414           pcre_exec() function.  This works in the same was  as  Perl's  matching
415           function, and provides a Perl-compatible matching operation.
416    
417           An  alternative  algorithm is provided by the pcre_dfa_exec() function;
418           this operates in a different way, and is not  Perl-compatible.  It  has
419           advantages  and disadvantages compared with the standard algorithm, and
420           these are described below.
421    
422           When there is only one possible way in which a given subject string can
423           match  a pattern, the two algorithms give the same answer. A difference
424           arises, however, when there are multiple possibilities. For example, if
425           the pattern
426    
427             ^<.*>
428    
429           is matched against the string
430    
431             <something> <something else> <something further>
432    
433           there are three possible answers. The standard algorithm finds only one
434           of them, whereas the DFA algorithm finds all three.
435    
436    
437    REGULAR EXPRESSIONS AS TREES
438    
439           The set of strings that are matched by a regular expression can be rep-
440           resented  as  a  tree structure. An unlimited repetition in the pattern
441           makes the tree of infinite size, but it is still a tree.  Matching  the
442           pattern  to a given subject string (from a given starting point) can be
443           thought of as a search of the tree.  There are  two  standard  ways  to
444           search  a  tree: depth-first and breadth-first, and these correspond to
445           the two matching algorithms provided by PCRE.
446    
447    
448    THE STANDARD MATCHING ALGORITHM
449    
450           In the terminology of Jeffrey Friedl's book Mastering  Regular  Expres-
451           sions,  the  standard  algorithm  is  an "NFA algorithm". It conducts a
452           depth-first search of the pattern tree. That is, it  proceeds  along  a
453           single path through the tree, checking that the subject matches what is
454           required. When there is a mismatch, the algorithm  tries  any  alterna-
455           tives  at  the  current point, and if they all fail, it backs up to the
456           previous branch point in the  tree,  and  tries  the  next  alternative
457           branch  at  that  level.  This often involves backing up (moving to the
458           left) in the subject string as well.  The  order  in  which  repetition
459           branches  are  tried  is controlled by the greedy or ungreedy nature of
460           the quantifier.
461    
462           If a leaf node is reached, a matching string has  been  found,  and  at
463           that  point the algorithm stops. Thus, if there is more than one possi-
464           ble match, this algorithm returns the first one that it finds.  Whether
465           this  is the shortest, the longest, or some intermediate length depends
466           on the way the greedy and ungreedy repetition quantifiers are specified
467           in the pattern.
468    
469           Because  it  ends  up  with a single path through the tree, it is rela-
470           tively straightforward for this algorithm to keep  track  of  the  sub-
471           strings  that  are  matched  by portions of the pattern in parentheses.
472           This provides support for capturing parentheses and back references.
473    
474    
475    THE DFA MATCHING ALGORITHM
476    
477           DFA stands for "deterministic finite automaton", but you do not need to
478           understand the origins of that name. This algorithm conducts a breadth-
479           first search of the tree. Starting from the first matching point in the
480           subject,  it scans the subject string from left to right, once, charac-
481           ter by character, and as it does  this,  it  remembers  all  the  paths
482           through the tree that represent valid matches.
483    
484           The  scan  continues until either the end of the subject is reached, or
485           there are no more unterminated paths. At this point,  terminated  paths
486           represent  the different matching possibilities (if there are none, the
487           match has failed).  Thus, if there is more  than  one  possible  match,
488           this algorithm finds all of them, and in particular, it finds the long-
489           est. In PCRE, there is an option to stop the algorithm after the  first
490           match (which is necessarily the shortest) has been found.
491    
492           Note that all the matches that are found start at the same point in the
493           subject. If the pattern
494    
495             cat(er(pillar)?)
496    
497           is matched against the string "the caterpillar catchment",  the  result
498           will  be the three strings "cat", "cater", and "caterpillar" that start
499           at the fourth character of the subject. The algorithm does not automat-
500           ically move on to find matches that start at later positions.
501    
502           There are a number of features of PCRE regular expressions that are not
503           supported by the DFA matching algorithm. They are as follows:
504    
505           1. Because the algorithm finds all  possible  matches,  the  greedy  or
506           ungreedy  nature  of repetition quantifiers is not relevant. Greedy and
507           ungreedy quantifiers are treated in exactly the same way.
508    
509           2. When dealing with multiple paths through the tree simultaneously, it
510           is  not  straightforward  to  keep track of captured substrings for the
511           different matching possibilities, and  PCRE's  implementation  of  this
512           algorithm does not attempt to do this. This means that no captured sub-
513           strings are available.
514    
515           3. Because no substrings are captured, back references within the  pat-
516           tern are not supported, and cause errors if encountered.
517    
518           4.  For  the same reason, conditional expressions that use a backrefer-
519           ence as the condition are not supported.
520    
521           5. Callouts are supported, but the value of the  capture_top  field  is
522           always 1, and the value of the capture_last field is always -1.
523    
524           6.  The \C escape sequence, which (in the standard algorithm) matches a
525           single byte, even in UTF-8 mode, is not supported because the DFA algo-
526           rithm moves through the subject string one character at a time, for all
527           active paths through the tree.
528    
529    
530    ADVANTAGES OF THE DFA ALGORITHM
531    
532           Using the DFA matching algorithm provides the following advantages:
533    
534           1. All possible matches (at a single point in the subject) are automat-
535           ically  found,  and  in particular, the longest match is found. To find
536           more than one match using the standard algorithm, you have to do kludgy
537           things with callouts.
538    
539           2.  There is much better support for partial matching. The restrictions
540           on the content of the pattern that apply when using the standard  algo-
541           rithm  for partial matching do not apply to the DFA algorithm. For non-
542           anchored patterns, the starting position of a partial match  is  avail-
543           able.
544    
545           3.  Because  the  DFA algorithm scans the subject string just once, and
546           never needs to backtrack, it is possible  to  pass  very  long  subject
547           strings  to  the matching function in several pieces, checking for par-
548           tial matching each time.
549    
550    
551    DISADVANTAGES OF THE DFA ALGORITHM
552    
553           The DFA algorithm suffers from a number of disadvantages:
554    
555           1. It is substantially slower than  the  standard  algorithm.  This  is
556           partly  because  it has to search for all possible matches, but is also
557           because it is less susceptible to optimization.
558    
559           2. Capturing parentheses and back references are not supported.
560    
561           3. The "atomic group" feature of PCRE regular expressions is supported,
562           but  does not provide the advantage that it does for the standard algo-
563           rithm.
564    
565    Last updated: 28 February 2005
566    Copyright (c) 1997-2005 University of Cambridge.
567    ------------------------------------------------------------------------------
568    
569    
570    PCREAPI(3)                                                          PCREAPI(3)
571    
572    
573    NAME
574           PCRE - Perl-compatible regular expressions
575    
576    
577    PCRE NATIVE API
578    
579           #include <pcre.h>
580    
581           pcre *pcre_compile(const char *pattern, int options,
582                const char **errptr, int *erroffset,
583                const unsigned char *tableptr);
584    
585           pcre *pcre_compile2(const char *pattern, int options,
586                int *errorcodeptr,
587                const char **errptr, int *erroffset,
588                const unsigned char *tableptr);
589    
590           pcre_extra *pcre_study(const pcre *code, int options,
591                const char **errptr);
592    
593           int pcre_exec(const pcre *code, const pcre_extra *extra,
594                const char *subject, int length, int startoffset,
595                int options, int *ovector, int ovecsize);
596    
597           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
598                const char *subject, int length, int startoffset,
599                int options, int *ovector, int ovecsize,
600                int *workspace, int wscount);
601    
602           int pcre_copy_named_substring(const pcre *code,
603                const char *subject, int *ovector,
604                int stringcount, const char *stringname,
605                char *buffer, int buffersize);
606    
607           int pcre_copy_substring(const char *subject, int *ovector,
608                int stringcount, int stringnumber, char *buffer,
609                int buffersize);
610    
611           int pcre_get_named_substring(const pcre *code,
612                const char *subject, int *ovector,
613                int stringcount, const char *stringname,
614                const char **stringptr);
615    
616           int pcre_get_stringnumber(const pcre *code,
617                const char *name);
618    
619           int pcre_get_substring(const char *subject, int *ovector,
620                int stringcount, int stringnumber,
621                const char **stringptr);
622    
623           int pcre_get_substring_list(const char *subject,
624                int *ovector, int stringcount, const char ***listptr);
625    
626           void pcre_free_substring(const char *stringptr);
627    
628           void pcre_free_substring_list(const char **stringptr);
629    
630           const unsigned char *pcre_maketables(void);
631    
632           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
633                int what, void *where);
634    
635           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
636    
637           int pcre_refcount(pcre *code, int adjust);
638    
639           int pcre_config(int what, void *where);
640    
641           char *pcre_version(void);
642    
643           void *(*pcre_malloc)(size_t);
644    
645           void (*pcre_free)(void *);
646    
647           void *(*pcre_stack_malloc)(size_t);
648    
649           void (*pcre_stack_free)(void *);
650    
651           int (*pcre_callout)(pcre_callout_block *);
652    
653    
654    PCRE API OVERVIEW
655    
656           PCRE has its own native API, which is described in this document. There
657           is also a set of wrapper functions that correspond to the POSIX regular
658           expression  API.  These  are  described in the pcreposix documentation.
659           Both of these APIs define a set of C function calls. A C++  wrapper  is
660           distributed with PCRE. It is documented in the pcrecpp page.
661    
662           The  native  API  C  function prototypes are defined in the header file
663           pcre.h, and on Unix systems the library itself is called  libpcre.   It
664           can normally be accessed by adding -lpcre to the command for linking an
665           application  that  uses  PCRE.  The  header  file  defines  the  macros
666           PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
667           bers for the library.  Applications can use these  to  include  support
668           for different releases of PCRE.
669    
670           The   functions   pcre_compile(),  pcre_compile2(),  pcre_study(),  and
671           pcre_exec() are used for compiling and matching regular expressions  in
672           a  Perl-compatible  manner. A sample program that demonstrates the sim-
673           plest way of using them is provided in the file  called  pcredemo.c  in
674           the  source distribution. The pcresample documentation describes how to
675           run it.
676    
677           A second matching function, pcre_dfa_exec(), which is not Perl-compati-
678           ble,  is  also provided. This uses a different algorithm for the match-
679           ing. This allows it to find all possible matches (at a given  point  in
680           the  subject),  not  just  one. However, this algorithm does not return
681           captured substrings. A description of the two matching  algorithms  and
682           their  advantages  and disadvantages is given in the pcrematching docu-
683           mentation.
684    
685           In addition to the main compiling and  matching  functions,  there  are
686           convenience functions for extracting captured substrings from a subject
687           string that is matched by pcre_exec(). They are:
688    
689             pcre_copy_substring()
690             pcre_copy_named_substring()
691             pcre_get_substring()
692             pcre_get_named_substring()
693             pcre_get_substring_list()
694             pcre_get_stringnumber()
695    
696           pcre_free_substring() and pcre_free_substring_list() are also provided,
697           to free the memory used for extracted strings.
698    
699           The  function  pcre_maketables()  is  used  to build a set of character
700           tables  in  the  current  locale   for   passing   to   pcre_compile(),
701           pcre_exec(),  or  pcre_dfa_exec(). This is an optional facility that is
702           provided for specialist use.  Most  commonly,  no  special  tables  are
703           passed,  in  which case internal tables that are generated when PCRE is
704           built are used.
705    
706           The function pcre_fullinfo() is used to find out  information  about  a
707           compiled  pattern; pcre_info() is an obsolete version that returns only
708           some of the available information, but is retained for  backwards  com-
709           patibility.   The function pcre_version() returns a pointer to a string
710           containing the version of PCRE and its date of release.
711    
712           The function pcre_refcount() maintains a  reference  count  in  a  data
713           block  containing  a compiled pattern. This is provided for the benefit
714           of object-oriented applications.
715    
716           The global variables pcre_malloc and pcre_free  initially  contain  the
717           entry  points  of  the  standard malloc() and free() functions, respec-
718           tively. PCRE calls the memory management functions via these variables,
719           so  a  calling  program  can replace them if it wishes to intercept the
720           calls. This should be done before calling any PCRE functions.
721    
722           The global variables pcre_stack_malloc  and  pcre_stack_free  are  also
723           indirections  to  memory  management functions. These special functions
724           are used only when PCRE is compiled to use  the  heap  for  remembering
725           data, instead of recursive function calls, when running the pcre_exec()
726           function. This is a non-standard way of building PCRE, for use in envi-
727           ronments that have limited stacks. Because of the greater use of memory
728           management, it runs more slowly.  Separate functions  are  provided  so
729           that  special-purpose  external  code  can  be used for this case. When
730           used, these functions are always called in a  stack-like  manner  (last
731           obtained,  first freed), and always for memory blocks of the same size.
732    
733           The global variable pcre_callout initially contains NULL. It can be set
734           by  the  caller  to  a "callout" function, which PCRE will then call at
735           specified points during a matching operation. Details are given in  the
736           pcrecallout documentation.
737    
738    
739    MULTITHREADING
740    
741           The  PCRE  functions  can be used in multi-threading applications, with
742           the  proviso  that  the  memory  management  functions  pointed  to  by
743           pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
744           callout function pointed to by pcre_callout, are shared by all threads.
745    
746           The  compiled form of a regular expression is not altered during match-
747           ing, so the same compiled pattern can safely be used by several threads
748           at once.
749    
750    
751    SAVING PRECOMPILED PATTERNS FOR LATER USE
752    
753           The compiled form of a regular expression can be saved and re-used at a
754           later time, possibly by a different program, and even on a  host  other
755           than  the  one  on  which  it  was  compiled.  Details are given in the
756           pcreprecompile documentation.
757    
758    
759    CHECKING BUILD-TIME OPTIONS
760    
761           int pcre_config(int what, void *where);
762    
763           The function pcre_config() makes it possible for a PCRE client to  dis-
764           cover which optional features have been compiled into the PCRE library.
765           The pcrebuild documentation has more details about these optional  fea-
766           tures.
767    
768           The  first  argument  for pcre_config() is an integer, specifying which
769           information is required; the second argument is a pointer to a variable
770           into  which  the  information  is  placed. The following information is
771           available:
772    
773             PCRE_CONFIG_UTF8
774    
775           The output is an integer that is set to one if UTF-8 support is  avail-
776           able; otherwise it is set to zero.
777    
778             PCRE_CONFIG_UNICODE_PROPERTIES
779    
780           The  output  is  an  integer  that is set to one if support for Unicode
781           character properties is available; otherwise it is set to zero.
782    
783             PCRE_CONFIG_NEWLINE
784    
785           The output is an integer that is set to the value of the code  that  is
786           used  for the newline character. It is either linefeed (10) or carriage
787           return (13), and should normally be the  standard  character  for  your
788           operating system.
789    
790             PCRE_CONFIG_LINK_SIZE
791    
792           The  output  is  an  integer that contains the number of bytes used for
793           internal linkage in compiled regular expressions. The value is 2, 3, or
794           4.  Larger  values  allow larger regular expressions to be compiled, at
795           the expense of slower matching. The default value of  2  is  sufficient
796           for  all  but  the  most massive patterns, since it allows the compiled
797           pattern to be up to 64K in size.
798    
799             PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
800    
801           The output is an integer that contains the threshold  above  which  the
802           POSIX  interface  uses malloc() for output vectors. Further details are
803           given in the pcreposix documentation.
804    
805             PCRE_CONFIG_MATCH_LIMIT
806    
807           The output is an integer that gives the default limit for the number of
808           internal  matching  function  calls in a pcre_exec() execution. Further
809           details are given with pcre_exec() below.
810    
811             PCRE_CONFIG_STACKRECURSE
812    
813           The output is an integer that is set to one if internal recursion  when
814           running pcre_exec() is implemented by recursive function calls that use
815           the stack to remember their state. This is the usual way that  PCRE  is
816           compiled. The output is zero if PCRE was compiled to use blocks of data
817           on the  heap  instead  of  recursive  function  calls.  In  this  case,
818           pcre_stack_malloc  and  pcre_stack_free  are  called  to  manage memory
819           blocks on the heap, thus avoiding the use of the stack.
820    
821    
822  COMPILING A PATTERN  COMPILING A PATTERN
      The function pcre_compile() is called to compile  a  pattern  
      into  an internal form. The pattern is a C string terminated  
      by a binary zero, and is passed in the argument  pattern.  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 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  con-  
      tains  a  copy of the tableptr argument, which is an address  
      (see below).  
   
      The size of a compiled pattern is  roughly  proportional  to  
      the length of the pattern string, except that each character  
      class (other than those containing just a single  character,  
      negated  or  not)  requires 33 bytes, and repeat quantifiers  
      with a minimum greater than one or a bounded  maximum  cause  
      the  relevant  portions of the compiled pattern to be repli-  
      cated.  
   
      The options argument contains independent bits  that  affect  
      the  compilation.  It  should  be  zero  if  no  options are  
      required. Some of the options, in particular, those that are  
      compatible  with Perl, can also be set and unset from within  
      the pattern (see the detailed description of regular expres-  
      sions below). For these options, the contents of the options  
      argument specifies their initial settings at  the  start  of  
      compilation  and  execution. The PCRE_ANCHORED option 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 message. The offset from the start  
      of  the  pattern  to  the  character  where  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.  
   
      If the final  argument,  tableptr,  is  NULL,  PCRE  uses  a  
      default  set  of character tables which are built when it is  
      compiled, using the default C  locale.  Otherwise,  tableptr  
      must  be  the result of a call to pcre_maketables(). See the  
      section on locale support below.  
   
      This code fragment shows a typical straightforward  call  to  
      pcre_compile():  
   
        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 option bits are defined in the 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 start of the  
      string which 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_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.  
   
        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  the  final  
      character  if it is a newline (but not before any other new-  
      lines).  The  PCRE_DOLLAR_ENDONLY  option  is   ignored   if  
      PCRE_MULTILINE is set. There is no equivalent to this option  
      in Perl.  
   
        PCRE_DOTALL  
   
      If this bit is  set,  a  dot  metacharater  in  the  pattern  
      matches all characters, including newlines. Without it, new-  
      lines are excluded. This option is equivalent to  Perl's  /s  
      option.  A negative class such as [^a] always matches a new-  
      line character, independent of the setting of this option.  
   
        PCRE_EXTENDED  
   
      If this bit is set, whitespace data characters in  the  pat-  
      tern  are  totally  ignored  except when escaped or inside a  
      character class, and characters between an unescaped #  out-  
      side  a  character  class  and  the  next newline character,  
      inclusive, are also ignored. This is equivalent to Perl's /x  
      option,  and  makes  it  possible to include comments inside  
      complicated patterns. Note, however, that this applies  only  
      to  data  characters. Whitespace characters may never appear  
      within special character sequences in a pattern, for example  
      within  the sequence (?( which introduces a conditional sub-  
      pattern.  
   
        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.  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_MULTILINE  
   
      By default, PCRE treats the subject string as consisting  of  
      a  single "line" of characters (even if it actually contains  
      several 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 immedi-  
      ately before any newline  in  the  subject  string,  respec-  
      tively,  as  well  as  at  the  very  start and end. This is  
      equivalent to Perl's /m option. If there are no "\n" charac-  
      ters  in  a subject string, or no occurrences of ^ or $ in a  
      pattern, setting PCRE_MULTILINE has no effect.  
   
        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 just byte  
      strings. However, it is available  only  if  PCRE  has  been  
      built  to  include  UTF-8  support.  If not, the use of this  
      option provokes an error. Support for UTF-8 is new,  experi-  
      mental,  and incomplete.  Details of exactly what it entails  
      are given below.  
823    
824           pcre *pcre_compile(const char *pattern, int options,
825                const char **errptr, int *erroffset,
826                const unsigned char *tableptr);
827    
828           pcre *pcre_compile2(const char *pattern, int options,
829                int *errorcodeptr,
830                const char **errptr, int *erroffset,
831                const unsigned char *tableptr);
832    
833           Either of the functions pcre_compile() or pcre_compile2() can be called
834           to compile a pattern into an internal form. The only difference between
835           the two interfaces is that pcre_compile2() has an additional  argument,
836           errorcodeptr, via which a numerical error code can be returned.
837    
838           The pattern is a C string terminated by a binary zero, and is passed in
839           the pattern argument. A pointer to a single block  of  memory  that  is
840           obtained  via  pcre_malloc is returned. This contains the compiled code
841           and related data. The pcre type is defined for the returned block; this
842           is a typedef for a structure whose contents are not externally defined.
843           It is up to the caller  to  free  the  memory  when  it  is  no  longer
844           required.
845    
846           Although  the compiled code of a PCRE regex is relocatable, that is, it
847           does not depend on memory location, the complete pcre data block is not
848           fully  relocatable, because it may contain a copy of the tableptr argu-
849           ment, which is an address (see below).
850    
851           The options argument contains independent bits that affect the compila-
852           tion.  It  should  be  zero  if  no options are required. The available
853           options are described below. Some of them, in  particular,  those  that
854           are  compatible  with  Perl,  can also be set and unset from within the
855           pattern (see the detailed description  in  the  pcrepattern  documenta-
856           tion).  For  these options, the contents of the options argument speci-
857           fies their initial settings at the start of compilation and  execution.
858           The  PCRE_ANCHORED option can be set at the time of matching as well as
859           at compile time.
860    
861           If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
862           if  compilation  of  a  pattern fails, pcre_compile() returns NULL, and
863           sets the variable pointed to by errptr to point to a textual error mes-
864           sage.  The  offset from the start of the pattern to the character where
865           the error was discovered is  placed  in  the  variable  pointed  to  by
866           erroffset,  which  must  not  be  NULL. If it is, an immediate error is
867           given.
868    
869           If pcre_compile2() is used instead of pcre_compile(),  and  the  error-
870           codeptr  argument is not NULL, a non-zero error code number is returned
871           via this argument in the event of an error. This is in addition to  the
872           textual error message. Error codes and messages are listed below.
873    
874           If  the  final  argument, tableptr, is NULL, PCRE uses a default set of
875           character tables that are  built  when  PCRE  is  compiled,  using  the
876           default  C  locale.  Otherwise, tableptr must be an address that is the
877           result of a call to pcre_maketables(). This value is  stored  with  the
878           compiled  pattern,  and used again by pcre_exec(), unless another table
879           pointer is passed to it. For more discussion, see the section on locale
880           support below.
881    
882           This  code  fragment  shows a typical straightforward call to pcre_com-
883           pile():
884    
885             pcre *re;
886             const char *error;
887             int erroffset;
888             re = pcre_compile(
889               "^A.*Z",          /* the pattern */
890               0,                /* default options */
891               &error,           /* for error message */
892               &erroffset,       /* for error offset */
893               NULL);            /* use default character tables */
894    
895           The following names for option bits are defined in  the  pcre.h  header
896           file:
897    
898             PCRE_ANCHORED
899    
900           If this bit is set, the pattern is forced to be "anchored", that is, it
901           is constrained to match only at the first matching point in the  string
902           that  is being searched (the "subject string"). This effect can also be
903           achieved by appropriate constructs in the pattern itself, which is  the
904           only way to do it in Perl.
905    
906             PCRE_AUTO_CALLOUT
907    
908           If this bit is set, pcre_compile() automatically inserts callout items,
909           all with number 255, before each pattern item. For  discussion  of  the
910           callout facility, see the pcrecallout documentation.
911    
912             PCRE_CASELESS
913    
914           If  this  bit is set, letters in the pattern match both upper and lower
915           case letters. It is equivalent to Perl's  /i  option,  and  it  can  be
916           changed  within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
917           always understands the concept of case for characters whose values  are
918           less  than 128, so caseless matching is always possible. For characters
919           with higher values, the concept of case is supported if  PCRE  is  com-
920           piled  with Unicode property support, but not otherwise. If you want to
921           use caseless matching for characters 128 and  above,  you  must  ensure
922           that  PCRE  is  compiled  with Unicode property support as well as with
923           UTF-8 support.
924    
925             PCRE_DOLLAR_ENDONLY
926    
927           If this bit is set, a dollar metacharacter in the pattern matches  only
928           at  the  end  of the subject string. Without this option, a dollar also
929           matches immediately before the final character if it is a newline  (but
930           not  before  any  other  newlines).  The  PCRE_DOLLAR_ENDONLY option is
931           ignored if PCRE_MULTILINE is set. There is no equivalent to this option
932           in Perl, and no way to set it within a pattern.
933    
934             PCRE_DOTALL
935    
936           If this bit is set, a dot metacharater in the pattern matches all char-
937           acters, including newlines. Without it,  newlines  are  excluded.  This
938           option  is equivalent to Perl's /s option, and it can be changed within
939           a pattern by a (?s) option setting.  A  negative  class  such  as  [^a]
940           always  matches a newline character, independent of the setting of this
941           option.
942    
943             PCRE_EXTENDED
944    
945           If this bit is set, whitespace  data  characters  in  the  pattern  are
946           totally ignored except when escaped or inside a character class. White-
947           space does not include the VT character (code 11). In addition, charac-
948           ters between an unescaped # outside a character class and the next new-
949           line character, inclusive, are also  ignored.  This  is  equivalent  to
950           Perl's  /x  option,  and  it  can be changed within a pattern by a (?x)
951           option setting.
952    
953           This option makes it possible to include  comments  inside  complicated
954           patterns.   Note,  however,  that this applies only to data characters.
955           Whitespace  characters  may  never  appear  within  special   character
956           sequences  in  a  pattern,  for  example  within the sequence (?( which
957           introduces a conditional subpattern.
958    
959             PCRE_EXTRA
960    
961           This option was invented in order to turn on  additional  functionality
962           of  PCRE  that  is  incompatible with Perl, but it is currently of very
963           little use. When set, any backslash in a pattern that is followed by  a
964           letter  that  has  no  special  meaning causes an error, thus reserving
965           these combinations for future expansion. By  default,  as  in  Perl,  a
966           backslash  followed by a letter with no special meaning is treated as a
967           literal. There are at present no  other  features  controlled  by  this
968           option. It can also be set by a (?X) option setting within a pattern.
969    
970             PCRE_FIRSTLINE
971    
972           If  this  option  is  set,  an  unanchored pattern is required to match
973           before or at the first newline character in the subject string,  though
974           the matched text may continue over the newline.
975    
976             PCRE_MULTILINE
977    
978           By  default,  PCRE  treats the subject string as consisting of a single
979           line of characters (even if it actually contains newlines). The  "start
980           of  line"  metacharacter  (^)  matches only at the start of the string,
981           while the "end of line" metacharacter ($) matches only at  the  end  of
982           the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
983           is set). This is the same as Perl.
984    
985           When PCRE_MULTILINE it is set, the "start of line" and  "end  of  line"
986           constructs  match  immediately following or immediately before any new-
987           line in the subject string, respectively, as well as at the very  start
988           and  end. This is equivalent to Perl's /m option, and it can be changed
989           within a pattern by a (?m) option setting. If there are no "\n" charac-
990           ters  in  a  subject  string, or no occurrences of ^ or $ in a pattern,
991           setting PCRE_MULTILINE has no effect.
992    
993             PCRE_NO_AUTO_CAPTURE
994    
995           If this option is set, it disables the use of numbered capturing paren-
996           theses  in the pattern. Any opening parenthesis that is not followed by
997           ? behaves as if it were followed by ?: but named parentheses can  still
998           be  used  for  capturing  (and  they acquire numbers in the usual way).
999           There is no equivalent of this option in Perl.
1000    
1001             PCRE_UNGREEDY
1002    
1003           This option inverts the "greediness" of the quantifiers  so  that  they
1004           are  not greedy by default, but become greedy if followed by "?". It is
1005           not compatible with Perl. It can also be set by a (?U)  option  setting
1006           within the pattern.
1007    
1008             PCRE_UTF8
1009    
1010           This  option  causes PCRE to regard both the pattern and the subject as
1011           strings of UTF-8 characters instead of single-byte  character  strings.
1012           However,  it is available only when PCRE is built to include UTF-8 sup-
1013           port. If not, the use of this option provokes an error. Details of  how
1014           this  option  changes the behaviour of PCRE are given in the section on
1015           UTF-8 support in the main pcre page.
1016    
1017             PCRE_NO_UTF8_CHECK
1018    
1019           When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1020           automatically  checked. If an invalid UTF-8 sequence of bytes is found,
1021           pcre_compile() returns an error. If you already know that your  pattern
1022           is  valid, and you want to skip this check for performance reasons, you
1023           can set the PCRE_NO_UTF8_CHECK option. When it is set,  the  effect  of
1024           passing an invalid UTF-8 string as a pattern is undefined. It may cause
1025           your program to crash.  Note that this option can  also  be  passed  to
1026           pcre_exec()  and pcre_dfa_exec(), to suppress the UTF-8 validity check-
1027           ing of subject strings.
1028    
1029    
1030    COMPILATION ERROR CODES
1031    
1032           The following table lists the error  codes  than  may  be  returned  by
1033           pcre_compile2(),  along with the error messages that may be returned by
1034           both compiling functions.
1035    
1036              0  no error
1037              1  \ at end of pattern
1038              2  \c at end of pattern
1039              3  unrecognized character follows \
1040              4  numbers out of order in {} quantifier
1041              5  number too big in {} quantifier
1042              6  missing terminating ] for character class
1043              7  invalid escape sequence in character class
1044              8  range out of order in character class
1045              9  nothing to repeat
1046             10  operand of unlimited repeat could match the empty string
1047             11  internal error: unexpected repeat
1048             12  unrecognized character after (?
1049             13  POSIX named classes are supported only within a class
1050             14  missing )
1051             15  reference to non-existent subpattern
1052             16  erroffset passed as NULL
1053             17  unknown option bit(s) set
1054             18  missing ) after comment
1055             19  parentheses nested too deeply
1056             20  regular expression too large
1057             21  failed to get memory
1058             22  unmatched parentheses
1059             23  internal error: code overflow
1060             24  unrecognized character after (?<
1061             25  lookbehind assertion is not fixed length
1062             26  malformed number after (?(
1063             27  conditional group contains more than two branches
1064             28  assertion expected after (?(
1065             29  (?R or (?digits must be followed by )
1066             30  unknown POSIX class name
1067             31  POSIX collating elements are not supported
1068             32  this version of PCRE is not compiled with PCRE_UTF8 support
1069             33  spare error
1070             34  character value in \x{...} sequence is too large
1071             35  invalid condition (?(0)
1072             36  \C not allowed in lookbehind assertion
1073             37  PCRE does not support \L, \l, \N, \U, or \u
1074             38  number after (?C is > 255
1075             39  closing ) for (?C expected
1076             40  recursive call could loop indefinitely
1077             41  unrecognized character after (?P
1078             42  syntax error after (?P
1079             43  two named groups have the same name
1080             44  invalid UTF-8 string
1081             45  support for \P, \p, and \X has not been compiled
1082             46  malformed \P or \p sequence
1083             47  unknown property name after \P or \p
1084    
1085    
1086  STUDYING A PATTERN  STUDYING A PATTERN
      When a 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 pattern as its first argument, and  
      returns a pointer to a pcre_extra block (another typedef for  
      a  structure  with  hidden  contents)  containing additional  
      information  about  the  pattern;  this  can  be  passed  to  
      pcre_exec(). If no additional information is available, NULL  
      is returned.  
   
      The second argument contains option  bits.  At  present,  no  
      options  are  defined  for  pcre_study(),  and this argument  
      should always be zero.  
   
      The third argument for pcre_study() is a pointer to an error  
      message. If studying succeeds (even if no data is returned),  
      the variable it points to  is  set  to  NULL.  Otherwise  it  
      points to a textual error message.  
   
      This is a typical call to pcre_study():  
   
        pcre_extra *pe;  
        pe = pcre_study(  
          re,             /* result of pcre_compile() */  
          0,              /* no options exist */  
          &error);        /* set to NULL or points to a message */  
   
      At present, studying a  pattern  is  useful  only  for  non-  
      anchored  patterns  that do not have a single fixed starting  
      character. A  bitmap  of  possible  starting  characters  is  
      created.  
1087    
1088           pcre_extra *pcre_study(const pcre *code, int options
1089                const char **errptr);
1090    
1091           If a compiled pattern is going to be used several times,  it  is  worth
1092           spending more time analyzing it in order to speed up the time taken for
1093           matching. The function pcre_study() takes a pointer to a compiled  pat-
1094           tern as its first argument. If studying the pattern produces additional
1095           information that will help speed up matching,  pcre_study()  returns  a
1096           pointer  to a pcre_extra block, in which the study_data field points to
1097           the results of the study.
1098    
1099           The  returned  value  from  pcre_study()  can  be  passed  directly  to
1100           pcre_exec().  However,  a  pcre_extra  block also contains other fields
1101           that can be set by the caller before the block  is  passed;  these  are
1102           described below in the section on matching a pattern.
1103    
1104           If  studying  the  pattern  does not produce any additional information
1105           pcre_study() returns NULL. In that circumstance, if the calling program
1106           wants  to  pass  any of the other fields to pcre_exec(), it must set up
1107           its own pcre_extra block.
1108    
1109           The second argument of pcre_study() contains option bits.  At  present,
1110           no options are defined, and this argument should always be zero.
1111    
1112           The  third argument for pcre_study() is a pointer for an error message.
1113           If studying succeeds (even if no data is  returned),  the  variable  it
1114           points  to  is set to NULL. Otherwise it points to a textual error mes-
1115           sage. You should therefore test the error pointer for NULL after  call-
1116           ing pcre_study(), to be sure that it has run successfully.
1117    
1118           This is a typical call to pcre_study():
1119    
1120             pcre_extra *pe;
1121             pe = pcre_study(
1122               re,             /* result of pcre_compile() */
1123               0,              /* no options exist */
1124               &error);        /* set to NULL or points to a message */
1125    
1126           At present, studying a pattern is useful only for non-anchored patterns
1127           that do not have a single fixed starting character. A bitmap of  possi-
1128           ble starting bytes is created.
1129    
1130    
1131  LOCALE SUPPORT  LOCALE SUPPORT
      PCRE handles caseless matching, and determines whether char-  
      acters  are  letters, digits, or whatever, by reference to a  
      set of tables. The library contains a default set of  tables  
      which  is  created in the default C locale when PCRE is com-  
      piled.  This  is   used   when   the   final   argument   of  
      pcre_compile()  is NULL, and is sufficient for many applica-  
      tions.  
   
      An alternative set of tables can, however, be supplied. Such  
      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() as often as necessary.  
      For example, to build and use tables  that  are  appropriate  
      for  the French locale (where accented characters with codes  
      greater than 128 are treated as letters), the following code  
      could be used:  
   
        setlocale(LC_CTYPE, "fr");  
        tables = pcre_maketables();  
        re = pcre_compile(..., tables);  
   
      The  tables  are  built  in  memory  that  is  obtained  via  
      pcre_malloc.  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 pcre_exec(). Thus  
      for any single pattern, compilation, studying  and  matching  
      all happen in the same locale, but different patterns can be  
      compiled in different locales. It is the caller's  responsi-  
      bility  to  ensure  that  the  memory  containing the tables  
      remains available for as long as it is needed.  
1132    
1133           PCRE  handles  caseless matching, and determines whether characters are
1134           letters digits, or whatever, by reference to a set of  tables,  indexed
1135           by  character  value.  When running in UTF-8 mode, this applies only to
1136           characters with codes less than 128. Higher-valued  codes  never  match
1137           escapes  such  as  \w or \d, but can be tested with \p if PCRE is built
1138           with Unicode character property support.
1139    
1140           An internal set of tables is created in the default C locale when  PCRE
1141           is  built.  This  is  used when the final argument of pcre_compile() is
1142           NULL, and is sufficient for many applications. An  alternative  set  of
1143           tables  can,  however, be supplied. These may be created in a different
1144           locale from the default. As more and more applications change to  using
1145           Unicode, the need for this locale support is expected to die away.
1146    
1147           External  tables  are  built by calling the pcre_maketables() function,
1148           which has no arguments, in the relevant locale. The result can then  be
1149           passed  to  pcre_compile()  or  pcre_exec()  as often as necessary. For
1150           example, to build and use tables that are appropriate  for  the  French
1151           locale  (where  accented  characters  with  values greater than 128 are
1152           treated as letters), the following code could be used:
1153    
1154             setlocale(LC_CTYPE, "fr_FR");
1155             tables = pcre_maketables();
1156             re = pcre_compile(..., tables);
1157    
1158           When pcre_maketables() runs, the tables are built  in  memory  that  is
1159           obtained  via  pcre_malloc. It is the caller's responsibility to ensure
1160           that the memory containing the tables remains available for as long  as
1161           it is needed.
1162    
1163           The pointer that is passed to pcre_compile() is saved with the compiled
1164           pattern, and the same tables are used via this pointer by  pcre_study()
1165           and normally also by pcre_exec(). Thus, by default, for any single pat-
1166           tern, compilation, studying and matching all happen in the same locale,
1167           but different patterns can be compiled in different locales.
1168    
1169           It  is  possible to pass a table pointer or NULL (indicating the use of
1170           the internal tables) to pcre_exec(). Although  not  intended  for  this
1171           purpose,  this facility could be used to match a pattern in a different
1172           locale from the one in which it was compiled. Passing table pointers at
1173           run time is discussed below in the section on matching a pattern.
1174    
1175    
1176  INFORMATION ABOUT A PATTERN  INFORMATION ABOUT A PATTERN
      The pcre_fullinfo() function  returns  information  about  a  
      compiled pattern. It replaces the obsolete pcre_info() func-  
      tion, which is nevertheless retained for backwards compabil-  
      ity (and is documented below).  
   
      The first argument for pcre_fullinfo() is a pointer  to  the  
      compiled  pattern.  The  second  argument  is  the result of  
      pcre_study(), or NULL if the pattern was  not  studied.  The  
      third  argument  specifies  which  piece  of  information is  
      required, while the fourth argument is a pointer to a  vari-  
      able  to receive the data. The yield of the function is zero  
      for success, or one of the following negative numbers:  
   
        PCRE_ERROR_NULL       the argument code was NULL  
                              the argument where was NULL  
        PCRE_ERROR_BADMAGIC   the "magic number" was not found  
        PCRE_ERROR_BADOPTION  the value of what was invalid  
   
      Here is a typical call of  pcre_fullinfo(),  to  obtain  the  
      length of the compiled pattern:  
   
        int rc;  
        unsigned long int length;  
        rc = pcre_fullinfo(  
          re,               /* result of pcre_compile() */  
          pe,               /* result of pcre_study(), or NULL */  
          PCRE_INFO_SIZE,   /* what is required */  
          &length);         /* where to put the data */  
   
      The possible values for the third argument  are  defined  in  
      pcre.h, and are as follows:  
   
        PCRE_INFO_OPTIONS  
   
      Return a copy of the options with which the pattern was com-  
      piled.  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  within  the   pattern   itself,   and   with   the  
      PCRE_ANCHORED  bit  forcibly  set if the form of the pattern  
      implies that it can match only at the  start  of  a  subject  
      string.  
   
        PCRE_INFO_SIZE  
   
      Return the size of the compiled pattern, that is, the  value  
      that  was  passed as the argument to pcre_malloc() when PCRE  
      was getting memory in which to place the compiled data.  The  
      fourth argument should point to a size_t variable.  
   
        PCRE_INFO_CAPTURECOUNT  
   
      Return the number of capturing subpatterns in  the  pattern.  
      The fourth argument should point to an int variable.  
   
        PCRE_INFO_BACKREFMAX  
   
      Return the number of the highest back reference in the  pat-  
      tern.  The  fourth argument should point to an int variable.  
      Zero is returned if there are no back references.  
   
        PCRE_INFO_FIRSTCHAR  
   
      Return information about the first character of any  matched  
      string,  for  a  non-anchored  pattern.  If there is a fixed  
      first   character,   e.g.   from   a   pattern    such    as  
      (cat|cow|coyote),  it  is returned in the integer pointed to  
      by where. Otherwise, if either  
   
      (a) the pattern was compiled with the PCRE_MULTILINE option,  
      and every branch starts with "^", or  
   
      (b) every  branch  of  the  pattern  starts  with  ".*"  and  
      PCRE_DOTALL is not set (if it were set, the pattern would be  
      anchored),  
   
      -1 is returned, indicating that the pattern matches only  at  
      the  start  of a subject string or after any "\n" within the  
      string. Otherwise -2 is returned.  For anchored patterns, -2  
      is returned.  
   
        PCRE_INFO_FIRSTTABLE  
   
      If the pattern was studied, and this resulted  in  the  con-  
      struction of a 256-bit table indicating a fixed set of char-  
      acters for the first character 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_LASTLITERAL  
   
      For a non-anchored pattern, return the value of  the  right-  
      most  literal  character  which  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 character, or  
      if the pattern is anchored, -1 is returned. For example, for  
      the pattern /a\d+z\d+/ the returned value is 'z'.  
   
      The pcre_info() function is now obsolete because its  inter-  
      face  is  too  restrictive  to return all the available data  
      about  a  compiled  pattern.   New   programs   should   use  
      pcre_fullinfo()  instead.  The  yield  of pcre_info() is the  
      number of capturing subpatterns, or  one  of  the  following  
      negative numbers:  
   
        PCRE_ERROR_NULL       the argument code was NULL  
        PCRE_ERROR_BADMAGIC   the "magic number" was not found  
   
      If the optptr argument is not NULL, a copy  of  the  options  
      with which the pattern was compiled is placed in the integer  
      it points to (see PCRE_INFO_OPTIONS above).  
   
      If the pattern is not anchored and the firstcharptr argument  
      is  not  NULL, it is used to pass back information about the  
      first    character    of    any    matched    string    (see  
      PCRE_INFO_FIRSTCHAR above).  
1177    
1178           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1179                int what, void *where);
1180    
1181           The  pcre_fullinfo() function returns information about a compiled pat-
1182           tern. It replaces the obsolete pcre_info() function, which is neverthe-
1183           less retained for backwards compability (and is documented below).
1184    
1185           The  first  argument  for  pcre_fullinfo() is a pointer to the compiled
1186           pattern. The second argument is the result of pcre_study(), or NULL  if
1187           the  pattern  was not studied. The third argument specifies which piece
1188           of information is required, and the fourth argument is a pointer  to  a
1189           variable  to  receive  the  data. The yield of the function is zero for
1190           success, or one of the following negative numbers:
1191    
1192             PCRE_ERROR_NULL       the argument code was NULL
1193                                   the argument where was NULL
1194             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1195             PCRE_ERROR_BADOPTION  the value of what was invalid
1196    
1197           The "magic number" is placed at the start of each compiled  pattern  as
1198           an  simple check against passing an arbitrary memory pointer. Here is a
1199           typical call of pcre_fullinfo(), to obtain the length of  the  compiled
1200           pattern:
1201    
1202             int rc;
1203             unsigned long int length;
1204             rc = pcre_fullinfo(
1205               re,               /* result of pcre_compile() */
1206               pe,               /* result of pcre_study(), or NULL */
1207               PCRE_INFO_SIZE,   /* what is required */
1208               &length);         /* where to put the data */
1209    
1210           The  possible  values for the third argument are defined in pcre.h, and
1211           are as follows:
1212    
1213             PCRE_INFO_BACKREFMAX
1214    
1215           Return the number of the highest back reference  in  the  pattern.  The
1216           fourth  argument  should  point to an int variable. Zero is returned if
1217           there are no back references.
1218    
1219             PCRE_INFO_CAPTURECOUNT
1220    
1221           Return the number of capturing subpatterns in the pattern.  The  fourth
1222           argument should point to an int variable.
1223    
1224             PCRE_INFO_DEFAULT_TABLES
1225    
1226           Return  a pointer to the internal default character tables within PCRE.
1227           The fourth argument should point to an unsigned char *  variable.  This
1228           information call is provided for internal use by the pcre_study() func-
1229           tion. External callers can cause PCRE to use  its  internal  tables  by
1230           passing a NULL table pointer.
1231    
1232             PCRE_INFO_FIRSTBYTE
1233    
1234           Return  information  about  the first byte of any matched string, for a
1235           non-anchored   pattern.   (This    option    used    to    be    called
1236           PCRE_INFO_FIRSTCHAR;  the  old  name  is still recognized for backwards
1237           compatibility.)
1238    
1239           If there is a fixed first byte, for example, from  a  pattern  such  as
1240           (cat|cow|coyote),  it  is  returned in the integer pointed to by where.
1241           Otherwise, if either
1242    
1243           (a) the pattern was compiled with the PCRE_MULTILINE option, and  every
1244           branch starts with "^", or
1245    
1246           (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1247           set (if it were set, the pattern would be anchored),
1248    
1249           -1 is returned, indicating that the pattern matches only at  the  start
1250           of  a  subject string or after any newline within the string. Otherwise
1251           -2 is returned. For anchored patterns, -2 is returned.
1252    
1253             PCRE_INFO_FIRSTTABLE
1254    
1255           If the pattern was studied, and this resulted in the construction of  a
1256           256-bit table indicating a fixed set of bytes for the first byte in any
1257           matching string, a pointer to the table is returned. Otherwise NULL  is
1258           returned.  The fourth argument should point to an unsigned char * vari-
1259           able.
1260    
1261             PCRE_INFO_LASTLITERAL
1262    
1263           Return the value of the rightmost literal byte that must exist  in  any
1264           matched  string,  other  than  at  its  start,  if such a byte has been
1265           recorded. The fourth argument should point to an int variable. If there
1266           is  no such byte, -1 is returned. For anchored patterns, a last literal
1267           byte is recorded only if it follows something of variable  length.  For
1268           example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1269           /^a\dz\d/ the returned value is -1.
1270    
1271             PCRE_INFO_NAMECOUNT
1272             PCRE_INFO_NAMEENTRYSIZE
1273             PCRE_INFO_NAMETABLE
1274    
1275           PCRE supports the use of named as well as numbered capturing  parenthe-
1276           ses.  The names are just an additional way of identifying the parenthe-
1277           ses,  which  still  acquire  numbers.  A  convenience  function  called
1278           pcre_get_named_substring()  is  provided  for  extracting an individual
1279           captured substring by name. It is also possible  to  extract  the  data
1280           directly,  by  first converting the name to a number in order to access
1281           the correct pointers in the output vector (described  with  pcre_exec()
1282           below).  To  do the conversion, you need to use the name-to-number map,
1283           which is described by these three values.
1284    
1285           The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1286           gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1287           of each entry; both of these  return  an  int  value.  The  entry  size
1288           depends  on the length of the longest name. PCRE_INFO_NAMETABLE returns
1289           a pointer to the first entry of the table  (a  pointer  to  char).  The
1290           first two bytes of each entry are the number of the capturing parenthe-
1291           sis, most significant byte first. The rest of the entry is  the  corre-
1292           sponding  name,  zero  terminated. The names are in alphabetical order.
1293           For example, consider the following pattern  (assume  PCRE_EXTENDED  is
1294           set, so white space - including newlines - is ignored):
1295    
1296             (?P<date> (?P<year>(\d\d)?\d\d) -
1297             (?P<month>\d\d) - (?P<day>\d\d) )
1298    
1299           There  are  four  named subpatterns, so the table has four entries, and
1300           each entry in the table is eight bytes long. The table is  as  follows,
1301           with non-printing bytes shows in hexadecimal, and undefined bytes shown
1302           as ??:
1303    
1304             00 01 d  a  t  e  00 ??
1305             00 05 d  a  y  00 ?? ??
1306             00 04 m  o  n  t  h  00
1307             00 02 y  e  a  r  00 ??
1308    
1309           When writing code to extract data  from  named  subpatterns  using  the
1310           name-to-number map, remember that the length of each entry is likely to
1311           be different for each compiled pattern.
1312    
1313             PCRE_INFO_OPTIONS
1314    
1315           Return a copy of the options with which the pattern was  compiled.  The
1316           fourth  argument  should  point to an unsigned long int variable. These
1317           option bits are those specified in the call to pcre_compile(), modified
1318           by any top-level option settings within the pattern itself.
1319    
1320           A  pattern  is  automatically  anchored by PCRE if all of its top-level
1321           alternatives begin with one of the following:
1322    
1323             ^     unless PCRE_MULTILINE is set
1324             \A    always
1325             \G    always
1326             .*    if PCRE_DOTALL is set and there are no back
1327                     references to the subpattern in which .* appears
1328    
1329           For such patterns, the PCRE_ANCHORED bit is set in the options returned
1330           by pcre_fullinfo().
1331    
1332             PCRE_INFO_SIZE
1333    
1334           Return  the  size  of the compiled pattern, that is, the value that was
1335           passed as the argument to pcre_malloc() when PCRE was getting memory in
1336           which to place the compiled data. The fourth argument should point to a
1337           size_t variable.
1338    
1339             PCRE_INFO_STUDYSIZE
1340    
1341           Return the size of the data block pointed to by the study_data field in
1342           a  pcre_extra  block.  That  is,  it  is  the  value that was passed to
1343           pcre_malloc() when PCRE was getting memory into which to place the data
1344           created  by  pcre_study(). The fourth argument should point to a size_t
1345           variable.
1346    
1347    
1348    OBSOLETE INFO FUNCTION
1349    
1350           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1351    
1352           The pcre_info() function is now obsolete because its interface  is  too
1353           restrictive  to return all the available data about a compiled pattern.
1354           New  programs  should  use  pcre_fullinfo()  instead.  The   yield   of
1355           pcre_info()  is the number of capturing subpatterns, or one of the fol-
1356           lowing negative numbers:
1357    
1358             PCRE_ERROR_NULL       the argument code was NULL
1359             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1360    
1361           If the optptr argument is not NULL, a copy of the  options  with  which
1362           the  pattern  was  compiled  is placed in the integer it points to (see
1363           PCRE_INFO_OPTIONS above).
1364    
1365           If the pattern is not anchored and the  firstcharptr  argument  is  not
1366           NULL,  it is used to pass back information about the first character of
1367           any matched string (see PCRE_INFO_FIRSTBYTE above).
1368    
1369    
1370    REFERENCE COUNTS
1371    
1372           int pcre_refcount(pcre *code, int adjust);
1373    
1374           The pcre_refcount() function is used to maintain a reference  count  in
1375           the data block that contains a compiled pattern. It is provided for the
1376           benefit of applications that  operate  in  an  object-oriented  manner,
1377           where different parts of the application may be using the same compiled
1378           pattern, but you want to free the block when they are all done.
1379    
1380           When a pattern is compiled, the reference count field is initialized to
1381           zero.   It is changed only by calling this function, whose action is to
1382           add the adjust value (which may be positive or  negative)  to  it.  The
1383           yield of the function is the new value. However, the value of the count
1384           is constrained to lie between 0 and 65535, inclusive. If the new  value
1385           is outside these limits, it is forced to the appropriate limit value.
1386    
1387           Except  when it is zero, the reference count is not correctly preserved
1388           if a pattern is compiled on one host and then  transferred  to  a  host
1389           whose byte-order is different. (This seems a highly unlikely scenario.)
1390    
1391    
1392    MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1393    
1394           int pcre_exec(const pcre *code, const pcre_extra *extra,
1395                const char *subject, int length, int startoffset,
1396                int options, int *ovector, int ovecsize);
1397    
1398           The function pcre_exec() is called to match a subject string against  a
1399           compiled  pattern, which is passed in the code argument. If the pattern
1400           has been studied, the result of the study should be passed in the extra
1401           argument.  This  function is the main matching facility of the library,
1402           and it operates in a Perl-like manner. For specialist use there is also
1403           an  alternative matching function, which is described below in the sec-
1404           tion about the pcre_dfa_exec() function.
1405    
1406           In most applications, the pattern will have been compiled (and  option-
1407           ally  studied)  in the same process that calls pcre_exec(). However, it
1408           is possible to save compiled patterns and study data, and then use them
1409           later  in  different processes, possibly even on different hosts. For a
1410           discussion about this, see the pcreprecompile documentation.
1411    
1412           Here is an example of a simple call to pcre_exec():
1413    
1414             int rc;
1415             int ovector[30];
1416             rc = pcre_exec(
1417               re,             /* result of pcre_compile() */
1418               NULL,           /* we didn't study the pattern */
1419               "some string",  /* the subject string */
1420               11,             /* the length of the subject string */
1421               0,              /* start at offset 0 in the subject */
1422               0,              /* default options */
1423               ovector,        /* vector of integers for substring information */
1424               30);            /* number of elements (NOT size in bytes) */
1425    
1426       Extra data for pcre_exec()
1427    
1428           If the extra argument is not NULL, it must point to a  pcre_extra  data
1429           block.  The pcre_study() function returns such a block (when it doesn't
1430           return NULL), but you can also create one for yourself, and pass  addi-
1431           tional  information in it. The fields in a pcre_extra block are as fol-
1432           lows:
1433    
1434             unsigned long int flags;
1435             void *study_data;
1436             unsigned long int match_limit;
1437             void *callout_data;
1438             const unsigned char *tables;
1439    
1440           The flags field is a bitmap that specifies which of  the  other  fields
1441           are set. The flag bits are:
1442    
1443             PCRE_EXTRA_STUDY_DATA
1444             PCRE_EXTRA_MATCH_LIMIT
1445             PCRE_EXTRA_CALLOUT_DATA
1446             PCRE_EXTRA_TABLES
1447    
1448           Other  flag  bits should be set to zero. The study_data field is set in
1449           the pcre_extra block that is returned by  pcre_study(),  together  with
1450           the appropriate flag bit. You should not set this yourself, but you may
1451           add to the block by setting the other fields  and  their  corresponding
1452           flag bits.
1453    
1454           The match_limit field provides a means of preventing PCRE from using up
1455           a vast amount of resources when running patterns that are not going  to
1456           match,  but  which  have  a very large number of possibilities in their
1457           search trees. The classic  example  is  the  use  of  nested  unlimited
1458           repeats.
1459    
1460           Internally,  PCRE uses a function called match() which it calls repeat-
1461           edly (sometimes recursively). The limit is imposed  on  the  number  of
1462           times  this  function is called during a match, which has the effect of
1463           limiting the amount of recursion and backtracking that can take  place.
1464           For patterns that are not anchored, the count starts from zero for each
1465           position in the subject string.
1466    
1467           The default limit for the library can be set when PCRE  is  built;  the
1468           default  default  is 10 million, which handles all but the most extreme
1469           cases. You can reduce  the  default  by  suppling  pcre_exec()  with  a
1470           pcre_extra  block  in  which match_limit is set to a smaller value, and
1471           PCRE_EXTRA_MATCH_LIMIT is set in the  flags  field.  If  the  limit  is
1472           exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1473    
1474           The  pcre_callout  field is used in conjunction with the "callout" fea-
1475           ture, which is described in the pcrecallout documentation.
1476    
1477           The tables field  is  used  to  pass  a  character  tables  pointer  to
1478           pcre_exec();  this overrides the value that is stored with the compiled
1479           pattern. A non-NULL value is stored with the compiled pattern  only  if
1480           custom  tables  were  supplied to pcre_compile() via its tableptr argu-
1481           ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
1482           PCRE's  internal  tables  to be used. This facility is helpful when re-
1483           using patterns that have been saved after compiling  with  an  external
1484           set  of  tables,  because  the  external tables might be at a different
1485           address when pcre_exec() is called. See the  pcreprecompile  documenta-
1486           tion for a discussion of saving compiled patterns for later use.
1487    
1488       Option bits for pcre_exec()
1489    
1490           The  unused  bits of the options argument for pcre_exec() must be zero.
1491           The  only  bits  that  may  be  set  are  PCRE_ANCHORED,   PCRE_NOTBOL,
1492           PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and PCRE_PARTIAL.
1493    
1494             PCRE_ANCHORED
1495    
1496           The  PCRE_ANCHORED  option  limits pcre_exec() to matching at the first
1497           matching position. If a pattern was  compiled  with  PCRE_ANCHORED,  or
1498           turned  out to be anchored by virtue of its contents, it cannot be made
1499           unachored at matching time.
1500    
1501             PCRE_NOTBOL
1502    
1503           This option specifies that first character of the subject string is not
1504           the  beginning  of  a  line, so the circumflex metacharacter should not
1505           match before it. Setting this without PCRE_MULTILINE (at compile  time)
1506           causes  circumflex  never to match. This option affects only the behav-
1507           iour of the circumflex metacharacter. It does not affect \A.
1508    
1509             PCRE_NOTEOL
1510    
1511           This option specifies that the end of the subject string is not the end
1512           of  a line, so the dollar metacharacter should not match it nor (except
1513           in multiline mode) a newline immediately before it. Setting this  with-
1514           out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1515           option affects only the behaviour of the dollar metacharacter. It  does
1516           not affect \Z or \z.
1517    
1518             PCRE_NOTEMPTY
1519    
1520           An empty string is not considered to be a valid match if this option is
1521           set. If there are alternatives in the pattern, they are tried.  If  all
1522           the  alternatives  match  the empty string, the entire match fails. For
1523           example, if the pattern
1524    
1525             a?b?
1526    
1527           is applied to a string not beginning with "a" or "b",  it  matches  the
1528           empty  string at the start of the subject. With PCRE_NOTEMPTY set, this
1529           match is not valid, so PCRE searches further into the string for occur-
1530           rences of "a" or "b".
1531    
1532           Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1533           cial case of a pattern match of the empty  string  within  its  split()
1534           function,  and  when  using  the /g modifier. It is possible to emulate
1535           Perl's behaviour after matching a null string by first trying the match
1536           again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1537           if that fails by advancing the starting offset (see below)  and  trying
1538           an ordinary match again. There is some code that demonstrates how to do
1539           this in the pcredemo.c sample program.
1540    
1541             PCRE_NO_UTF8_CHECK
1542    
1543           When PCRE_UTF8 is set at compile time, the validity of the subject as a
1544           UTF-8  string is automatically checked when pcre_exec() is subsequently
1545           called.  The value of startoffset is also checked  to  ensure  that  it
1546           points  to the start of a UTF-8 character. If an invalid UTF-8 sequence
1547           of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If
1548           startoffset  contains  an  invalid  value, PCRE_ERROR_BADUTF8_OFFSET is
1549           returned.
1550    
1551           If you already know that your subject is valid, and you  want  to  skip
1552           these    checks    for   performance   reasons,   you   can   set   the
1553           PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might  want  to
1554           do  this  for the second and subsequent calls to pcre_exec() if you are
1555           making repeated calls to find all  the  matches  in  a  single  subject
1556           string.  However,  you  should  be  sure  that the value of startoffset
1557           points to the start of a UTF-8 character.  When  PCRE_NO_UTF8_CHECK  is
1558           set,  the  effect of passing an invalid UTF-8 string as a subject, or a
1559           value of startoffset that does not point to the start of a UTF-8  char-
1560           acter, is undefined. Your program may crash.
1561    
1562             PCRE_PARTIAL
1563    
1564           This  option  turns  on  the  partial  matching feature. If the subject
1565           string fails to match the pattern, but at some point during the  match-
1566           ing  process  the  end of the subject was reached (that is, the subject
1567           partially matches the pattern and the failure to  match  occurred  only
1568           because  there were not enough subject characters), pcre_exec() returns
1569           PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL  is
1570           used,  there  are restrictions on what may appear in the pattern. These
1571           are discussed in the pcrepartial documentation.
1572    
1573       The string to be matched by pcre_exec()
1574    
1575           The subject string is passed to pcre_exec() as a pointer in subject,  a
1576           length  in  length, and a starting byte offset in startoffset. In UTF-8
1577           mode, the byte offset must point to the start  of  a  UTF-8  character.
1578           Unlike  the  pattern string, the subject may contain binary zero bytes.
1579           When the starting offset is zero, the search for a match starts at  the
1580           beginning of the subject, and this is by far the most common case.
1581    
1582           A  non-zero  starting offset is useful when searching for another match
1583           in the same subject by calling pcre_exec() again after a previous  suc-
1584           cess.   Setting  startoffset differs from just passing over a shortened
1585           string and setting PCRE_NOTBOL in the case of  a  pattern  that  begins
1586           with any kind of lookbehind. For example, consider the pattern
1587    
1588             \Biss\B
1589    
1590           which  finds  occurrences  of "iss" in the middle of words. (\B matches
1591           only if the current position in the subject is not  a  word  boundary.)
1592           When  applied  to the string "Mississipi" the first call to pcre_exec()
1593           finds the first occurrence. If pcre_exec() is called  again  with  just
1594           the  remainder  of  the  subject,  namely  "issipi", it does not match,
1595           because \B is always false at the start of the subject, which is deemed
1596           to  be  a  word  boundary. However, if pcre_exec() is passed the entire
1597           string again, but with startoffset set to 4, it finds the second occur-
1598           rence  of "iss" because it is able to look behind the starting point to
1599           discover that it is preceded by a letter.
1600    
1601           If a non-zero starting offset is passed when the pattern  is  anchored,
1602           one attempt to match at the given offset is made. This can only succeed
1603           if the pattern does not require the match to be at  the  start  of  the
1604           subject.
1605    
1606       How pcre_exec() returns captured substrings
1607    
1608           In  general, a pattern matches a certain portion of the subject, and in
1609           addition, further substrings from the subject  may  be  picked  out  by
1610           parts  of  the  pattern.  Following the usage in Jeffrey Friedl's book,
1611           this is called "capturing" in what follows, and the  phrase  "capturing
1612           subpattern"  is  used for a fragment of a pattern that picks out a sub-
1613           string. PCRE supports several other kinds of  parenthesized  subpattern
1614           that do not cause substrings to be captured.
1615    
1616           Captured  substrings are returned to the caller via a vector of integer
1617           offsets whose address is passed in ovector. The number of  elements  in
1618           the  vector is passed in ovecsize, which must be a non-negative number.
1619           Note: this argument is NOT the size of ovector in bytes.
1620    
1621           The first two-thirds of the vector is used to pass back  captured  sub-
1622           strings,  each  substring using a pair of integers. The remaining third
1623           of the vector is used as workspace by pcre_exec() while  matching  cap-
1624           turing  subpatterns, and is not available for passing back information.
1625           The length passed in ovecsize should always be a multiple of three.  If
1626           it is not, it is rounded down.
1627    
1628           When  a  match  is successful, information about captured substrings is
1629           returned in pairs of integers, starting at the  beginning  of  ovector,
1630           and  continuing  up  to two-thirds of its length at the most. The first
1631           element of a pair is set to the offset of the first character in a sub-
1632           string,  and  the  second  is  set to the offset of the first character
1633           after the end of a substring. The  first  pair,  ovector[0]  and  ovec-
1634           tor[1],  identify  the  portion  of  the  subject string matched by the
1635           entire pattern. The next pair is used for the first  capturing  subpat-
1636           tern,  and  so  on.  The value returned by pcre_exec() is the number of
1637           pairs that have been set. If there are no  capturing  subpatterns,  the
1638           return  value  from  a  successful match is 1, indicating that just the
1639           first pair of offsets has been set.
1640    
1641           Some convenience functions are provided  for  extracting  the  captured
1642           substrings  as  separate  strings. These are described in the following
1643           section.
1644    
1645           It is possible for an capturing subpattern number  n+1  to  match  some
1646           part  of  the  subject  when subpattern n has not been used at all. For
1647           example, if the string "abc" is matched against the pattern (a|(z))(bc)
1648           subpatterns  1 and 3 are matched, but 2 is not. When this happens, both
1649           offset values corresponding to the unused subpattern are set to -1.
1650    
1651           If a capturing subpattern is matched repeatedly, it is the last portion
1652           of the string that it matched that is returned.
1653    
1654           If  the vector is too small to hold all the captured substring offsets,
1655           it is used as far as possible (up to two-thirds of its length), and the
1656           function  returns a value of zero. In particular, if the substring off-
1657           sets are not of interest, pcre_exec() may be called with ovector passed
1658           as  NULL  and  ovecsize  as zero. However, if the pattern contains back
1659           references and the ovector is not big enough to  remember  the  related
1660           substrings,  PCRE has to get additional memory for use during matching.
1661           Thus it is usually advisable to supply an ovector.
1662    
1663           Note that pcre_info() can be used to find out how many  capturing  sub-
1664           patterns there are in a compiled pattern. The smallest size for ovector
1665           that will allow for n captured substrings, in addition to  the  offsets
1666           of the substring matched by the whole pattern, is (n+1)*3.
1667    
1668       Return values from pcre_exec()
1669    
1670           If  pcre_exec()  fails, it returns a negative number. The following are
1671           defined in the header file:
1672    
1673             PCRE_ERROR_NOMATCH        (-1)
1674    
1675           The subject string did not match the pattern.
1676    
1677             PCRE_ERROR_NULL           (-2)
1678    
1679           Either code or subject was passed as NULL,  or  ovector  was  NULL  and
1680           ovecsize was not zero.
1681    
1682             PCRE_ERROR_BADOPTION      (-3)
1683    
1684           An unrecognized bit was set in the options argument.
1685    
1686             PCRE_ERROR_BADMAGIC       (-4)
1687    
1688           PCRE  stores a 4-byte "magic number" at the start of the compiled code,
1689           to catch the case when it is passed a junk pointer and to detect when a
1690           pattern that was compiled in an environment of one endianness is run in
1691           an environment with the other endianness. This is the error  that  PCRE
1692           gives when the magic number is not present.
1693    
1694             PCRE_ERROR_UNKNOWN_NODE   (-5)
1695    
1696           While running the pattern match, an unknown item was encountered in the
1697           compiled pattern. This error could be caused by a bug  in  PCRE  or  by
1698           overwriting of the compiled pattern.
1699    
1700             PCRE_ERROR_NOMEMORY       (-6)
1701    
1702           If  a  pattern contains back references, but the ovector that is passed
1703           to pcre_exec() is not big enough to remember the referenced substrings,
1704           PCRE  gets  a  block of memory at the start of matching to use for this
1705           purpose. If the call via pcre_malloc() fails, this error is given.  The
1706           memory is automatically freed at the end of matching.
1707    
1708             PCRE_ERROR_NOSUBSTRING    (-7)
1709    
1710           This  error is used by the pcre_copy_substring(), pcre_get_substring(),
1711           and  pcre_get_substring_list()  functions  (see  below).  It  is  never
1712           returned by pcre_exec().
1713    
1714             PCRE_ERROR_MATCHLIMIT     (-8)
1715    
1716           The  recursion  and backtracking limit, as specified by the match_limit
1717           field in a pcre_extra structure (or defaulted)  was  reached.  See  the
1718           description above.
1719    
1720             PCRE_ERROR_CALLOUT        (-9)
1721    
1722           This error is never generated by pcre_exec() itself. It is provided for
1723           use by callout functions that want to yield a distinctive  error  code.
1724           See the pcrecallout documentation for details.
1725    
1726             PCRE_ERROR_BADUTF8        (-10)
1727    
1728           A  string  that contains an invalid UTF-8 byte sequence was passed as a
1729           subject.
1730    
1731             PCRE_ERROR_BADUTF8_OFFSET (-11)
1732    
1733           The UTF-8 byte sequence that was passed as a subject was valid, but the
1734           value  of startoffset did not point to the beginning of a UTF-8 charac-
1735           ter.
1736    
1737             PCRE_ERROR_PARTIAL        (-12)
1738    
1739           The subject string did not match, but it did match partially.  See  the
1740           pcrepartial documentation for details of partial matching.
1741    
1742             PCRE_ERROR_BADPARTIAL     (-13)
1743    
1744           The  PCRE_PARTIAL  option  was  used with a compiled pattern containing
1745           items that are not supported for partial matching. See the  pcrepartial
1746           documentation for details of partial matching.
1747    
1748             PCRE_ERROR_INTERNAL       (-14)
1749    
1750           An  unexpected  internal error has occurred. This error could be caused
1751           by a bug in PCRE or by overwriting of the compiled pattern.
1752    
1753             PCRE_ERROR_BADCOUNT       (-15)
1754    
1755           This error is given if the value of the ovecsize argument is  negative.
1756    
1757    
1758    EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
1759    
1760           int pcre_copy_substring(const char *subject, int *ovector,
1761                int stringcount, int stringnumber, char *buffer,
1762                int buffersize);
1763    
1764           int pcre_get_substring(const char *subject, int *ovector,
1765                int stringcount, int stringnumber,
1766                const char **stringptr);
1767    
1768           int pcre_get_substring_list(const char *subject,
1769                int *ovector, int stringcount, const char ***listptr);
1770    
1771           Captured  substrings  can  be  accessed  directly  by using the offsets
1772           returned by pcre_exec() in  ovector.  For  convenience,  the  functions
1773           pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
1774           string_list() are provided for extracting captured substrings  as  new,
1775           separate,  zero-terminated strings. These functions identify substrings
1776           by number. The next section describes functions  for  extracting  named
1777           substrings.  A  substring  that  contains  a  binary  zero is correctly
1778           extracted and has a further zero added on the end, but  the  result  is
1779           not, of course, a C string.
1780    
1781           The  first  three  arguments  are the same for all three of these func-
1782           tions: subject is the subject string that has  just  been  successfully
1783           matched, ovector is a pointer to the vector of integer offsets that was
1784           passed to pcre_exec(), and stringcount is the number of substrings that
1785           were  captured  by  the match, including the substring that matched the
1786           entire regular expression. This is the value returned by pcre_exec() if
1787           it  is greater than zero. If pcre_exec() returned zero, indicating that
1788           it ran out of space in ovector, the value passed as stringcount  should
1789           be the number of elements in the vector divided by three.
1790    
1791           The  functions pcre_copy_substring() and pcre_get_substring() extract a
1792           single substring, whose number is given as  stringnumber.  A  value  of
1793           zero  extracts  the  substring that matched the entire pattern, whereas
1794           higher values  extract  the  captured  substrings.  For  pcre_copy_sub-
1795           string(),  the  string  is  placed  in buffer, whose length is given by
1796           buffersize, while for pcre_get_substring() a new  block  of  memory  is
1797           obtained  via  pcre_malloc,  and its address is returned via stringptr.
1798           The yield of the function is the length of the  string,  not  including
1799           the terminating zero, or one of
1800    
1801             PCRE_ERROR_NOMEMORY       (-6)
1802    
1803           The  buffer  was too small for pcre_copy_substring(), or the attempt to
1804           get memory failed for pcre_get_substring().
1805    
1806             PCRE_ERROR_NOSUBSTRING    (-7)
1807    
1808           There is no substring whose number is stringnumber.
1809    
1810           The pcre_get_substring_list()  function  extracts  all  available  sub-
1811           strings  and  builds  a list of pointers to them. All this is done in a
1812           single block of memory that is obtained via pcre_malloc. The address of
1813           the  memory  block  is returned via listptr, which is also the start of
1814           the list of string pointers. The end of the list is marked  by  a  NULL
1815           pointer. The yield of the function is zero if all went well, or
1816    
1817             PCRE_ERROR_NOMEMORY       (-6)
1818    
1819           if the attempt to get the memory block failed.
1820    
1821           When  any of these functions encounter a substring that is unset, which
1822           can happen when capturing subpattern number n+1 matches  some  part  of
1823           the  subject, but subpattern n has not been used at all, they return an
1824           empty string. This can be distinguished from a genuine zero-length sub-
1825           string  by inspecting the appropriate offset in ovector, which is nega-
1826           tive for unset substrings.
1827    
1828           The two convenience functions pcre_free_substring() and  pcre_free_sub-
1829           string_list()  can  be  used  to free the memory returned by a previous
1830           call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
1831           tively.  They  do  nothing  more  than  call the function pointed to by
1832           pcre_free, which of course could be called directly from a  C  program.
1833           However,  PCRE is used in some situations where it is linked via a spe-
1834           cial  interface  to  another  programming  language  which  cannot  use
1835           pcre_free  directly;  it is for these cases that the functions are pro-
1836           vided.
1837    
1838    
1839    EXTRACTING CAPTURED SUBSTRINGS BY NAME
1840    
1841           int pcre_get_stringnumber(const pcre *code,
1842                const char *name);
1843    
1844           int pcre_copy_named_substring(const pcre *code,
1845                const char *subject, int *ovector,
1846                int stringcount, const char *stringname,
1847                char *buffer, int buffersize);
1848    
1849           int pcre_get_named_substring(const pcre *code,
1850                const char *subject, int *ovector,
1851                int stringcount, const char *stringname,
1852                const char **stringptr);
1853    
1854           To extract a substring by name, you first have to find associated  num-
1855           ber.  For example, for this pattern
1856    
1857             (a+)b(?P<xxx>\d+)...
1858    
1859           the number of the subpattern called "xxx" is 2. You can find the number
1860           from the name by calling pcre_get_stringnumber(). The first argument is
1861           the  compiled  pattern,  and  the  second is the name. The yield of the
1862           function is the subpattern number, or  PCRE_ERROR_NOSUBSTRING  (-7)  if
1863           there is no subpattern of that name.
1864    
1865           Given the number, you can extract the substring directly, or use one of
1866           the functions described in the previous section. For convenience, there
1867           are also two functions that do the whole job.
1868    
1869           Most    of    the    arguments   of   pcre_copy_named_substring()   and
1870           pcre_get_named_substring() are the same  as  those  for  the  similarly
1871           named  functions  that extract by number. As these are described in the
1872           previous section, they are not re-described here. There  are  just  two
1873           differences:
1874    
1875           First,  instead  of a substring number, a substring name is given. Sec-
1876           ond, there is an extra argument, given at the start, which is a pointer
1877           to  the compiled pattern. This is needed in order to gain access to the
1878           name-to-number translation table.
1879    
1880           These functions call pcre_get_stringnumber(), and if it succeeds,  they
1881           then  call  pcre_copy_substring() or pcre_get_substring(), as appropri-
1882           ate.
1883    
1884    
1885    FINDING ALL POSSIBLE MATCHES
1886    
1887           The traditional matching function uses a  similar  algorithm  to  Perl,
1888           which stops when it finds the first match, starting at a given point in
1889           the subject. If you want to find all possible matches, or  the  longest
1890           possible  match,  consider using the alternative matching function (see
1891           below) instead. If you cannot use the alternative function,  but  still
1892           need  to  find all possible matches, you can kludge it up by making use
1893           of the callout facility, which is described in the pcrecallout documen-
1894           tation.
1895    
1896           What you have to do is to insert a callout right at the end of the pat-
1897           tern.  When your callout function is called, extract and save the  cur-
1898           rent  matched  substring.  Then  return  1, which forces pcre_exec() to
1899           backtrack and try other alternatives. Ultimately, when it runs  out  of
1900           matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
1901    
1902    
1903    MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
1904    
1905           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
1906                const char *subject, int length, int startoffset,
1907                int options, int *ovector, int ovecsize,
1908                int *workspace, int wscount);
1909    
1910           The  function  pcre_dfa_exec()  is  called  to  match  a subject string
1911           against a compiled pattern, using a "DFA" matching algorithm. This  has
1912           different  characteristics to the normal algorithm, and is not compati-
1913           ble with Perl. Some of the features of PCRE patterns are not supported.
1914           Nevertheless, there are times when this kind of matching can be useful.
1915           For a discussion of the two matching algorithms, see  the  pcrematching
1916           documentation.
1917    
1918           The  arguments  for  the  pcre_dfa_exec()  function are the same as for
1919           pcre_exec(), plus two extras. The ovector argument is used in a differ-
1920           ent  way,  and  this is described below. The other common arguments are
1921           used in the same way as for pcre_exec(), so their  description  is  not
1922           repeated here.
1923    
1924           The  two  additional  arguments provide workspace for the function. The
1925           workspace vector should contain at least 20 elements. It  is  used  for
1926           keeping  track  of  multiple  paths  through  the  pattern  tree.  More
1927           workspace will be needed for patterns and subjects where  there  are  a
1928           lot of possible matches.
1929    
1930           Here is an example of a simple call to pcre_exec():
1931    
1932             int rc;
1933             int ovector[10];
1934             int wspace[20];
1935             rc = pcre_exec(
1936               re,             /* result of pcre_compile() */
1937               NULL,           /* we didn't study the pattern */
1938               "some string",  /* the subject string */
1939               11,             /* the length of the subject string */
1940               0,              /* start at offset 0 in the subject */
1941               0,              /* default options */
1942               ovector,        /* vector of integers for substring information */
1943               10,             /* number of elements (NOT size in bytes) */
1944               wspace,         /* working space vector */
1945               20);            /* number of elements (NOT size in bytes) */
1946    
1947       Option bits for pcre_dfa_exec()
1948    
1949           The  unused  bits  of  the options argument for pcre_dfa_exec() must be
1950           zero. The only bits that may be  set  are  PCRE_ANCHORED,  PCRE_NOTBOL,
1951           PCRE_NOTEOL,     PCRE_NOTEMPTY,    PCRE_NO_UTF8_CHECK,    PCRE_PARTIAL,
1952           PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All  but  the  last  three  of
1953           these  are  the  same  as  for pcre_exec(), so their description is not
1954           repeated here.
1955    
1956             PCRE_PARTIAL
1957    
1958           This has the same general effect as it does for  pcre_exec(),  but  the
1959           details   are   slightly   different.  When  PCRE_PARTIAL  is  set  for
1960           pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is  converted  into
1961           PCRE_ERROR_PARTIAL  if  the  end  of the subject is reached, there have
1962           been no complete matches, but there is still at least one matching pos-
1963           sibility.  The portion of the string that provided the partial match is
1964           set as the first matching string.
1965    
1966             PCRE_DFA_SHORTEST
1967    
1968           Setting the PCRE_DFA_SHORTEST option causes the matching  algorithm  to
1969           stop  as  soon  as  it  has found one match. Because of the way the DFA
1970           algorithm works, this is necessarily the shortest possible match at the
1971           first possible matching point in the subject string.
1972    
1973             PCRE_DFA_RESTART
1974    
1975           When  pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option, and
1976           returns a partial match, it is possible to call it  again,  with  addi-
1977           tional  subject  characters,  and have it continue with the same match.
1978           The PCRE_DFA_RESTART option requests this action; when it is  set,  the
1979           workspace  and wscount options must reference the same vector as before
1980           because data about the match so far is left in  them  after  a  partial
1981           match.  There  is  more  discussion of this facility in the pcrepartial
1982           documentation.
1983    
1984       Successful returns from pcre_dfa_exec()
1985    
1986           When pcre_dfa_exec() succeeds, it may have matched more than  one  sub-
1987           string in the subject. Note, however, that all the matches from one run
1988           of the function start at the same point in  the  subject.  The  shorter
1989           matches  are all initial substrings of the longer matches. For example,
1990           if the pattern
1991    
1992             <.*>
1993    
1994           is matched against the string
1995    
1996             This is <something> <something else> <something further> no more
1997    
1998           the three matched strings are
1999    
2000             <something>
2001             <something> <something else>
2002             <something> <something else> <something further>
2003    
2004           On success, the yield of the function is a number  greater  than  zero,
2005           which  is  the  number of matched substrings. The substrings themselves
2006           are returned in ovector. Each string uses two elements;  the  first  is
2007           the  offset  to the start, and the second is the offset to the end. All
2008           the strings have the same start offset. (Space could have been saved by
2009           giving  this only once, but it was decided to retain some compatibility
2010           with the way pcre_exec() returns data, even though the meaning  of  the
2011           strings is different.)
2012    
2013           The strings are returned in reverse order of length; that is, the long-
2014           est matching string is given first. If there were too many  matches  to
2015           fit  into ovector, the yield of the function is zero, and the vector is
2016           filled with the longest matches.
2017    
2018       Error returns from pcre_dfa_exec()
2019    
2020           The pcre_dfa_exec() function returns a negative number when  it  fails.
2021           Many  of  the  errors  are  the  same as for pcre_exec(), and these are
2022           described above.  There are in addition the following errors  that  are
2023           specific to pcre_dfa_exec():
2024    
2025             PCRE_ERROR_DFA_UITEM      (-16)
2026    
2027           This  return is given if pcre_dfa_exec() encounters an item in the pat-
2028           tern that it does not support, for instance, the use of \C  or  a  back
2029           reference.
2030    
2031             PCRE_ERROR_DFA_UCOND      (-17)
2032    
2033           This  return is given if pcre_dfa_exec() encounters a condition item in
2034           a pattern that uses a back reference for the  condition.  This  is  not
2035           supported.
2036    
2037             PCRE_ERROR_DFA_UMLIMIT    (-18)
2038    
2039           This  return  is given if pcre_dfa_exec() is called with an extra block
2040           that contains a setting of the match_limit field. This is not supported
2041           (it is meaningless).
2042    
2043             PCRE_ERROR_DFA_WSSIZE     (-19)
2044    
2045           This  return  is  given  if  pcre_dfa_exec()  runs  out of space in the
2046           workspace vector.
2047    
2048             PCRE_ERROR_DFA_RECURSE    (-20)
2049    
2050           When a recursive subpattern is processed, the matching  function  calls
2051           itself  recursively,  using  private vectors for ovector and workspace.
2052           This error is given if the output vector  is  not  large  enough.  This
2053           should be extremely rare, as a vector of size 1000 is used.
2054    
2055    Last updated: 16 May 2005
2056    Copyright (c) 1997-2005 University of Cambridge.
2057    ------------------------------------------------------------------------------
2058    
 MATCHING A PATTERN  
      The function pcre_exec() is called to match a subject string  
2059    
2060    PCRECALLOUT(3)                                                  PCRECALLOUT(3)
2061    
2062    
2063    NAME
2064           PCRE - Perl-compatible regular expressions
2065    
2066    
2067  SunOS 5.8                 Last change:                          9  PCRE CALLOUTS
2068    
2069           int (*pcre_callout)(pcre_callout_block *);
2070    
2071           PCRE provides a feature called "callout", which is a means of temporar-
2072           ily passing control to the caller of PCRE  in  the  middle  of  pattern
2073           matching.  The  caller of PCRE provides an external function by putting
2074           its entry point in the global variable pcre_callout. By  default,  this
2075           variable contains NULL, which disables all calling out.
2076    
2077           Within  a  regular  expression,  (?C) indicates the points at which the
2078           external function is to be called.  Different  callout  points  can  be
2079           identified  by  putting  a number less than 256 after the letter C. The
2080           default value is zero.  For  example,  this  pattern  has  two  callout
2081           points:
2082    
2083             (?C1)eabc(?C2)def
2084    
2085           If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is
2086           called, PCRE automatically  inserts  callouts,  all  with  number  255,
2087           before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2088           used with the pattern
2089    
2090             A(\d{2}|--)
2091    
2092           it is processed as if it were
2093    
2094           (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2095    
2096           Notice that there is a callout before and after  each  parenthesis  and
2097           alternation  bar.  Automatic  callouts  can  be  used  for tracking the
2098           progress of pattern matching. The pcretest command has an  option  that
2099           sets  automatic callouts; when it is used, the output indicates how the
2100           pattern is matched. This is useful information when you are  trying  to
2101           optimize the performance of a particular pattern.
2102    
2103    
2104    MISSING CALLOUTS
2105    
2106           You  should  be  aware  that,  because of optimizations in the way PCRE
2107           matches patterns, callouts sometimes do not happen. For example, if the
2108           pattern is
2109    
2110             ab(?C4)cd
2111    
2112           PCRE knows that any matching string must contain the letter "d". If the
2113           subject string is "abyz", the lack of "d" means that  matching  doesn't
2114           ever  start,  and  the  callout is never reached. However, with "abyd",
2115           though the result is still no match, the callout is obeyed.
2116    
2117    
2118    THE CALLOUT INTERFACE
2119    
2120           During matching, when PCRE reaches a callout point, the external  func-
2121           tion  defined by pcre_callout is called (if it is set). This applies to
2122           both the pcre_exec() and the pcre_dfa_exec()  matching  functions.  The
2123           only  argument  to  the callout function is a pointer to a pcre_callout
2124           block. This structure contains the following fields:
2125    
2126             int          version;
2127             int          callout_number;
2128             int         *offset_vector;
2129             const char  *subject;
2130             int          subject_length;
2131             int          start_match;
2132             int          current_position;
2133             int          capture_top;
2134             int          capture_last;
2135             void        *callout_data;
2136             int          pattern_position;
2137             int          next_item_length;
2138    
2139           The version field is an integer containing the version  number  of  the
2140           block  format. The initial version was 0; the current version is 1. The
2141           version number will change again in future  if  additional  fields  are
2142           added, but the intention is never to remove any of the existing fields.
2143    
2144           The callout_number field contains the number of the  callout,  as  com-
2145           piled  into  the pattern (that is, the number after ?C for manual call-
2146           outs, and 255 for automatically generated callouts).
2147    
2148           The offset_vector field is a pointer to the vector of offsets that  was
2149           passed   by   the   caller  to  pcre_exec()  or  pcre_dfa_exec().  When
2150           pcre_exec() is used, the contents can be inspected in order to  extract
2151           substrings  that  have  been  matched  so  far,  in the same way as for
2152           extracting substrings after a match has completed. For  pcre_dfa_exec()
2153           this field is not useful.
2154    
2155           The subject and subject_length fields contain copies of the values that
2156           were passed to pcre_exec().
2157    
2158           The start_match field contains the offset within the subject  at  which
2159           the  current match attempt started. If the pattern is not anchored, the
2160           callout function may be called several times from the same point in the
2161           pattern for different starting points in the subject.
2162    
2163           The  current_position  field  contains the offset within the subject of
2164           the current match pointer.
2165    
2166           When the pcre_exec() function is used, the capture_top  field  contains
2167           one  more than the number of the highest numbered captured substring so
2168           far. If no substrings have been captured, the value of  capture_top  is
2169           one.  This  is always the case when pcre_dfa_exec() is used, because it
2170           does not support captured substrings.
2171    
2172           The capture_last field contains the number of the  most  recently  cap-
2173           tured  substring. If no substrings have been captured, its value is -1.
2174           This is always the case when pcre_dfa_exec() is used.
2175    
2176           The callout_data field contains a value that is passed  to  pcre_exec()
2177           or  pcre_dfa_exec() specifically so that it can be passed back in call-
2178           outs. It is passed in the pcre_callout field  of  the  pcre_extra  data
2179           structure.  If  no such data was passed, the value of callout_data in a
2180           pcre_callout block is NULL. There is a description  of  the  pcre_extra
2181           structure in the pcreapi documentation.
2182    
2183           The  pattern_position field is present from version 1 of the pcre_call-
2184           out structure. It contains the offset to the next item to be matched in
2185           the pattern string.
2186    
2187           The  next_item_length field is present from version 1 of the pcre_call-
2188           out structure. It contains the length of the next item to be matched in
2189           the  pattern  string. When the callout immediately precedes an alterna-
2190           tion bar, a closing parenthesis, or the end of the pattern, the  length
2191           is  zero.  When the callout precedes an opening parenthesis, the length
2192           is that of the entire subpattern.
2193    
2194           The pattern_position and next_item_length fields are intended  to  help
2195           in  distinguishing between different automatic callouts, which all have
2196           the same callout number. However, they are set for all callouts.
2197    
2198    
2199    RETURN VALUES
2200    
2201           The external callout function returns an integer to PCRE. If the  value
2202           is  zero,  matching  proceeds  as  normal. If the value is greater than
2203           zero, matching fails at the current point, but  the  testing  of  other
2204           matching possibilities goes ahead, just as if a lookahead assertion had
2205           failed. If the value is less than zero, the  match  is  abandoned,  and
2206           pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2207    
2208           Negative   values   should   normally   be   chosen  from  the  set  of
2209           PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2210           dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
2211           reserved for use by callout functions; it will never be  used  by  PCRE
2212           itself.
2213    
2214    Last updated: 28 February 2005
2215    Copyright (c) 1997-2005 University of Cambridge.
2216    ------------------------------------------------------------------------------
2217    
      against  a pre-compiled pattern, which is passed in the code  
      argument. If the pattern has been studied, the result of the  
      study should be passed in the extra argument. Otherwise this  
      must be NULL.  
   
      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 for substring information */  
          30);            /* number of elements in the vector */  
   
      The PCRE_ANCHORED option can be passed in the options  argu-  
      ment,  whose unused bits must be zero. However, 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.  
   
      There are also three further options that can be set only at  
      matching time:  
   
        PCRE_NOTBOL  
   
      The first character of the 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.  
   
        PCRE_NOTEOL  
   
      The end of the string is not the end of a line, so the  dol-  
      lar  metacharacter should not match it nor (except in multi-  
      line mode) a newline immediately  before  it.  Setting  this  
      without PCRE_MULTILINE (at compile time) causes dollar never  
      to match.  
   
        PCRE_NOTEMPTY  
   
      An empty string is not considered to be  a  valid  match  if  
      this  option  is  set. If there are alternatives in the pat-  
      tern, 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  the  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 occurrences of "a" or "b".  
   
      Perl has no direct equivalent of PCRE_NOTEMPTY, 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  
      offset  with  PCRE_NOTEMPTY  set,  and then if that fails by  
      advancing the starting offset  (see  below)  and  trying  an  
      ordinary match again.  
   
      The subject string is passed as  a  pointer  in  subject,  a  
      length  in  length,  and  a  starting offset in startoffset.  
      Unlike the pattern string, the subject  may  contain  binary  
      zero  characters.  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.  
   
      A non-zero starting offset  is  useful  when  searching  for  
      another  match  in  the  same subject by calling pcre_exec()  
      again after a previous success.  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  occurrence  of "iss" because it is able to look  
      behind the starting point to discover that it is preceded by  
      a letter.  
   
      If a non-zero starting offset is passed when the pattern  is  
      anchored, one attempt to match at the given offset is tried.  
      This can only succeed if the pattern does  not  require  the  
      match to be at the start of the subject.  
   
      In general, a pattern matches a certain portion of the  sub-  
      ject,  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 substring.  
      PCRE supports several other kinds of  parenthesized  subpat-  
      tern that do not cause substrings to be captured.  
   
      Captured substrings are returned to the caller via a  vector  
      of  integer  offsets whose address is passed in ovector. The  
      number of elements in the vector is passed in ovecsize.  The  
      first two-thirds of the vector is used to pass back captured  
      substrings, each substring using a  pair  of  integers.  The  
      remaining  third  of  the  vector  is  used  as workspace by  
      pcre_exec() while matching capturing subpatterns, and is not  
      available for passing back information. The length passed in  
      ovecsize should always be a multiple of three. If it is not,  
      it is rounded down.  
   
      When a match has been 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 a pair is set to  
      the offset of the first character in a  substring,  and  the  
      second is set to the offset of the first character after the  
      end of a substring. The first  pair,  ovector[0]  and  ovec-  
      tor[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 the number of pairs that have  been  set.  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.  
   
      Some convenience functions are provided for  extracting  the  
      captured substrings as separate strings. These are described  
      in the following section.  
   
      It is possible for an 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) subpatterns 1 and 3  
      are matched, but 2 is not. When this  happens,  both  offset  
      values corresponding to the unused subpattern are set to -1.  
   
      If a capturing subpattern is matched repeatedly, it  is  the  
      last  portion  of  the  string  that  it  matched  that gets  
      returned.  
   
      If the vector is too small to hold  all  the  captured  sub-  
      strings,  it is used as far as possible (up to two-thirds of  
      its length), and the function returns a value  of  zero.  In  
      particular,  if  the  substring offsets are not of interest,  
      pcre_exec() may be called with ovector passed  as  NULL  and  
      ovecsize  as  zero.  However,  if  the pattern contains back  
      references and the ovector isn't 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.  
   
      Note that pcre_info() can be used to find out how many  cap-  
      turing  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.  
   
      If pcre_exec() fails, it returns a negative number. The fol-  
      lowing 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 com-  
      piled  code,  to  catch  the  case  when it is passed a junk  
      pointer. This is the error it gives when  the  magic  number  
      isn't present.  
   
        PCRE_ERROR_UNKNOWN_NODE   (-5)  
   
      While running the pattern match, an unknown item was encoun-  
      tered 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  
      freed at the end of matching.  
   
   
   
   
 EXTRACTING CAPTURED SUBSTRINGS  
      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_substring_list()  are  provided for extracting  
      captured  substrings  as  new,   separate,   zero-terminated  
      strings.   A  substring  that  contains  a  binary  zero  is  
      correctly extracted and has a further zero added on the end,  
      but the result does not, of course, function as a C string.  
   
      The first three arguments are the same for all  three  func-  
      tions:  subject  is  the  subject string which 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  ovec-  
      tor,  the  value passed as stringcount should be the size of  
      the vector divided by three.  
   
      The functions pcre_copy_substring() and pcre_get_substring()  
      extract a single substring, whose number is given as string-  
      number. A value of zero extracts the substring that  matched  
      the entire pattern, while higher values extract the captured  
      substrings. For pcre_copy_substring(), 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  
   
        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  avail-  
      able  substrings  and builds a list of pointers to them. All  
      this is done in a single block of memory which  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  
   
        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  substring   by  
      inspecting the appropriate offset in ovector, which is nega-  
      tive for unset substrings.  
   
      The  two  convenience  functions  pcre_free_substring()  and  
      pcre_free_substring_list()  can  be  used to free the memory  
      returned by  a  previous  call  of  pcre_get_substring()  or  
      pcre_get_substring_list(),  respectively.  They  do  nothing  
      more than call the function pointed to by  pcre_free,  which  
      of  course  could  be called directly from a C program. How-  
      ever, PCRE is used in some situations where it is linked via  
      a  special  interface  to another programming language which  
      cannot use pcre_free directly; it is for  these  cases  that  
      the functions are provided.  
2218    
2219    PCRECOMPAT(3)                                                    PCRECOMPAT(3)
2220    
2221    
2222  LIMITATIONS  NAME
2223       There are some size limitations in PCRE but it is hoped that         PCRE - Perl-compatible regular expressions
2224       they will never in practice be relevant.  The maximum length  
2225       of a compiled pattern is 65539 (sic) bytes.  All  values  in  
2226       repeating  quantifiers  must be less than 65536.  There max-  DIFFERENCES BETWEEN PCRE AND PERL
2227       imum number of capturing subpatterns is 65535.  There is  no  
2228       limit  to  the  number of non-capturing subpatterns, but the         This  document describes the differences in the ways that PCRE and Perl
2229       maximum depth of nesting of all kinds of parenthesized  sub-         handle regular expressions. The differences  described  here  are  with
2230       pattern,  including  capturing  subpatterns, assertions, and         respect to Perl 5.8.
2231       other types of subpattern, is 200.  
2232           1.  PCRE does not have full UTF-8 support. Details of what it does have
2233       The maximum length of a subject string is the largest  posi-         are given in the section on UTF-8 support in the main pcre page.
2234       tive number that an integer variable can hold. However, PCRE  
2235       uses recursion to handle subpatterns and indefinite  repeti-         2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2236       tion.  This  means  that the available stack space may limit         permits  them,  but they do not mean what you might think. For example,
2237       the size of a subject string that can be processed  by  cer-         (?!a){3} does not assert that the next three characters are not "a". It
2238       tain patterns.         just asserts that the next character is not "a" three times.
2239    
2240           3.  Capturing  subpatterns  that occur inside negative lookahead asser-
2241           tions are counted, but their entries in the offsets  vector  are  never
2242  DIFFERENCES FROM PERL         set.  Perl sets its numerical variables from any such patterns that are
2243       The differences described here  are  with  respect  to  Perl         matched before the assertion fails to match something (thereby succeed-
2244       5.005.         ing),  but  only  if the negative lookahead assertion contains just one
2245           branch.
2246       1. By default, a whitespace character is any character  that  
2247       the  C  library  function isspace() recognizes, though it is         4. Though binary zero characters are supported in the  subject  string,
2248       possible to compile PCRE  with  alternative  character  type         they are not allowed in a pattern string because it is passed as a nor-
2249       tables. Normally isspace() matches space, formfeed, newline,         mal C string, terminated by zero. The escape sequence \0 can be used in
2250       carriage return, horizontal tab, and vertical tab. Perl 5 no         the pattern to represent a binary zero.
2251       longer  includes vertical tab in its set of whitespace char-  
2252       acters. The \v escape that was in the Perl documentation for         5.  The  following Perl escape sequences are not supported: \l, \u, \L,
2253       a long time was never in fact recognized. However, the char-         \U, and \N. In fact these are implemented by Perl's general string-han-
2254       acter itself was treated as whitespace at least up to 5.002.         dling  and are not part of its pattern matching engine. If any of these
2255       In 5.004 and 5.005 it does not match \s.         are encountered by PCRE, an error is generated.
2256    
2257       2. PCRE does  not  allow  repeat  quantifiers  on  lookahead         6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE
2258       assertions. Perl permits them, but they do not mean what you         is  built  with Unicode character property support. The properties that
2259       might think. For example, (?!a){3} does not assert that  the         can be tested with \p and \P are limited to the general category  prop-
2260       next  three characters are not "a". It just asserts that the         erties such as Lu and Nd.
2261       next character is not "a" three times.  
2262           7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2263       3. Capturing subpatterns that occur inside  negative  looka-         ters in between are treated as literals.  This  is  slightly  different
2264       head  assertions  are  counted,  but  their  entries  in the         from  Perl  in  that  $  and  @ are also handled as literals inside the
2265       offsets vector are never set. Perl sets its numerical  vari-         quotes. In Perl, they cause variable interpolation (but of course  PCRE
2266       ables  from  any  such  patterns that are matched before the         does not have variables). Note the following examples:
2267       assertion fails to match something (thereby succeeding), but  
2268       only  if  the negative lookahead assertion contains just one             Pattern            PCRE matches      Perl matches
2269       branch.  
2270               \Qabc$xyz\E        abc$xyz           abc followed by the
2271       4. Though binary zero characters are supported in  the  sub-                                                    contents of $xyz
2272       ject  string,  they  are  not  allowed  in  a pattern string             \Qabc\$xyz\E       abc\$xyz          abc\$xyz
2273       because it is passed as a normal  C  string,  terminated  by             \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
2274       zero. The escape sequence "\0" can be used in the pattern to  
2275       represent a binary zero.         The  \Q...\E  sequence  is recognized both inside and outside character
2276           classes.
2277       5. The following Perl escape sequences  are  not  supported:  
2278       \l,  \u,  \L,  \U,  \E, \Q. In fact these are implemented by         8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
2279       Perl's general string-handling and are not part of its  pat-         constructions.  However,  there is support for recursive patterns using
2280       tern matching engine.         the non-Perl items (?R),  (?number),  and  (?P>name).  Also,  the  PCRE
2281           "callout"  feature allows an external function to be called during pat-
2282       6. The Perl \G assertion is  not  supported  as  it  is  not         tern matching. See the pcrecallout documentation for details.
2283       relevant to single pattern matches.  
2284           9. There are some differences that are concerned with the  settings  of
2285       7. Fairly obviously, PCRE does not support the (?{code}) and         captured  strings  when  part  of  a  pattern is repeated. For example,
2286       (?p{code})  constructions. However, there is some experimen-         matching "aba" against the  pattern  /^(a(b)?)+$/  in  Perl  leaves  $2
2287       tal support for recursive patterns using the  non-Perl  item         unset, but in PCRE it is set to "b".
2288       (?R).  
2289           10. PCRE provides some extensions to the Perl regular expression facil-
2290       8. There are at the time of writing some  oddities  in  Perl         ities:
      5.005_02  concerned  with  the  settings of captured strings  
      when part of a pattern is repeated.  For  example,  matching  
      "aba"  against the pattern /^(a(b)?)+$/ sets $2 to the value  
      "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves  $2  
      unset.    However,    if   the   pattern   is   changed   to  
      /^(aa(b(b))?)+$/ then $2 (and $3) are set.  
   
      In Perl 5.004 $2 is set in both cases, and that is also true  
      of PCRE. If in the future Perl changes to a consistent state  
      that is different, PCRE may change to follow.  
   
      9. Another as yet unresolved discrepancy  is  that  in  Perl  
      5.005_02  the  pattern /^(a)?(?(1)a|b)+$/ matches the string  
      "a", whereas in PCRE it does not.  However, in both Perl and  
      PCRE /^(a)?a/ matched against "a" leaves $1 unset.  
   
      10. PCRE  provides  some  extensions  to  the  Perl  regular  
      expression facilities:  
   
      (a) Although lookbehind assertions must match  fixed  length  
      strings,  each  alternative branch of a lookbehind assertion  
      can match a different length of string. Perl 5.005  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 special meaning is faulted.  
   
      (d) If PCRE_UNGREEDY is set, the greediness of  the  repeti-  
      tion  quantifiers  is inverted, that is, by default they are  
      not greedy, but if followed by a question mark they are.  
   
      (e) PCRE_ANCHORED can be used to force a pattern to be tried  
      only at the start of the subject.  
   
      (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY  options  
      for pcre_exec() have no Perl equivalents.  
   
      (g) The (?R) construct allows for recursive pattern matching  
      (Perl  5.6 can do this using the (?p{code}) construct, which  
      PCRE cannot of course support.)  
   
   
   
 REGULAR EXPRESSION DETAILS  
      The syntax and semantics of  the  regular  expressions  sup-  
      ported  by PCRE are described below. Regular expressions are  
      also described in the Perl documentation and in a number  of  
      other  books,  some  of which have copious examples. Jeffrey  
      Friedl's  "Mastering  Regular  Expressions",  published   by  
      O'Reilly (ISBN 1-56592-257), covers them in great detail.  
   
      The description here is intended as reference documentation.  
      The basic operation of PCRE is on strings of bytes. However,  
      there is the beginnings of some support for UTF-8  character  
      strings.  To  use  this  support  you must configure PCRE to  
      include it, and then call pcre_compile() with the  PCRE_UTF8  
      option.  How  this affects the pattern matching is described  
      in the final section of this document.  
   
      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 charac-  
      ters 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.  The  power  of  regular  expressions comes from the  
      ability to include alternatives and repetitions in the  pat-  
      tern.  These  are encoded in the pattern by the use of meta-  
      characters, which do not stand for  themselves  but  instead  
      are interpreted in some special way.  
   
      There are two different sets of meta-characters: those  that  
      are  recognized anywhere in the pattern except within square  
      brackets, and those that are recognized in square  brackets.  
      Outside square brackets, the meta-characters are as follows:  
   
        \      general escape character with several uses  
        ^      assert start of  subject  (or  line,  in  multiline  
      mode)  
        $      assert end of subject (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  
        {      start min/max quantifier  
   
      Part of a pattern that is in square  brackets  is  called  a  
      "character  class".  In  a  character  class  the only meta-  
      characters are:  
   
        \      general escape character  
        ^      negate the class, but only if the first character  
        -      indicates character range  
        ]      terminates the character class  
2291    
2292       The following sections describe  the  use  of  each  of  the         (a) Although lookbehind assertions must  match  fixed  length  strings,
2293       meta-characters.         each alternative branch of a lookbehind assertion can match a different
2294           length of string. Perl requires them all to have the same length.
2295    
2296           (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the  $
2297           meta-character matches only at the very end of the string.
2298    
2299           (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2300           cial meaning is faulted.
2301    
2302           (d) If PCRE_UNGREEDY is set, the greediness of the  repetition  quanti-
2303           fiers is inverted, that is, by default they are not greedy, but if fol-
2304           lowed by a question mark they are.
2305    
2306           (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2307           tried only at the first matching position in the subject string.
2308    
2309           (f)  The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2310           TURE options for pcre_exec() have no Perl equivalents.
2311    
2312           (g) The (?R), (?number), and (?P>name) constructs allows for  recursive
2313           pattern  matching  (Perl  can  do  this using the (?p{code}) construct,
2314           which PCRE cannot support.)
2315    
2316           (h) PCRE supports named capturing substrings, using the Python  syntax.
2317    
2318           (i)  PCRE  supports  the  possessive quantifier "++" syntax, taken from
2319           Sun's Java package.
2320    
2321           (j) The (R) condition, for testing recursion, is a PCRE extension.
2322    
2323           (k) The callout facility is PCRE-specific.
2324    
2325           (l) The partial matching facility is PCRE-specific.
2326    
2327           (m) Patterns compiled by PCRE can be saved and re-used at a later time,
2328           even on different hosts that have the other endianness.
2329    
2330           (n)  The  alternative  matching function (pcre_dfa_exec()) matches in a
2331           different way and is not Perl-compatible.
2332    
2333    Last updated: 28 February 2005
2334    Copyright (c) 1997-2005 University of Cambridge.
2335    ------------------------------------------------------------------------------
2336    
2337    
2338    PCREPATTERN(3)                                                  PCREPATTERN(3)
2339    
2340    
2341    NAME
2342           PCRE - Perl-compatible regular expressions
2343    
2344    
2345    PCRE REGULAR EXPRESSION DETAILS
2346    
2347           The  syntax  and semantics of the regular expressions supported by PCRE
2348           are described below. Regular expressions are also described in the Perl
2349           documentation  and  in  a  number  of books, some of which have copious
2350           examples.  Jeffrey Friedl's "Mastering Regular Expressions",  published
2351           by  O'Reilly, covers regular expressions in great detail. This descrip-
2352           tion of PCRE's regular expressions is intended as reference material.
2353    
2354           The original operation of PCRE was on strings of  one-byte  characters.
2355           However,  there is now also support for UTF-8 character strings. To use
2356           this, you must build PCRE to  include  UTF-8  support,  and  then  call
2357           pcre_compile()  with  the  PCRE_UTF8  option.  How this affects pattern
2358           matching is mentioned in several places below. There is also a  summary
2359           of  UTF-8  features  in  the  section on UTF-8 support in the main pcre
2360           page.
2361    
2362           The remainder of this document discusses the  patterns  that  are  sup-
2363           ported  by  PCRE when its main matching function, pcre_exec(), is used.
2364           From  release  6.0,   PCRE   offers   a   second   matching   function,
2365           pcre_dfa_exec(),  which matches using a different algorithm that is not
2366           Perl-compatible. The advantages and disadvantages  of  the  alternative
2367           function, and how it differs from the normal function, are discussed in
2368           the pcrematching page.
2369    
2370           A regular expression is a pattern that is  matched  against  a  subject
2371           string  from  left  to right. Most characters stand for themselves in a
2372           pattern, and match the corresponding characters in the  subject.  As  a
2373           trivial example, the pattern
2374    
2375             The quick brown fox
2376    
2377           matches a portion of a subject string that is identical to itself. When
2378           caseless matching is specified (the PCRE_CASELESS option), letters  are
2379           matched  independently  of case. In UTF-8 mode, PCRE always understands
2380           the concept of case for characters whose values are less than  128,  so
2381           caseless  matching  is always possible. For characters with higher val-
2382           ues, the concept of case is supported if PCRE is compiled with  Unicode
2383           property  support,  but  not  otherwise.   If  you want to use caseless
2384           matching for characters 128 and above, you must  ensure  that  PCRE  is
2385           compiled with Unicode property support as well as with UTF-8 support.
2386    
2387           The  power  of  regular  expressions  comes from the ability to include
2388           alternatives and repetitions in the pattern. These are encoded  in  the
2389           pattern by the use of metacharacters, which do not stand for themselves
2390           but instead are interpreted in some special way.
2391    
2392           There are two different sets of metacharacters: those that  are  recog-
2393           nized  anywhere in the pattern except within square brackets, and those
2394           that are recognized in square brackets. Outside  square  brackets,  the
2395           metacharacters are as follows:
2396    
2397             \      general escape character with several uses
2398             ^      assert start of string (or line, in multiline mode)
2399             $      assert end of string (or line, in multiline mode)
2400             .      match any character except newline (by default)
2401             [      start character class definition
2402             |      start of alternative branch
2403             (      start subpattern
2404             )      end subpattern
2405             ?      extends the meaning of (
2406                    also 0 or 1 quantifier
2407                    also quantifier minimizer
2408             *      0 or more quantifier
2409             +      1 or more quantifier
2410                    also "possessive quantifier"
2411             {      start min/max quantifier
2412    
2413           Part  of  a  pattern  that is in square brackets is called a "character
2414           class". In a character class the only metacharacters are:
2415    
2416             \      general escape character
2417             ^      negate the class, but only if the first character
2418             -      indicates character range
2419             [      POSIX character class (only if followed by POSIX
2420                      syntax)
2421             ]      terminates the character class
2422    
2423           The following sections describe the use of each of the  metacharacters.
2424    
2425    
2426  BACKSLASH  BACKSLASH
      The backslash character has several uses. Firstly, if it  is  
      followed  by  a  non-alphameric character, 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 applies whether or not the follow-  
      ing character would otherwise  be  interpreted  as  a  meta-  
      character,  so it is always safe to precede a non-alphameric  
      with "\" to specify that it stands for itself.  In  particu-  
      lar, if you want to match a backslash, you write "\\".  
   
      If a pattern is compiled with the PCRE_EXTENDED option, whi-  
      tespace in the pattern (other than in a character class) and  
      characters between a "#" outside a character class  and  the  
      next  newline  character  are ignored. An escaping backslash  
      can be used to include a whitespace or "#" character as part  
      of the pattern.  
   
      A second use of backslash provides a way  of  encoding  non-  
      printing  characters  in patterns in a visible manner. There  
      is no restriction on the appearance of non-printing  charac-  
      ters,  apart from the binary zero that terminates a pattern,  
      but when a pattern is being prepared by text editing, it  is  
      usually  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 character  
        \e     escape (hex 1B)  
        \f     formfeed (hex 0C)  
        \n     newline (hex 0A)  
        \r     carriage return (hex 0D)  
        \t     tab (hex 09)  
        \xhh   character with hex code hh  
        \ddd   character with octal code ddd, or backreference  
   
      The precise effect of "\cx" is as follows: if "x" is a lower  
      case  letter,  it  is converted to upper case. Then bit 6 of  
      the character (hex 40) is inverted.  Thus "\cz" becomes  hex  
      1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B.  
   
      After "\x", up to two hexadecimal digits are  read  (letters  
      can be in upper or lower case).  
   
      After "\0" up to two further octal digits are read. In  both  
      cases,  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.  Make sure you  
      supply two digits after the initial zero  if  the  character  
      that follows is itself an octal digit.  
   
      The handling of a backslash followed by a digit other than 0  
      is  complicated.   Outside  a character class, PCRE reads it  
      and any following digits 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  follow-  
      ing  the  backslash,  and  generates  a single byte from the  
      least significant 8 bits of the value. Any subsequent digits  
      stand for themselves.  For example:  
   
        \040   is another way of writing a space  
        \40    is the same, provided there are fewer than 40  
                  previous capturing subpatterns  
        \7     is always a back reference  
        \11    might be a back reference, or another way of  
                  writing a tab  
        \011   is always a tab  
        \0113  is a tab followed by the character "3"  
        \113   is the character with octal code 113 (since there  
                  can be no more than 99 back references)  
        \377   is a byte consisting entirely of 1 bits  
        \81    is either a back reference, or a binary zero  
                  followed by the two characters "8" and "1"  
   
      Note that octal values of 100 or greater must not be  intro-  
      duced  by  a  leading zero, because no more than three octal  
      digits are ever read.  
   
      All the sequences that define a single  byte  value  can  be  
      used both inside and outside character classes. In addition,  
      inside a character class, the sequence "\b"  is  interpreted  
      as  the  backspace  character  (hex 08). Outside a character  
      class it has a different meaning (see below).  
   
      The third use of backslash is for specifying generic charac-  
      ter types:  
   
        \d     any decimal digit  
        \D     any character that is not a decimal digit  
        \s     any whitespace character  
        \S     any character that is not a whitespace character  
        \w     any "word" character  
        \W     any "non-word" character  
   
      Each pair of escape sequences partitions the complete set of  
      characters  into  two  disjoint  sets.  Any  given character  
      matches one, and only one, of each pair.  
   
      A "word" character is any letter or digit or the  underscore  
      character,  that  is,  any  character which can be part of a  
      Perl "word". The definition of letters and  digits  is  con-  
      trolled  by PCRE's character tables, and may vary if locale-  
      specific matching is  taking  place  (see  "Locale  support"  
      above). For example, in the "fr" (French) locale, some char-  
      acter codes greater than 128 are used for accented  letters,  
      and these are matched by \w.  
   
      These character type 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, since there  
      is no character to match.  
   
      The fourth use of backslash is  for  certain  simple  asser-  
      tions. An assertion 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     word boundary  
        \B     not a word boundary  
        \A     start of subject (independent of multiline mode)  
        \Z     end of subject or newline at  end  (independent  of  
      multiline mode)  
        \z     end of subject (independent of multiline mode)  
   
      These assertions may not appear in  character  classes  (but  
      note that "\b" has a different meaning, namely the backspace  
      character, inside a character class).  
   
      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.  
   
      The \A, \Z, and \z assertions differ  from  the  traditional  
      circumflex  and  dollar  (described below) in that they only  
      ever match at the very start and end of the subject  string,  
      whatever  options  are  set.  They  are  not affected by the  
      PCRE_NOTBOL or PCRE_NOTEOL options. If the startoffset argu-  
      ment  of  pcre_exec()  is  non-zero, \A can never match. The  
      difference between \Z and \z is that  \Z  matches  before  a  
      newline  that is the last character of the string as well as  
      at the end of the string, whereas \z  matches  only  at  the  
      end.  
2427    
2428           The backslash character has several uses. Firstly, if it is followed by
2429           a non-alphanumeric character, it takes away any  special  meaning  that
2430           character  may  have.  This  use  of  backslash  as an escape character
2431           applies both inside and outside character classes.
2432    
2433           For example, if you want to match a * character, you write  \*  in  the
2434           pattern.   This  escaping  action  applies whether or not the following
2435           character would otherwise be interpreted as a metacharacter, so  it  is
2436           always  safe  to  precede  a non-alphanumeric with backslash to specify
2437           that it stands for itself. In particular, if you want to match a  back-
2438           slash, you write \\.
2439    
2440           If  a  pattern is compiled with the PCRE_EXTENDED option, whitespace in
2441           the pattern (other than in a character class) and characters between  a
2442           # outside a character class and the next newline character are ignored.
2443           An escaping backslash can be used to include a whitespace or #  charac-
2444           ter as part of the pattern.
2445    
2446           If  you  want  to remove the special meaning from a sequence of charac-
2447           ters, you can do so by putting them between \Q and \E. This is  differ-
2448           ent  from  Perl  in  that  $  and  @ are handled as literals in \Q...\E
2449           sequences in PCRE, whereas in Perl, $ and @ cause  variable  interpola-
2450           tion. Note the following examples:
2451    
2452             Pattern            PCRE matches   Perl matches
2453    
2454             \Qabc$xyz\E        abc$xyz        abc followed by the
2455                                                 contents of $xyz
2456             \Qabc\$xyz\E       abc\$xyz       abc\$xyz
2457             \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
2458    
2459           The  \Q...\E  sequence  is recognized both inside and outside character
2460           classes.
2461    
2462       Non-printing characters
2463    
2464           A second use of backslash provides a way of encoding non-printing char-
2465           acters  in patterns in a visible manner. There is no restriction on the
2466           appearance of non-printing characters, apart from the binary zero  that
2467           terminates  a  pattern,  but  when  a pattern is being prepared by text
2468           editing, it is usually easier  to  use  one  of  the  following  escape
2469           sequences than the binary character it represents:
2470    
2471             \a        alarm, that is, the BEL character (hex 07)
2472             \cx       "control-x", where x is any character
2473             \e        escape (hex 1B)
2474             \f        formfeed (hex 0C)
2475             \n        newline (hex 0A)
2476             \r        carriage return (hex 0D)
2477             \t        tab (hex 09)
2478             \ddd      character with octal code ddd, or backreference
2479             \xhh      character with hex code hh
2480             \x{hhh..} character with hex code hhh... (UTF-8 mode only)
2481    
2482           The  precise  effect of \cx is as follows: if x is a lower case letter,
2483           it is converted to upper case. Then bit 6 of the character (hex 40)  is
2484           inverted.   Thus  \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
2485           becomes hex 7B.
2486    
2487           After \x, from zero to two hexadecimal digits are read (letters can  be
2488           in  upper or lower case). In UTF-8 mode, any number of hexadecimal dig-
2489           its may appear between \x{ and }, but the value of the  character  code
2490           must  be  less  than  2**31  (that is, the maximum hexadecimal value is
2491           7FFFFFFF). If characters other than hexadecimal digits  appear  between
2492           \x{  and }, or if there is no terminating }, this form of escape is not
2493           recognized. Instead, the initial \x will  be  interpreted  as  a  basic
2494           hexadecimal  escape, with no following digits, giving a character whose
2495           value is zero.
2496    
2497           Characters whose value is less than 256 can be defined by either of the
2498           two  syntaxes for \x when PCRE is in UTF-8 mode. There is no difference
2499           in the way they are handled. For example, \xdc is exactly the  same  as
2500           \x{dc}.
2501    
2502           After  \0  up  to  two further octal digits are read. In both cases, if
2503           there are fewer than two digits, just those that are present are  used.
2504           Thus  the sequence \0\x\07 specifies two binary zeros followed by a BEL
2505           character (code value 7). Make sure you supply  two  digits  after  the
2506           initial  zero  if the pattern character that follows is itself an octal
2507           digit.
2508    
2509           The handling of a backslash followed by a digit other than 0 is compli-
2510           cated.  Outside a character class, PCRE reads it and any following dig-
2511           its as a decimal number. If the number is less than  10,  or  if  there
2512           have been at least that many previous capturing left parentheses in the
2513           expression, the entire  sequence  is  taken  as  a  back  reference.  A
2514           description  of how this works is given later, following the discussion
2515           of parenthesized subpatterns.
2516    
2517           Inside a character class, or if the decimal number is  greater  than  9
2518           and  there have not been that many capturing subpatterns, PCRE re-reads
2519           up to three octal digits following the backslash, and generates a  sin-
2520           gle byte from the least significant 8 bits of the value. Any subsequent
2521           digits stand for themselves.  For example:
2522    
2523             \040   is another way of writing a space
2524             \40    is the same, provided there are fewer than 40
2525                       previous capturing subpatterns
2526             \7     is always a back reference
2527             \11    might be a back reference, or another way of
2528                       writing a tab
2529             \011   is always a tab
2530             \0113  is a tab followed by the character "3"
2531             \113   might be a back reference, otherwise the
2532                       character with octal code 113
2533             \377   might be a back reference, otherwise
2534                       the byte consisting entirely of 1 bits
2535             \81    is either a back reference, or a binary zero
2536                       followed by the two characters "8" and "1"
2537    
2538           Note that octal values of 100 or greater must not be  introduced  by  a
2539           leading zero, because no more than three octal digits are ever read.
2540    
2541           All  the  sequences  that  define a single byte value or a single UTF-8
2542           character (in UTF-8 mode) can be used both inside and outside character
2543           classes.  In  addition,  inside  a  character class, the sequence \b is
2544           interpreted as the backspace character (hex 08), and the sequence \X is
2545           interpreted  as  the  character  "X".  Outside a character class, these
2546           sequences have different meanings (see below).
2547    
2548       Generic character types
2549    
2550           The third use of backslash is for specifying generic  character  types.
2551           The following are always recognized:
2552    
2553             \d     any decimal digit
2554             \D     any character that is not a decimal digit
2555             \s     any whitespace character
2556             \S     any character that is not a whitespace character
2557             \w     any "word" character
2558             \W     any "non-word" character
2559    
2560           Each pair of escape sequences partitions the complete set of characters
2561           into two disjoint sets. Any given character matches one, and only  one,
2562           of each pair.
2563    
2564           These character type sequences can appear both inside and outside char-
2565           acter classes. They each match one character of the  appropriate  type.
2566           If  the current matching point is at the end of the subject string, all
2567           of them fail, since there is no character to match.
2568    
2569           For compatibility with Perl, \s does not match the VT  character  (code
2570           11).   This makes it different from the the POSIX "space" class. The \s
2571           characters are HT (9), LF (10), FF (12), CR (13), and space (32).
2572    
2573           A "word" character is an underscore or any character less than 256 that
2574           is  a  letter  or  digit.  The definition of letters and digits is con-
2575           trolled by PCRE's low-valued character tables, and may vary if  locale-
2576           specific  matching is taking place (see "Locale support" in the pcreapi
2577           page). For example, in the  "fr_FR"  (French)  locale,  some  character
2578           codes  greater  than  128  are used for accented letters, and these are
2579           matched by \w.
2580    
2581           In UTF-8 mode, characters with values greater than 128 never match  \d,
2582           \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
2583           code character property support is available.
2584    
2585       Unicode character properties
2586    
2587           When PCRE is built with Unicode character property support, three addi-
2588           tional  escape sequences to match generic character types are available
2589           when UTF-8 mode is selected. They are:
2590    
2591            \p{xx}   a character with the xx property
2592            \P{xx}   a character without the xx property
2593            \X       an extended Unicode sequence
2594    
2595           The property names represented by xx above are limited to  the  Unicode
2596           general  category properties. Each character has exactly one such prop-
2597           erty, specified by a two-letter abbreviation.  For  compatibility  with
2598           Perl,  negation  can be specified by including a circumflex between the
2599           opening brace and the property name. For example, \p{^Lu} is  the  same
2600           as \P{Lu}.
2601    
2602           If  only  one  letter  is  specified with \p or \P, it includes all the
2603           properties that start with that letter. In this case, in the absence of
2604           negation, the curly brackets in the escape sequence are optional; these
2605           two examples have the same effect:
2606    
2607             \p{L}
2608             \pL
2609    
2610           The following property codes are supported:
2611    
2612             C     Other
2613             Cc    Control
2614             Cf    Format
2615             Cn    Unassigned
2616             Co    Private use
2617             Cs    Surrogate
2618    
2619             L     Letter
2620             Ll    Lower case letter
2621             Lm    Modifier letter
2622             Lo    Other letter
2623             Lt    Title case letter
2624             Lu    Upper case letter
2625    
2626             M     Mark
2627             Mc    Spacing mark
2628             Me    Enclosing mark
2629             Mn    Non-spacing mark
2630    
2631             N     Number
2632             Nd    Decimal number
2633             Nl    Letter number
2634             No    Other number
2635    
2636             P     Punctuation
2637             Pc    Connector punctuation
2638             Pd    Dash punctuation
2639             Pe    Close punctuation
2640             Pf    Final punctuation
2641             Pi    Initial punctuation
2642             Po    Other punctuation
2643             Ps    Open punctuation
2644    
2645             S     Symbol
2646             Sc    Currency symbol
2647             Sk    Modifier symbol
2648             Sm    Mathematical symbol
2649             So    Other symbol
2650    
2651             Z     Separator
2652             Zl    Line separator
2653             Zp    Paragraph separator
2654             Zs    Space separator
2655    
2656           Extended properties such as "Greek" or "InMusicalSymbols" are not  sup-
2657           ported by PCRE.
2658    
2659           Specifying  caseless  matching  does not affect these escape sequences.
2660           For example, \p{Lu} always matches only upper case letters.
2661    
2662           The \X escape matches any number of Unicode  characters  that  form  an
2663           extended Unicode sequence. \X is equivalent to
2664    
2665             (?>\PM\pM*)
2666    
2667           That  is,  it matches a character without the "mark" property, followed
2668           by zero or more characters with the "mark"  property,  and  treats  the
2669           sequence  as  an  atomic group (see below).  Characters with the "mark"
2670           property are typically accents that affect the preceding character.
2671    
2672           Matching characters by Unicode property is not fast, because  PCRE  has
2673           to  search  a  structure  that  contains data for over fifteen thousand
2674           characters. That is why the traditional escape sequences such as \d and
2675           \w do not use Unicode properties in PCRE.
2676    
2677       Simple assertions
2678    
2679           The fourth use of backslash is for certain simple assertions. An asser-
2680           tion specifies a condition that has to be met at a particular point  in
2681           a  match, without consuming any characters from the subject string. The
2682           use of subpatterns for more complicated assertions is described  below.
2683           The backslashed assertions are:
2684    
2685             \b     matches at a word boundary
2686             \B     matches when not at a word boundary
2687             \A     matches at start of subject
2688             \Z     matches at end of subject or before newline at end
2689             \z     matches at end of subject
2690             \G     matches at first matching position in subject
2691    
2692           These  assertions may not appear in character classes (but note that \b
2693           has a different meaning, namely the backspace character, inside a char-
2694           acter class).
2695    
2696           A  word  boundary is a position in the subject string where the current
2697           character and the previous character do not both match \w or  \W  (i.e.
2698           one  matches  \w  and the other matches \W), or the start or end of the
2699           string if the first or last character matches \w, respectively.
2700    
2701           The \A, \Z, and \z assertions differ from  the  traditional  circumflex
2702           and dollar (described in the next section) in that they only ever match
2703           at the very start and end of the subject string, whatever  options  are
2704           set.  Thus,  they are independent of multiline mode. These three asser-
2705           tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
2706           affect  only the behaviour of the circumflex and dollar metacharacters.
2707           However, if the startoffset argument of pcre_exec() is non-zero,  indi-
2708           cating that matching is to start at a point other than the beginning of
2709           the subject, \A can never match. The difference between \Z  and  \z  is
2710           that  \Z  matches  before  a  newline that is the last character of the
2711           string as well as at the end of the string, whereas \z matches only  at
2712           the end.
2713    
2714           The  \G assertion is true only when the current matching position is at
2715           the start point of the match, as specified by the startoffset  argument
2716           of  pcre_exec().  It  differs  from \A when the value of startoffset is
2717           non-zero. By calling pcre_exec() multiple times with appropriate  argu-
2718           ments, you can mimic Perl's /g option, and it is in this kind of imple-
2719           mentation where \G can be useful.
2720    
2721           Note, however, that PCRE's interpretation of \G, as the  start  of  the
2722           current match, is subtly different from Perl's, which defines it as the
2723           end of the previous match. In Perl, these can  be  different  when  the
2724           previously  matched  string was empty. Because PCRE does just one match
2725           at a time, it cannot reproduce this behaviour.
2726    
2727           If all the alternatives of a pattern begin with \G, the  expression  is
2728           anchored to the starting match position, and the "anchored" flag is set
2729           in the compiled regular expression.
2730    
2731    
2732  CIRCUMFLEX AND DOLLAR  CIRCUMFLEX AND DOLLAR
      Outside a character class, in the default matching mode, the  
      circumflex  character  is an assertion which is true only if  
      the current matching point is at the start  of  the  subject  
      string.  If  the startoffset argument of pcre_exec() is non-  
      zero, circumflex can never match. 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 alter-  
      natives start with a circumflex, that is, if the pattern  is  
      constrained to match only at the start of the subject, it is  
      said to be an "anchored" pattern. (There are also other con-  
      structs that can cause a pattern to be anchored.)  
   
      A dollar character is an assertion which is true only if the  
      current  matching point is at the end of the subject string,  
      or immediately before a newline character that is  the  last  
      character in the string (by default). Dollar need not be the  
      last character of the pattern if a  number  of  alternatives  
      are  involved,  but it should be the last item in any branch  
      in which it appears.  Dollar has no  special  meaning  in  a  
      character class.  
   
      The meaning of dollar can be changed so that it matches only  
      at   the   very   end   of   the   string,  by  setting  the  
      PCRE_DOLLAR_ENDONLY option at compile or matching 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,  they  match  immediately  after  and  immediately  
      before an internal "\n" character, respectively, in addition  
      to matching at the start and end of the subject string.  For  
      example,  the  pattern  /^abc$/  matches  the subject string  
      "def\nabc" in multiline  mode,  but  not  otherwise.  Conse-  
      quently,  patterns  that  are  anchored  in single line mode  
      because all branches start with "^" are not anchored in mul-  
      tiline 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 PCRE_MULTILINE is set or not.  
2733    
2734           Outside a character class, in the default matching mode, the circumflex
2735           character  is  an  assertion  that is true only if the current matching
2736           point is at the start of the subject string. If the  startoffset  argu-
2737           ment  of  pcre_exec()  is  non-zero,  circumflex can never match if the
2738           PCRE_MULTILINE option is unset. Inside a  character  class,  circumflex
2739           has an entirely different meaning (see below).
2740    
2741           Circumflex  need  not be the first character of the pattern if a number
2742           of alternatives are involved, but it should be the first thing in  each
2743           alternative  in  which  it appears if the pattern is ever to match that
2744           branch. If all possible alternatives start with a circumflex, that  is,
2745           if  the  pattern  is constrained to match only at the start of the sub-
2746           ject, it is said to be an "anchored" pattern.  (There  are  also  other
2747           constructs that can cause a pattern to be anchored.)
2748    
2749           A  dollar  character  is  an assertion that is true only if the current
2750           matching point is at the end of  the  subject  string,  or  immediately
2751           before a newline character that is the last character in the string (by
2752           default). Dollar need not be the last character of  the  pattern  if  a
2753           number  of alternatives are involved, but it should be the last item in
2754           any branch in which it appears.  Dollar has no  special  meaning  in  a
2755           character class.
2756    
2757           The  meaning  of  dollar  can be changed so that it matches only at the
2758           very end of the string, by setting the  PCRE_DOLLAR_ENDONLY  option  at
2759           compile time. This does not affect the \Z assertion.
2760    
2761           The meanings of the circumflex and dollar characters are changed if the
2762           PCRE_MULTILINE option is set. When this is the case, they match immedi-
2763           ately  after  and  immediately  before  an  internal newline character,
2764           respectively, in addition to matching at the start and end of the  sub-
2765           ject  string.  For  example,  the  pattern  /^abc$/ matches the subject
2766           string "def\nabc" (where \n represents a newline character)  in  multi-
2767           line mode, but not otherwise.  Consequently, patterns that are anchored
2768           in single line mode because all branches start with ^ are not  anchored
2769           in  multiline  mode,  and  a  match for circumflex is possible when the
2770           startoffset  argument  of  pcre_exec()  is  non-zero.   The   PCRE_DOL-
2771           LAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
2772    
2773           Note  that  the sequences \A, \Z, and \z can be used to match the start
2774           and end of the subject in both modes, and if all branches of a  pattern
2775           start  with  \A it is always anchored, whether PCRE_MULTILINE is set or
2776           not.
2777    
2778    
2779  FULL STOP (PERIOD, DOT)  FULL STOP (PERIOD, DOT)
      Outside a character class, a dot in the pattern matches  any  
      one character in the subject, including a non-printing char-  
      acter, but not (by default)  newline.   If  the  PCRE_DOTALL  
      option  is set, dots match newlines as well. The handling of  
      dot is entirely independent of the  handling  of  circumflex  
      and  dollar,  the  only  relationship  being  that they both  
      involve newline characters. Dot has no special meaning in  a  
      character class.  
   
   
   
 SQUARE BRACKETS  
      An opening square bracket introduces a character class, ter-  
      minated  by  a  closing  square  bracket.  A  closing square  
      bracket on its own is  not  special.  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 cir-  
      cumflex, if present) or escaped with a backslash.  
   
      A character class matches a single character in the subject;  
      the  character  must  be in the set of characters defined by  
      the class, unless the first character in the class is a cir-  
      cumflex,  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  con-  
      venient  notation for specifying the characters which are in  
      the class by enumerating those that are not. It  is  not  an  
      assertion:  it  still  consumes a character from the subject  
      string, and fails if the current pointer is at  the  end  of  
      the string.  
   
      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 case-  
      ful version would.  
   
      The newline character is never treated in any special way in  
      character  classes,  whatever the setting of the PCRE_DOTALL  
      or PCRE_MULTILINE options is. A  class  such  as  [^a]  will  
      always match a newline.  
   
      The minus (hyphen) character can be used to specify a  range  
      of  characters  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  inter-  
      preted 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  character  of  a  range.  A  pattern such as [W-]46] is  
      interpreted as a class of two characters ("W" and "-")  fol-  
      lowed 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 single class containing a range followed by  two  
      separate characters. The octal or hexadecimal representation  
      of "]" can also be used to end a range.  
   
      Ranges operate in ASCII collating sequence. They can also be  
      used  for  characters  specified  numerically,  for  example  
      [\000-\037]. 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  if  character tables for the "fr"  
      locale are in use, [\xc8-\xcb] matches accented E characters  
      in both cases.  
   
      The character types \d, \D, \s, \S,  \w,  and  \W  may  also  
      appear  in  a  character  class, and add the characters that  
      they match to the class. For example, [\dABCDEF] matches any  
      hexadecimal  digit.  A  circumflex  can conveniently be used  
      with the upper case character types to specify a  more  res-  
      tricted set of characters than the matching lower case type.  
      For example, the class [^\W_] matches any letter  or  digit,  
      but not underscore.  
   
      All non-alphameric characters other than \,  -,  ^  (at  the  
      start)  and  the  terminating ] are non-special in character  
      classes, but it does no harm if they are escaped.  
2780    
2781           Outside a character class, a dot in the pattern matches any one charac-
2782           ter  in  the  subject,  including a non-printing character, but not (by
2783           default) newline.  In UTF-8 mode, a dot matches  any  UTF-8  character,
2784           which might be more than one byte long, except (by default) newline. If
2785           the PCRE_DOTALL option is set, dots match newlines as  well.  The  han-
2786           dling  of dot is entirely independent of the handling of circumflex and
2787           dollar, the only relationship being  that  they  both  involve  newline
2788           characters. Dot has no special meaning in a character class.
2789