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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. (Certain features that appeared in Python and
22           PCRE before they appeared in Perl are also available using  the  Python
23           syntax.)
24    
25           The  current  implementation of PCRE (release 7.x) corresponds approxi-
26           mately with Perl 5.10, including support for UTF-8 encoded strings  and
27           Unicode general category properties. However, UTF-8 and Unicode support
28           has to be explicitly enabled; it is not the default. The Unicode tables
29           correspond to Unicode release 5.0.0.
30    
31           In  addition to the Perl-compatible matching function, PCRE contains an
32           alternative matching function that matches the same  compiled  patterns
33           in  a different way. In certain circumstances, the alternative function
34           has some advantages. For a discussion of the two  matching  algorithms,
35           see the pcrematching page.
36    
37           PCRE  is  written  in C and released as a C library. A number of people
38           have written wrappers and interfaces of various kinds.  In  particular,
39           Google  Inc.   have  provided  a comprehensive C++ wrapper. This is now
40           included as part of the PCRE distribution. The pcrecpp page has details
41           of  this  interface.  Other  people's contributions can be found in the
42           Contrib directory at the primary FTP site, which is:
43    
44           ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
45    
46           Details of exactly which Perl regular expression features are  and  are
47           not supported by PCRE are given in separate documents. See the pcrepat-
48           tern and pcrecompat pages. There is a syntax summary in the  pcresyntax
49           page.
50    
51           Some  features  of  PCRE can be included, excluded, or changed when the
52           library is built. The pcre_config() function makes it  possible  for  a
53           client  to  discover  which  features are available. The features them-
54           selves are described in the pcrebuild page. Documentation about  build-
55           ing  PCRE for various operating systems can be found in the README file
56           in the source distribution.
57    
58           The library contains a number of undocumented  internal  functions  and
59           data  tables  that  are  used by more than one of the exported external
60           functions, but which are not intended  for  use  by  external  callers.
61           Their  names  all begin with "_pcre_", which hopefully will not provoke
62           any name clashes. In some environments, it is possible to control which
63           external  symbols  are  exported when a shared library is built, and in
64           these cases the undocumented symbols are not exported.
65    
66    
67    USER DOCUMENTATION
68    
69           The user documentation for PCRE comprises a number  of  different  sec-
70           tions.  In the "man" format, each of these is a separate "man page". In
71           the HTML format, each is a separate page, linked from the  index  page.
72           In  the  plain text format, all the sections are concatenated, for ease
73           of searching. The sections are as follows:
74    
75             pcre              this document
76             pcre-config       show PCRE installation configuration information
77             pcreapi           details of PCRE's native C API
78             pcrebuild         options for building PCRE
79             pcrecallout       details of the callout feature
80             pcrecompat        discussion of Perl compatibility
81             pcrecpp           details of the C++ wrapper
82             pcregrep          description of the pcregrep command
83             pcrematching      discussion of the two matching algorithms
84             pcrepartial       details of the partial matching facility
85             pcrepattern       syntax and semantics of supported
86                                 regular expressions
87             pcresyntax        quick syntax reference
88             pcreperform       discussion of performance issues
89             pcreposix         the POSIX-compatible C API
90             pcreprecompile    details of saving and re-using precompiled patterns
91             pcresample        discussion of the sample program
92             pcrestack         discussion of stack usage
93             pcretest          description of the pcretest testing command
94    
95       pcre *pcre_compile(const char *pattern, int options,         In  addition,  in the "man" and HTML formats, there is a short page for
96            const char **errptr, int *erroffset,         each C library function, listing its arguments and results.
           const unsigned char *tableptr);  
97    
      pcre_extra *pcre_study(const pcre *code, int options,  
           const char **errptr);  
98    
99       int pcre_exec(const pcre *code, const pcre_extra *extra,  LIMITATIONS
           const char *subject, int length, int startoffset,  
           int options, int *ovector, int ovecsize);  
100    
101       int pcre_copy_substring(const char *subject, int *ovector,         There are some size limitations in PCRE but it is hoped that they  will
102            int stringcount, int stringnumber, char *buffer,         never in practice be relevant.
           int buffersize);  
103    
104       int pcre_get_substring(const char *subject, int *ovector,         The  maximum  length of a compiled pattern is 65539 (sic) bytes if PCRE
105            int stringcount, int stringnumber,         is compiled with the default internal linkage size of 2. If you want to
106            const char **stringptr);         process  regular  expressions  that are truly enormous, you can compile
107           PCRE with an internal linkage size of 3 or 4 (see the  README  file  in
108           the  source  distribution and the pcrebuild documentation for details).
109           In these cases the limit is substantially larger.  However,  the  speed
110           of execution is slower.
111    
112           All values in repeating quantifiers must be less than 65536.
113    
114           There is no limit to the number of parenthesized subpatterns, but there
115           can be no more than 65535 capturing subpatterns.
116    
117           The maximum length of name for a named subpattern is 32 characters, and
118           the maximum number of named subpatterns is 10000.
119    
120           The  maximum  length of a subject string is the largest positive number
121           that an integer variable can hold. However, when using the  traditional
122           matching function, PCRE uses recursion to handle subpatterns and indef-
123           inite repetition.  This means that the available stack space may  limit
124           the size of a subject string that can be processed by certain patterns.
125           For a discussion of stack issues, see the pcrestack documentation.
126    
127    
128    UTF-8 AND UNICODE PROPERTY SUPPORT
129    
130           From release 3.3, PCRE has  had  some  support  for  character  strings
131           encoded  in the UTF-8 format. For release 4.0 this was greatly extended
132           to cover most common requirements, and in release 5.0  additional  sup-
133           port for Unicode general category properties was added.
134    
135           In  order  process  UTF-8 strings, you must build PCRE to include UTF-8
136           support in the code, and, in addition,  you  must  call  pcre_compile()
137           with  the PCRE_UTF8 option flag. When you do this, both the pattern and
138           any subject strings that are matched against it are  treated  as  UTF-8
139           strings instead of just strings of bytes.
140    
141           If  you compile PCRE with UTF-8 support, but do not use it at run time,
142           the library will be a bit bigger, but the additional run time  overhead
143           is limited to testing the PCRE_UTF8 flag occasionally, so should not be
144           very big.
145    
146           If PCRE is built with Unicode character property support (which implies
147           UTF-8  support),  the  escape sequences \p{..}, \P{..}, and \X are sup-
148           ported.  The available properties that can be tested are limited to the
149           general  category  properties such as Lu for an upper case letter or Nd
150           for a decimal number, the Unicode script names such as Arabic  or  Han,
151           and  the  derived  properties  Any  and L&. A full list is given in the
152           pcrepattern documentation. Only the short names for properties are sup-
153           ported.  For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
154           ter}, is not supported.  Furthermore,  in  Perl,  many  properties  may
155           optionally  be  prefixed by "Is", for compatibility with Perl 5.6. PCRE
156           does not support this.
157    
158       Validity of UTF-8 strings
159    
160           When you set the PCRE_UTF8 flag, the strings  passed  as  patterns  and
161           subjects are (by default) checked for validity on entry to the relevant
162           functions. From release 7.3 of PCRE, the check is according  the  rules
163           of  RFC  3629, which are themselves derived from the Unicode specifica-
164           tion. Earlier releases of PCRE followed the rules of  RFC  2279,  which
165           allows  the  full range of 31-bit values (0 to 0x7FFFFFFF). The current
166           check allows only values in the range U+0 to U+10FFFF, excluding U+D800
167           to U+DFFF.
168    
169           The  excluded  code  points are the "Low Surrogate Area" of Unicode, of
170           which the Unicode Standard says this: "The Low Surrogate Area does  not
171           contain  any  character  assignments,  consequently  no  character code
172           charts or namelists are provided for this area. Surrogates are reserved
173           for  use  with  UTF-16 and then must be used in pairs." The code points
174           that are encoded by UTF-16 pairs  are  available  as  independent  code
175           points  in  the  UTF-8  encoding.  (In other words, the whole surrogate
176           thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
177    
178           If an  invalid  UTF-8  string  is  passed  to  PCRE,  an  error  return
179           (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
180           that your strings are valid, and therefore want to skip these checks in
181           order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
182           compile time or at run time, PCRE assumes that the pattern  or  subject
183           it  is  given  (respectively)  contains only valid UTF-8 codes. In this
184           case, it does not diagnose an invalid UTF-8 string.
185    
186           If you pass an invalid UTF-8 string  when  PCRE_NO_UTF8_CHECK  is  set,
187           what  happens  depends on why the string is invalid. If the string con-
188           forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
189           string  of  characters  in  the  range 0 to 0x7FFFFFFF. In other words,
190           apart from the initial validity test, PCRE (when in UTF-8 mode) handles
191           strings  according  to  the more liberal rules of RFC 2279. However, if
192           the string does not even conform to RFC 2279, the result is  undefined.
193           Your program may crash.
194    
195           If  you  want  to  process  strings  of  values  in the full range 0 to
196           0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you  can
197           set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
198           this situation, you will have to apply your own validity check.
199    
200       General comments about UTF-8 mode
201    
202           1. An unbraced hexadecimal escape sequence (such  as  \xb3)  matches  a
203           two-byte UTF-8 character if the value is greater than 127.
204    
205           2.  Octal  numbers  up to \777 are recognized, and match two-byte UTF-8
206           characters for values greater than \177.
207    
208           3. Repeat quantifiers apply to complete UTF-8 characters, not to  indi-
209           vidual bytes, for example: \x{100}{3}.
210    
211           4.  The dot metacharacter matches one UTF-8 character instead of a sin-
212           gle byte.
213    
214           5. The escape sequence \C can be used to match a single byte  in  UTF-8
215           mode,  but  its  use can lead to some strange effects. This facility is
216           not available in the alternative matching function, pcre_dfa_exec().
217    
218           6. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly
219           test  characters of any code value, but the characters that PCRE recog-
220           nizes as digits, spaces, or word characters  remain  the  same  set  as
221           before, all with values less than 256. This remains true even when PCRE
222           includes Unicode property support, because to do otherwise  would  slow
223           down  PCRE in many common cases. If you really want to test for a wider
224           sense of, say, "digit", you must use Unicode  property  tests  such  as
225           \p{Nd}.
226    
227           7.  Similarly,  characters that match the POSIX named character classes
228           are all low-valued characters.
229    
230           8. However, the Perl 5.10 horizontal and vertical  whitespace  matching
231           escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
232           acters.
233    
234           9. Case-insensitive matching applies only to  characters  whose  values
235           are  less than 128, unless PCRE is built with Unicode property support.
236           Even when Unicode property support is available, PCRE  still  uses  its
237           own  character  tables when checking the case of low-valued characters,
238           so as not to degrade performance.  The Unicode property information  is
239           used only for characters with higher values. Even when Unicode property
240           support is available, PCRE supports case-insensitive matching only when
241           there  is  a  one-to-one  mapping between a letter's cases. There are a
242           small number of many-to-one mappings in Unicode;  these  are  not  sup-
243           ported by PCRE.
244    
      int pcre_get_substring_list(const char *subject,  
           int *ovector, int stringcount, const char ***listptr);  
245    
246       void pcre_free_substring(const char *stringptr);  AUTHOR
247    
248       void pcre_free_substring_list(const char **stringptr);         Philip Hazel
249           University Computing Service
250           Cambridge CB2 3QH, England.
251    
252       const unsigned char *pcre_maketables(void);         Putting  an actual email address here seems to have been a spam magnet,
253           so I've taken it away. If you want to email me, use  my  two  initials,
254           followed by the two digits 10, at the domain cam.ac.uk.
255    
      int pcre_fullinfo(const pcre *code, const pcre_extra *extra,  
           int what, void *where);  
256    
257       int pcre_info(const pcre *code, int *optptr, *firstcharptr);  REVISION
258    
259       char *pcre_version(void);         Last updated: 09 August 2007
260           Copyright (c) 1997-2007 University of Cambridge.
261    ------------------------------------------------------------------------------
262    
      void *(*pcre_malloc)(size_t);  
263    
264       void (*pcre_free)(void *);  PCREBUILD(3)                                                      PCREBUILD(3)
265    
266    
267    NAME
268           PCRE - Perl-compatible regular expressions
269    
270    
271  DESCRIPTION  PCRE BUILD-TIME OPTIONS
272       The PCRE library is a set of functions that implement  regu-  
273       lar  expression  pattern  matching using the same syntax and         This  document  describes  the  optional  features  of PCRE that can be
274       semantics as Perl  5,  with  just  a  few  differences  (see         selected when the library is compiled. They are all selected, or  dese-
275           lected, by providing options to the configure script that is run before
276           the make command. The complete list of  options  for  configure  (which
277           includes  the  standard  ones such as the selection of the installation
278           directory) can be obtained by running
279    
280             ./configure --help
281    
282           The following sections include  descriptions  of  options  whose  names
283           begin with --enable or --disable. These settings specify changes to the
284           defaults for the configure command. Because of the way  that  configure
285           works,  --enable  and --disable always come in pairs, so the complemen-
286           tary option always exists as well, but as it specifies the default,  it
287           is not described.
288    
289    
290    C++ SUPPORT
291    
292           By default, the configure script will search for a C++ compiler and C++
293           header files. If it finds them, it automatically builds the C++ wrapper
294           library for PCRE. You can disable this by adding
295    
296             --disable-cpp
297    
298           to the configure command.
299    
300    
301    UTF-8 SUPPORT
302    
303           To build PCRE with support for UTF-8 character strings, add
304    
305             --enable-utf8
306    
307           to  the  configure  command.  Of  itself, this does not make PCRE treat
308           strings as UTF-8. As well as compiling PCRE with this option, you  also
309           have  have to set the PCRE_UTF8 option when you call the pcre_compile()
310           function.
311    
312    
313    UNICODE CHARACTER PROPERTY SUPPORT
314    
315           UTF-8 support allows PCRE to process character values greater than  255
316           in  the  strings that it handles. On its own, however, it does not pro-
317           vide any facilities for accessing the properties of such characters. If
318           you  want  to  be able to use the pattern escapes \P, \p, and \X, which
319           refer to Unicode character properties, you must add
320    
321             --enable-unicode-properties
322    
323           to the configure command. This implies UTF-8 support, even if you  have
324           not explicitly requested it.
325    
326           Including  Unicode  property  support  adds around 30K of tables to the
327           PCRE library. Only the general category properties such as  Lu  and  Nd
328           are supported. Details are given in the pcrepattern documentation.
329    
330    
331    CODE VALUE OF NEWLINE
332    
333           By  default,  PCRE interprets character 10 (linefeed, LF) as indicating
334           the end of a line. This is the normal newline  character  on  Unix-like
335           systems. You can compile PCRE to use character 13 (carriage return, CR)
336           instead, by adding
337    
338             --enable-newline-is-cr
339    
340           to the  configure  command.  There  is  also  a  --enable-newline-is-lf
341           option, which explicitly specifies linefeed as the newline character.
342    
343           Alternatively, you can specify that line endings are to be indicated by
344           the two character sequence CRLF. If you want this, add
345    
346             --enable-newline-is-crlf
347    
348           to the configure command. There is a fourth option, specified by
349    
350             --enable-newline-is-anycrlf
351    
352           which causes PCRE to recognize any of the three sequences  CR,  LF,  or
353           CRLF as indicating a line ending. Finally, a fifth option, specified by
354    
355             --enable-newline-is-any
356    
357           causes PCRE to recognize any Unicode newline sequence.
358    
359           Whatever line ending convention is selected when PCRE is built  can  be
360           overridden  when  the library functions are called. At build time it is
361           conventional to use the standard for your operating system.
362    
363    
364    BUILDING SHARED AND STATIC LIBRARIES
365    
366           The PCRE building process uses libtool to build both shared and  static
367           Unix  libraries by default. You can suppress one of these by adding one
368           of
369    
370             --disable-shared
371             --disable-static
372    
373           to the configure command, as required.
374    
375    
376    POSIX MALLOC USAGE
377    
378           When PCRE is called through the POSIX interface (see the pcreposix doc-
379           umentation),  additional  working  storage  is required for holding the
380           pointers to capturing substrings, because PCRE requires three  integers
381           per  substring,  whereas  the POSIX interface provides only two. If the
382           number of expected substrings is small, the wrapper function uses space
383           on the stack, because this is faster than using malloc() for each call.
384           The default threshold above which the stack is no longer used is 10; it
385           can be changed by adding a setting such as
386    
387             --with-posix-malloc-threshold=20
388    
389           to the configure command.
390    
391    
392    HANDLING VERY LARGE PATTERNS
393    
394           Within  a  compiled  pattern,  offset values are used to point from one
395           part to another (for example, from an opening parenthesis to an  alter-
396           nation  metacharacter).  By default, two-byte values are used for these
397           offsets, leading to a maximum size for a  compiled  pattern  of  around
398           64K.  This  is sufficient to handle all but the most gigantic patterns.
399           Nevertheless, some people do want to process enormous patterns,  so  it
400           is  possible  to compile PCRE to use three-byte or four-byte offsets by
401           adding a setting such as
402    
403             --with-link-size=3
404    
405           to the configure command. The value given must be 2,  3,  or  4.  Using
406           longer  offsets slows down the operation of PCRE because it has to load
407           additional bytes when handling them.
408    
409    
410    AVOIDING EXCESSIVE STACK USAGE
411    
412           When matching with the pcre_exec() function, PCRE implements backtrack-
413           ing  by  making recursive calls to an internal function called match().
414           In environments where the size of the stack is limited,  this  can  se-
415           verely  limit  PCRE's operation. (The Unix environment does not usually
416           suffer from this problem, but it may sometimes be necessary to increase
417           the  maximum  stack size.  There is a discussion in the pcrestack docu-
418           mentation.) An alternative approach to recursion that uses memory  from
419           the  heap  to remember data, instead of using recursive function calls,
420           has been implemented to work round the problem of limited  stack  size.
421           If you want to build a version of PCRE that works this way, add
422    
423             --disable-stack-for-recursion
424    
425           to  the  configure  command. With this configuration, PCRE will use the
426           pcre_stack_malloc and pcre_stack_free variables to call memory  manage-
427           ment  functions. By default these point to malloc() and free(), but you
428           can replace the pointers so that your own functions are used.
429    
430           Separate functions are  provided  rather  than  using  pcre_malloc  and
431           pcre_free  because  the  usage  is  very  predictable:  the block sizes
432           requested are always the same, and  the  blocks  are  always  freed  in
433           reverse  order.  A calling program might be able to implement optimized
434           functions that perform better  than  malloc()  and  free().  PCRE  runs
435           noticeably more slowly when built in this way. This option affects only
436           the  pcre_exec()  function;  it   is   not   relevant   for   the   the
437           pcre_dfa_exec() function.
438    
439    
440    LIMITING PCRE RESOURCE USAGE
441    
442           Internally,  PCRE has a function called match(), which it calls repeat-
443           edly  (sometimes  recursively)  when  matching  a  pattern   with   the
444           pcre_exec()  function.  By controlling the maximum number of times this
445           function may be called during a single matching operation, a limit  can
446           be  placed  on  the resources used by a single call to pcre_exec(). The
447           limit can be changed at run time, as described in the pcreapi  documen-
448           tation.  The default is 10 million, but this can be changed by adding a
449           setting such as
450    
451             --with-match-limit=500000
452    
453           to  the  configure  command.  This  setting  has  no  effect   on   the
454           pcre_dfa_exec() matching function.
455    
456           In  some  environments  it is desirable to limit the depth of recursive
457           calls of match() more strictly than the total number of calls, in order
458           to  restrict  the maximum amount of stack (or heap, if --disable-stack-
459           for-recursion is specified) that is used. A second limit controls this;
460           it  defaults  to  the  value  that is set for --with-match-limit, which
461           imposes no additional constraints. However, you can set a  lower  limit
462           by adding, for example,
463    
464             --with-match-limit-recursion=10000
465    
466           to  the  configure  command.  This  value can also be overridden at run
467           time.
468    
469    
470    CREATING CHARACTER TABLES AT BUILD TIME
471    
472           PCRE uses fixed tables for processing characters whose code values  are
473           less  than 256. By default, PCRE is built with a set of tables that are
474           distributed in the file pcre_chartables.c.dist. These  tables  are  for
475           ASCII codes only. If you add
476    
477             --enable-rebuild-chartables
478    
479           to  the  configure  command, the distributed tables are no longer used.
480           Instead, a program called dftables is compiled and  run.  This  outputs
481           the source for new set of tables, created in the default locale of your
482           C runtime system. (This method of replacing the tables does not work if
483           you  are cross compiling, because dftables is run on the local host. If
484           you need to create alternative tables when cross  compiling,  you  will
485           have to do so "by hand".)
486    
487    
488    USING EBCDIC CODE
489    
490           PCRE  assumes  by  default that it will run in an environment where the
491           character code is ASCII (or Unicode, which is  a  superset  of  ASCII).
492           This  is  the  case for most computer operating systems. PCRE can, how-
493           ever, be compiled to run in an EBCDIC environment by adding
494    
495             --enable-ebcdic
496    
497           to the configure command. This setting implies --enable-rebuild-charta-
498           bles.  You  should  only  use  it if you know that you are in an EBCDIC
499           environment (for example, an IBM mainframe operating system).
500    
501    
502    SEE ALSO
503    
504           pcreapi(3), pcre_config(3).
505    
506    
507    AUTHOR
508    
509           Philip Hazel
510           University Computing Service
511           Cambridge CB2 3QH, England.
512    
513    
514    REVISION
515    
516           Last updated: 30 July 2007
517           Copyright (c) 1997-2007 University of Cambridge.
518    ------------------------------------------------------------------------------
519    
520    
521    PCREMATCHING(3)                                                PCREMATCHING(3)
522    
523    
524    NAME
525           PCRE - Perl-compatible regular expressions
526    
527    
528    PCRE MATCHING ALGORITHMS
529    
530           This document describes the two different algorithms that are available
531           in PCRE for matching a compiled regular expression against a given sub-
532           ject  string.  The  "standard"  algorithm  is  the  one provided by the
533           pcre_exec() function.  This works in the same was  as  Perl's  matching
534           function, and provides a Perl-compatible matching operation.
535    
536           An  alternative  algorithm is provided by the pcre_dfa_exec() function;
537           this operates in a different way, and is not  Perl-compatible.  It  has
538           advantages  and disadvantages compared with the standard algorithm, and
539           these are described below.
540    
541           When there is only one possible way in which a given subject string can
542           match  a pattern, the two algorithms give the same answer. A difference
543           arises, however, when there are multiple possibilities. For example, if
544           the pattern
545    
546             ^<.*>
547    
548           is matched against the string
549    
550             <something> <something else> <something further>
551    
552           there are three possible answers. The standard algorithm finds only one
553           of them, whereas the alternative algorithm finds all three.
554    
555    
556    REGULAR EXPRESSIONS AS TREES
557    
558           The set of strings that are matched by a regular expression can be rep-
559           resented  as  a  tree structure. An unlimited repetition in the pattern
560           makes the tree of infinite size, but it is still a tree.  Matching  the
561           pattern  to a given subject string (from a given starting point) can be
562           thought of as a search of the tree.  There are two  ways  to  search  a
563           tree:  depth-first  and  breadth-first, and these correspond to the two
564           matching algorithms provided by PCRE.
565    
566    
567    THE STANDARD MATCHING ALGORITHM
568    
569           In the terminology of Jeffrey Friedl's book "Mastering Regular  Expres-
570           sions",  the  standard  algorithm  is an "NFA algorithm". It conducts a
571           depth-first search of the pattern tree. That is, it  proceeds  along  a
572           single path through the tree, checking that the subject matches what is
573           required. When there is a mismatch, the algorithm  tries  any  alterna-
574           tives  at  the  current point, and if they all fail, it backs up to the
575           previous branch point in the  tree,  and  tries  the  next  alternative
576           branch  at  that  level.  This often involves backing up (moving to the
577           left) in the subject string as well.  The  order  in  which  repetition
578           branches  are  tried  is controlled by the greedy or ungreedy nature of
579           the quantifier.
580    
581           If a leaf node is reached, a matching string has  been  found,  and  at
582           that  point the algorithm stops. Thus, if there is more than one possi-
583           ble match, this algorithm returns the first one that it finds.  Whether
584           this  is the shortest, the longest, or some intermediate length depends
585           on the way the greedy and ungreedy repetition quantifiers are specified
586           in the pattern.
587    
588           Because  it  ends  up  with a single path through the tree, it is rela-
589           tively straightforward for this algorithm to keep  track  of  the  sub-
590           strings  that  are  matched  by portions of the pattern in parentheses.
591           This provides support for capturing parentheses and back references.
592    
593    
594    THE ALTERNATIVE MATCHING ALGORITHM
595    
596           This algorithm conducts a breadth-first search of  the  tree.  Starting
597           from  the  first  matching  point  in the subject, it scans the subject
598           string from left to right, once, character by character, and as it does
599           this,  it remembers all the paths through the tree that represent valid
600           matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
601           though  it is not implemented as a traditional finite state machine (it
602           keeps multiple states active simultaneously).
603    
604           The scan continues until either the end of the subject is  reached,  or
605           there  are  no more unterminated paths. At this point, terminated paths
606           represent the different matching possibilities (if there are none,  the
607           match  has  failed).   Thus,  if there is more than one possible match,
608           this algorithm finds all of them, and in particular, it finds the long-
609           est.  In PCRE, there is an option to stop the algorithm after the first
610           match (which is necessarily the shortest) has been found.
611    
612           Note that all the matches that are found start at the same point in the
613           subject. If the pattern
614    
615             cat(er(pillar)?)
616    
617           is  matched  against the string "the caterpillar catchment", the result
618           will be the three strings "cat", "cater", and "caterpillar" that  start
619           at the fourth character of the subject. The algorithm does not automat-
620           ically move on to find matches that start at later positions.
621    
622           There are a number of features of PCRE regular expressions that are not
623           supported by the alternative matching algorithm. They are as follows:
624    
625           1.  Because  the  algorithm  finds  all possible matches, the greedy or
626           ungreedy nature of repetition quantifiers is not relevant.  Greedy  and
627           ungreedy quantifiers are treated in exactly the same way. However, pos-
628           sessive quantifiers can make a difference when what follows could  also
629           match what is quantified, for example in a pattern like this:
630    
631             ^a++\w!
632    
633           This  pattern matches "aaab!" but not "aaa!", which would be matched by
634           a non-possessive quantifier. Similarly, if an atomic group is  present,
635           it  is matched as if it were a standalone pattern at the current point,
636           and the longest match is then "locked in" for the rest of  the  overall
637           pattern.
638    
639           2. When dealing with multiple paths through the tree simultaneously, it
640           is not straightforward to keep track of  captured  substrings  for  the
641           different  matching  possibilities,  and  PCRE's implementation of this
642           algorithm does not attempt to do this. This means that no captured sub-
643           strings are available.
644    
645           3.  Because no substrings are captured, back references within the pat-
646           tern are not supported, and cause errors if encountered.
647    
648           4. For the same reason, conditional expressions that use  a  backrefer-
649           ence  as  the  condition or test for a specific group recursion are not
650           supported.
651    
652           5. Because many paths through the tree may be  active,  the  \K  escape
653           sequence, which resets the start of the match when encountered (but may
654           be on some paths and not on others), is not  supported.  It  causes  an
655           error if encountered.
656    
657           6.  Callouts  are  supported, but the value of the capture_top field is
658           always 1, and the value of the capture_last field is always -1.
659    
660       below).  The  current  implementation  corresponds  to  Perl         7. The \C escape sequence, which (in the standard algorithm) matches  a
661       5.005, with some additional features  from  later  versions.         single  byte, even in UTF-8 mode, is not supported because the alterna-
662       This  includes  some  experimental,  incomplete  support for         tive algorithm moves through the subject  string  one  character  at  a
663       UTF-8 encoded strings. Details of exactly what is  and  what         time, for all active paths through the tree.
      is not supported are given below.  
664    
665       PCRE has its own native API,  which  is  described  in  this         8.  None  of  the  backtracking control verbs such as (*PRUNE) are sup-
666       document.  There  is  also  a  set of wrapper functions that         ported.
      correspond to the POSIX regular expression API.   These  are  
      described in the pcreposix documentation.  
667    
      The native API function prototypes are defined in the header  
      file  pcre.h,  and  on  Unix  systems  the library itself is  
      called libpcre.a, so can be accessed by adding -lpcre to the  
      command  for  linking  an  application  which  calls it. The  
      header file defines the macros PCRE_MAJOR and PCRE_MINOR  to  
      contain the major and minor release numbers for the library.  
      Applications can use these to include support for  different  
      releases.  
668    
669       The functions pcre_compile(), pcre_study(), and  pcre_exec()  ADVANTAGES OF THE ALTERNATIVE ALGORITHM
      are used for compiling and matching regular expressions.  
670    
671       The functions  pcre_copy_substring(),  pcre_get_substring(),         Using the alternative matching algorithm provides the following  advan-
672       and  pcre_get_substring_list() are convenience functions for         tages:
      extracting  captured  substrings  from  a  matched   subject  
      string; pcre_free_substring() and pcre_free_substring_list()  
      are also provided, to free the  memory  used  for  extracted  
      strings.  
673    
674       The function pcre_maketables() is used (optionally) to build         1. All possible matches (at a single point in the subject) are automat-
675       a  set of character tables in the current locale for passing         ically found, and in particular, the longest match is  found.  To  find
676       to pcre_compile().         more than one match using the standard algorithm, you have to do kludgy
677           things with callouts.
678    
679       The function pcre_fullinfo() is used to find out information         2. There is much better support for partial matching. The  restrictions
680       about a compiled pattern; pcre_info() is an obsolete version         on  the content of the pattern that apply when using the standard algo-
681       which returns only some of the available information, but is         rithm for partial matching do not apply to the  alternative  algorithm.
682       retained   for   backwards   compatibility.    The  function         For  non-anchored patterns, the starting position of a partial match is
683       pcre_version() returns a pointer to a string containing  the         available.
      version of PCRE and its date of release.  
684    
685       The global variables  pcre_malloc  and  pcre_free  initially         3. Because the alternative algorithm  scans  the  subject  string  just
686       contain the entry points of the standard malloc() and free()         once,  and  never  needs to backtrack, it is possible to pass very long
687       functions respectively. PCRE  calls  the  memory  management         subject strings to the matching function in  several  pieces,  checking
688       functions  via  these  variables,  so  a calling program can         for partial matching each time.
      replace them if it  wishes  to  intercept  the  calls.  This  
      should be done before calling any PCRE functions.  
689    
690    
691    DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
692    
693  MULTI-THREADING         The alternative algorithm suffers from a number of disadvantages:
      The  PCRE  functions  can   be   used   in   multi-threading  
694    
695           1.  It  is  substantially  slower  than the standard algorithm. This is
696           partly because it has to search for all possible matches, but  is  also
697           because it is less susceptible to optimization.
698    
699           2. Capturing parentheses and back references are not supported.
700    
701           3. Although atomic groups are supported, their use does not provide the
702           performance advantage that it does for the standard algorithm.
703    
704    
705  SunOS 5.8                 Last change:                          2  AUTHOR
706    
707           Philip Hazel
708           University Computing Service
709           Cambridge CB2 3QH, England.
710    
711    
712    REVISION
713    
714           Last updated: 08 August 2007
715           Copyright (c) 1997-2007 University of Cambridge.
716    ------------------------------------------------------------------------------
717    
718    
719    PCREAPI(3)                                                          PCREAPI(3)
720    
721    
722    NAME
723           PCRE - Perl-compatible regular expressions
724    
725    
726    PCRE NATIVE API
727    
728           #include <pcre.h>
729    
730           pcre *pcre_compile(const char *pattern, int options,
731                const char **errptr, int *erroffset,
732                const unsigned char *tableptr);
733    
734           pcre *pcre_compile2(const char *pattern, int options,
735                int *errorcodeptr,
736                const char **errptr, int *erroffset,
737                const unsigned char *tableptr);
738    
739           pcre_extra *pcre_study(const pcre *code, int options,
740                const char **errptr);
741    
742           int pcre_exec(const pcre *code, const pcre_extra *extra,
743                const char *subject, int length, int startoffset,
744                int options, int *ovector, int ovecsize);
745    
746           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
747                const char *subject, int length, int startoffset,
748                int options, int *ovector, int ovecsize,
749                int *workspace, int wscount);
750    
751           int pcre_copy_named_substring(const pcre *code,
752                const char *subject, int *ovector,
753                int stringcount, const char *stringname,
754                char *buffer, int buffersize);
755    
756           int pcre_copy_substring(const char *subject, int *ovector,
757                int stringcount, int stringnumber, char *buffer,
758                int buffersize);
759    
760           int pcre_get_named_substring(const pcre *code,
761                const char *subject, int *ovector,
762                int stringcount, const char *stringname,
763                const char **stringptr);
764    
765           int pcre_get_stringnumber(const pcre *code,
766                const char *name);
767    
768           int pcre_get_stringtable_entries(const pcre *code,
769                const char *name, char **first, char **last);
770    
771           int pcre_get_substring(const char *subject, int *ovector,
772                int stringcount, int stringnumber,
773                const char **stringptr);
774    
775           int pcre_get_substring_list(const char *subject,
776                int *ovector, int stringcount, const char ***listptr);
777    
778           void pcre_free_substring(const char *stringptr);
779    
780           void pcre_free_substring_list(const char **stringptr);
781    
782           const unsigned char *pcre_maketables(void);
783    
784           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
785                int what, void *where);
786    
787           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
788    
789           int pcre_refcount(pcre *code, int adjust);
790    
791           int pcre_config(int what, void *where);
792    
793           char *pcre_version(void);
794    
795           void *(*pcre_malloc)(size_t);
796    
797           void (*pcre_free)(void *);
798    
799           void *(*pcre_stack_malloc)(size_t);
800    
801           void (*pcre_stack_free)(void *);
802    
803           int (*pcre_callout)(pcre_callout_block *);
804    
805    
806    PCRE API OVERVIEW
807    
808           PCRE has its own native API, which is described in this document. There
809           are also some wrapper functions that correspond to  the  POSIX  regular
810           expression  API.  These  are  described in the pcreposix documentation.
811           Both of these APIs define a set of C function calls. A C++  wrapper  is
812           distributed with PCRE. It is documented in the pcrecpp page.
813    
814           The  native  API  C  function prototypes are defined in the header file
815           pcre.h, and on Unix systems the library itself is called  libpcre.   It
816           can normally be accessed by adding -lpcre to the command for linking an
817           application  that  uses  PCRE.  The  header  file  defines  the  macros
818           PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
819           bers for the library.  Applications can use these  to  include  support
820           for different releases of PCRE.
821    
822           The   functions   pcre_compile(),  pcre_compile2(),  pcre_study(),  and
823           pcre_exec() are used for compiling and matching regular expressions  in
824           a  Perl-compatible  manner. A sample program that demonstrates the sim-
825           plest way of using them is provided in the file  called  pcredemo.c  in
826           the  source distribution. The pcresample documentation describes how to
827           run it.
828    
829           A second matching function, pcre_dfa_exec(), which is not Perl-compati-
830           ble,  is  also provided. This uses a different algorithm for the match-
831           ing. The alternative algorithm finds all possible matches (at  a  given
832           point  in  the subject), and scans the subject just once. However, this
833           algorithm does not return captured substrings. A description of the two
834           matching  algorithms and their advantages and disadvantages is given in
835           the pcrematching documentation.
836    
837           In addition to the main compiling and  matching  functions,  there  are
838           convenience functions for extracting captured substrings from a subject
839           string that is matched by pcre_exec(). They are:
840    
841             pcre_copy_substring()
842             pcre_copy_named_substring()
843             pcre_get_substring()
844             pcre_get_named_substring()
845             pcre_get_substring_list()
846             pcre_get_stringnumber()
847             pcre_get_stringtable_entries()
848    
849           pcre_free_substring() and pcre_free_substring_list() are also provided,
850           to free the memory used for extracted strings.
851    
852           The  function  pcre_maketables()  is  used  to build a set of character
853           tables  in  the  current  locale   for   passing   to   pcre_compile(),
854           pcre_exec(),  or  pcre_dfa_exec(). This is an optional facility that is
855           provided for specialist use.  Most  commonly,  no  special  tables  are
856           passed,  in  which case internal tables that are generated when PCRE is
857           built are used.
858    
859           The function pcre_fullinfo() is used to find out  information  about  a
860           compiled  pattern; pcre_info() is an obsolete version that returns only
861           some of the available information, but is retained for  backwards  com-
862           patibility.   The function pcre_version() returns a pointer to a string
863           containing the version of PCRE and its date of release.
864    
865           The function pcre_refcount() maintains a  reference  count  in  a  data
866           block  containing  a compiled pattern. This is provided for the benefit
867           of object-oriented applications.
868    
869           The global variables pcre_malloc and pcre_free  initially  contain  the
870           entry  points  of  the  standard malloc() and free() functions, respec-
871           tively. PCRE calls the memory management functions via these variables,
872           so  a  calling  program  can replace them if it wishes to intercept the
873           calls. This should be done before calling any PCRE functions.
874    
875           The global variables pcre_stack_malloc  and  pcre_stack_free  are  also
876           indirections  to  memory  management functions. These special functions
877           are used only when PCRE is compiled to use  the  heap  for  remembering
878           data, instead of recursive function calls, when running the pcre_exec()
879           function. See the pcrebuild documentation for  details  of  how  to  do
880           this.  It  is  a non-standard way of building PCRE, for use in environ-
881           ments that have limited stacks. Because of the greater  use  of  memory
882           management,  it  runs  more  slowly. Separate functions are provided so
883           that special-purpose external code can be  used  for  this  case.  When
884           used,  these  functions  are always called in a stack-like manner (last
885           obtained, first freed), and always for memory blocks of the same  size.
886           There  is  a discussion about PCRE's stack usage in the pcrestack docu-
887           mentation.
888    
889           The global variable pcre_callout initially contains NULL. It can be set
890           by  the  caller  to  a "callout" function, which PCRE will then call at
891           specified points during a matching operation. Details are given in  the
892           pcrecallout documentation.
893    
894    
895    NEWLINES
896    
897           PCRE  supports five different conventions for indicating line breaks in
898           strings: a single CR (carriage return) character, a  single  LF  (line-
899           feed) character, the two-character sequence CRLF, any of the three pre-
900           ceding, or any Unicode newline sequence. The Unicode newline  sequences
901           are  the  three just mentioned, plus the single characters VT (vertical
902           tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS  (line
903           separator, U+2028), and PS (paragraph separator, U+2029).
904    
905           Each  of  the first three conventions is used by at least one operating
906           system as its standard newline sequence. When PCRE is built, a  default
907           can  be  specified.  The default default is LF, which is the Unix stan-
908           dard. When PCRE is run, the default can be overridden,  either  when  a
909           pattern is compiled, or when it is matched.
910    
911           At compile time, the newline convention can be specified by the options
912           argument of pcre_compile(), or it can be specified by special  text  at
913           the start of the pattern itself; this overrides any other settings. See
914           the pcrepattern page for details of the special character sequences.
915    
916           In the PCRE documentation the word "newline" is used to mean "the char-
917           acter  or pair of characters that indicate a line break". The choice of
918           newline convention affects the handling of  the  dot,  circumflex,  and
919           dollar metacharacters, the handling of #-comments in /x mode, and, when
920           CRLF is a recognized line ending sequence, the match position  advance-
921           ment for a non-anchored pattern. There is more detail about this in the
922           section on pcre_exec() options below. The choice of newline  convention
923           does not affect the interpretation of the \n or \r escape sequences.
924    
925    
926    MULTITHREADING
927    
928           The  PCRE  functions  can be used in multi-threading applications, with
929           the  proviso  that  the  memory  management  functions  pointed  to  by
930           pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
931           callout function pointed to by pcre_callout, are shared by all threads.
932    
933           The  compiled form of a regular expression is not altered during match-
934           ing, so the same compiled pattern can safely be used by several threads
935           at once.
936    
937    
938    SAVING PRECOMPILED PATTERNS FOR LATER USE
939    
940           The compiled form of a regular expression can be saved and re-used at a
941           later time, possibly by a different program, and even on a  host  other
942           than  the  one  on  which  it  was  compiled.  Details are given in the
943           pcreprecompile documentation. However, compiling a  regular  expression
944           with  one version of PCRE for use with a different version is not guar-
945           anteed to work and may cause crashes.
946    
      applications,  with  the  proviso that the memory management  
      functions pointed to by pcre_malloc and pcre_free are shared  
      by all threads.  
947    
948       The compiled form of a regular  expression  is  not  altered  CHECKING BUILD-TIME OPTIONS
      during  matching, so the same compiled pattern can safely be  
      used by several threads at once.  
949    
950           int pcre_config(int what, void *where);
951    
952           The function pcre_config() makes it possible for a PCRE client to  dis-
953           cover which optional features have been compiled into the PCRE library.
954           The pcrebuild documentation has more details about these optional  fea-
955           tures.
956    
957           The  first  argument  for pcre_config() is an integer, specifying which
958           information is required; the second argument is a pointer to a variable
959           into  which  the  information  is  placed. The following information is
960           available:
961    
962             PCRE_CONFIG_UTF8
963    
964           The output is an integer that is set to one if UTF-8 support is  avail-
965           able; otherwise it is set to zero.
966    
967             PCRE_CONFIG_UNICODE_PROPERTIES
968    
969           The  output  is  an  integer  that is set to one if support for Unicode
970           character properties is available; otherwise it is set to zero.
971    
972             PCRE_CONFIG_NEWLINE
973    
974           The output is an integer whose value specifies  the  default  character
975           sequence  that is recognized as meaning "newline". The four values that
976           are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
977           and  -1  for  ANY. The default should normally be the standard sequence
978           for your operating system.
979    
980             PCRE_CONFIG_LINK_SIZE
981    
982           The output is an integer that contains the number  of  bytes  used  for
983           internal linkage in compiled regular expressions. The value is 2, 3, or
984           4. Larger values allow larger regular expressions to  be  compiled,  at
985           the  expense  of  slower matching. The default value of 2 is sufficient
986           for all but the most massive patterns, since  it  allows  the  compiled
987           pattern to be up to 64K in size.
988    
989             PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
990    
991           The  output  is  an integer that contains the threshold above which the
992           POSIX interface uses malloc() for output vectors. Further  details  are
993           given in the pcreposix documentation.
994    
995             PCRE_CONFIG_MATCH_LIMIT
996    
997           The output is an integer that gives the default limit for the number of
998           internal matching function calls in a  pcre_exec()  execution.  Further
999           details are given with pcre_exec() below.
1000    
1001             PCRE_CONFIG_MATCH_LIMIT_RECURSION
1002    
1003           The  output is an integer that gives the default limit for the depth of
1004           recursion when calling the internal matching function in a  pcre_exec()
1005           execution. Further details are given with pcre_exec() below.
1006    
1007             PCRE_CONFIG_STACKRECURSE
1008    
1009           The  output is an integer that is set to one if internal recursion when
1010           running pcre_exec() is implemented by recursive function calls that use
1011           the  stack  to remember their state. This is the usual way that PCRE is
1012           compiled. The output is zero if PCRE was compiled to use blocks of data
1013           on  the  heap  instead  of  recursive  function  calls.  In  this case,
1014           pcre_stack_malloc and  pcre_stack_free  are  called  to  manage  memory
1015           blocks on the heap, thus avoiding the use of the stack.
1016    
1017    
1018  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 this for conveni-  
      ence, but in fact pcre is just a typedef for void, since the  
      contents  of  the block are not externally defined. It is up  
      to the caller to free  the  memory  when  it  is  no  longer  
      required.  
   
      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.  
   
      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.  
1019    
1020           pcre *pcre_compile(const char *pattern, int options,
1021                const char **errptr, int *erroffset,
1022                const unsigned char *tableptr);
1023    
1024           pcre *pcre_compile2(const char *pattern, int options,
1025                int *errorcodeptr,
1026                const char **errptr, int *erroffset,
1027                const unsigned char *tableptr);
1028    
1029           Either of the functions pcre_compile() or pcre_compile2() can be called
1030           to compile a pattern into an internal form. The only difference between
1031           the  two interfaces is that pcre_compile2() has an additional argument,
1032           errorcodeptr, via which a numerical error code can be returned.
1033    
1034           The pattern is a C string terminated by a binary zero, and is passed in
1035           the  pattern  argument.  A  pointer to a single block of memory that is
1036           obtained via pcre_malloc is returned. This contains the  compiled  code
1037           and related data. The pcre type is defined for the returned block; this
1038           is a typedef for a structure whose contents are not externally defined.
1039           It is up to the caller to free the memory (via pcre_free) when it is no
1040           longer required.
1041    
1042           Although the compiled code of a PCRE regex is relocatable, that is,  it
1043           does not depend on memory location, the complete pcre data block is not
1044           fully relocatable, because it may contain a copy of the tableptr  argu-
1045           ment, which is an address (see below).
1046    
1047           The options argument contains various bit settings that affect the com-
1048           pilation. It should be zero if no options are required.  The  available
1049           options  are  described  below. Some of them, in particular, those that
1050           are compatible with Perl, can also be set and  unset  from  within  the
1051           pattern  (see  the  detailed  description in the pcrepattern documenta-
1052           tion). For these options, the contents of the options  argument  speci-
1053           fies  their initial settings at the start of compilation and execution.
1054           The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at  the  time
1055           of matching as well as at compile time.
1056    
1057           If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
1058           if compilation of a pattern fails,  pcre_compile()  returns  NULL,  and
1059           sets the variable pointed to by errptr to point to a textual error mes-
1060           sage. This is a static string that is part of the library. You must not
1061           try to free it. The offset from the start of the pattern to the charac-
1062           ter where the error was discovered is placed in the variable pointed to
1063           by  erroffset,  which must not be NULL. If it is, an immediate error is
1064           given.
1065    
1066           If pcre_compile2() is used instead of pcre_compile(),  and  the  error-
1067           codeptr  argument is not NULL, a non-zero error code number is returned
1068           via this argument in the event of an error. This is in addition to  the
1069           textual error message. Error codes and messages are listed below.
1070    
1071           If  the  final  argument, tableptr, is NULL, PCRE uses a default set of
1072           character tables that are  built  when  PCRE  is  compiled,  using  the
1073           default  C  locale.  Otherwise, tableptr must be an address that is the
1074           result of a call to pcre_maketables(). This value is  stored  with  the
1075           compiled  pattern,  and used again by pcre_exec(), unless another table
1076           pointer is passed to it. For more discussion, see the section on locale
1077           support below.
1078    
1079           This  code  fragment  shows a typical straightforward call to pcre_com-
1080           pile():
1081    
1082             pcre *re;
1083             const char *error;
1084             int erroffset;
1085             re = pcre_compile(
1086               "^A.*Z",          /* the pattern */
1087               0,                /* default options */
1088               &error,           /* for error message */
1089               &erroffset,       /* for error offset */
1090               NULL);            /* use default character tables */
1091    
1092           The following names for option bits are defined in  the  pcre.h  header
1093           file:
1094    
1095             PCRE_ANCHORED
1096    
1097           If this bit is set, the pattern is forced to be "anchored", that is, it
1098           is constrained to match only at the first matching point in the  string
1099           that  is being searched (the "subject string"). This effect can also be
1100           achieved by appropriate constructs in the pattern itself, which is  the
1101           only way to do it in Perl.
1102    
1103             PCRE_AUTO_CALLOUT
1104    
1105           If this bit is set, pcre_compile() automatically inserts callout items,
1106           all with number 255, before each pattern item. For  discussion  of  the
1107           callout facility, see the pcrecallout documentation.
1108    
1109             PCRE_CASELESS
1110    
1111           If  this  bit is set, letters in the pattern match both upper and lower
1112           case letters. It is equivalent to Perl's  /i  option,  and  it  can  be
1113           changed  within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1114           always understands the concept of case for characters whose values  are
1115           less  than 128, so caseless matching is always possible. For characters
1116           with higher values, the concept of case is supported if  PCRE  is  com-
1117           piled  with Unicode property support, but not otherwise. If you want to
1118           use caseless matching for characters 128 and  above,  you  must  ensure
1119           that  PCRE  is  compiled  with Unicode property support as well as with
1120           UTF-8 support.
1121    
1122             PCRE_DOLLAR_ENDONLY
1123    
1124           If this bit is set, a dollar metacharacter in the pattern matches  only
1125           at  the  end  of the subject string. Without this option, a dollar also
1126           matches immediately before a newline at the end of the string (but  not
1127           before  any  other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1128           if PCRE_MULTILINE is set.  There is no equivalent  to  this  option  in
1129           Perl, and no way to set it within a pattern.
1130    
1131             PCRE_DOTALL
1132    
1133           If this bit is set, a dot metacharater in the pattern matches all char-
1134           acters, including those that indicate newline. Without it, a  dot  does
1135           not  match  when  the  current position is at a newline. This option is
1136           equivalent to Perl's /s option, and it can be changed within a  pattern
1137           by  a (?s) option setting. A negative class such as [^a] always matches
1138           newline characters, independent of the setting of this option.
1139    
1140             PCRE_DUPNAMES
1141    
1142           If this bit is set, names used to identify capturing  subpatterns  need
1143           not be unique. This can be helpful for certain types of pattern when it
1144           is known that only one instance of the named  subpattern  can  ever  be
1145           matched.  There  are  more details of named subpatterns below; see also
1146           the pcrepattern documentation.
1147    
1148             PCRE_EXTENDED
1149    
1150           If this bit is set, whitespace  data  characters  in  the  pattern  are
1151           totally ignored except when escaped or inside a character class. White-
1152           space does not include the VT character (code 11). In addition, charac-
1153           ters between an unescaped # outside a character class and the next new-
1154           line, inclusive, are also ignored. This  is  equivalent  to  Perl's  /x
1155           option,  and  it  can be changed within a pattern by a (?x) option set-
1156           ting.
1157    
1158           This option makes it possible to include  comments  inside  complicated
1159           patterns.   Note,  however,  that this applies only to data characters.
1160           Whitespace  characters  may  never  appear  within  special   character
1161           sequences  in  a  pattern,  for  example  within the sequence (?( which
1162           introduces a conditional subpattern.
1163    
1164             PCRE_EXTRA
1165    
1166           This option was invented in order to turn on  additional  functionality
1167           of  PCRE  that  is  incompatible with Perl, but it is currently of very
1168           little use. When set, any backslash in a pattern that is followed by  a
1169           letter  that  has  no  special  meaning causes an error, thus reserving
1170           these combinations for future expansion. By  default,  as  in  Perl,  a
1171           backslash  followed by a letter with no special meaning is treated as a
1172           literal. (Perl can, however, be persuaded to give a warning for  this.)
1173           There  are  at  present no other features controlled by this option. It
1174           can also be set by a (?X) option setting within a pattern.
1175    
1176             PCRE_FIRSTLINE
1177    
1178           If this option is set, an  unanchored  pattern  is  required  to  match
1179           before  or  at  the  first  newline  in  the subject string, though the
1180           matched text may continue over the newline.
1181    
1182             PCRE_MULTILINE
1183    
1184           By default, PCRE treats the subject string as consisting  of  a  single
1185           line  of characters (even if it actually contains newlines). The "start
1186           of line" metacharacter (^) matches only at the  start  of  the  string,
1187           while  the  "end  of line" metacharacter ($) matches only at the end of
1188           the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1189           is set). This is the same as Perl.
1190    
1191           When  PCRE_MULTILINE  it  is set, the "start of line" and "end of line"
1192           constructs match immediately following or immediately  before  internal
1193           newlines  in  the  subject string, respectively, as well as at the very
1194           start and end. This is equivalent to Perl's /m option, and  it  can  be
1195           changed within a pattern by a (?m) option setting. If there are no new-
1196           lines in a subject string, or no occurrences of ^ or $  in  a  pattern,
1197           setting PCRE_MULTILINE has no effect.
1198    
1199             PCRE_NEWLINE_CR
1200             PCRE_NEWLINE_LF
1201             PCRE_NEWLINE_CRLF
1202             PCRE_NEWLINE_ANYCRLF
1203             PCRE_NEWLINE_ANY
1204    
1205           These  options  override the default newline definition that was chosen
1206           when PCRE was built. Setting the first or the second specifies  that  a
1207           newline  is  indicated  by a single character (CR or LF, respectively).
1208           Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by  the
1209           two-character  CRLF  sequence.  Setting  PCRE_NEWLINE_ANYCRLF specifies
1210           that any of the three preceding sequences should be recognized. Setting
1211           PCRE_NEWLINE_ANY  specifies that any Unicode newline sequence should be
1212           recognized. The Unicode newline sequences are the three just mentioned,
1213           plus  the  single  characters  VT (vertical tab, U+000B), FF (formfeed,
1214           U+000C), NEL (next line, U+0085), LS (line separator, U+2028),  and  PS
1215           (paragraph  separator,  U+2029).  The  last  two are recognized only in
1216           UTF-8 mode.
1217    
1218           The newline setting in the  options  word  uses  three  bits  that  are
1219           treated as a number, giving eight possibilities. Currently only six are
1220           used (default plus the five values above). This means that if  you  set
1221           more  than one newline option, the combination may or may not be sensi-
1222           ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1223           PCRE_NEWLINE_CRLF,  but other combinations may yield unused numbers and
1224           cause an error.
1225    
1226           The only time that a line break is specially recognized when  compiling
1227           a  pattern  is  if  PCRE_EXTENDED  is set, and an unescaped # outside a
1228           character class is encountered. This indicates  a  comment  that  lasts
1229           until  after the next line break sequence. In other circumstances, line
1230           break  sequences  are  treated  as  literal  data,   except   that   in
1231           PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1232           and are therefore ignored.
1233    
1234           The newline option that is set at compile time becomes the default that
1235           is  used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1236    
1237             PCRE_NO_AUTO_CAPTURE
1238    
1239           If this option is set, it disables the use of numbered capturing paren-
1240           theses  in the pattern. Any opening parenthesis that is not followed by
1241           ? behaves as if it were followed by ?: but named parentheses can  still
1242           be  used  for  capturing  (and  they acquire numbers in the usual way).
1243           There is no equivalent of this option in Perl.
1244    
1245             PCRE_UNGREEDY
1246    
1247           This option inverts the "greediness" of the quantifiers  so  that  they
1248           are  not greedy by default, but become greedy if followed by "?". It is
1249           not compatible with Perl. It can also be set by a (?U)  option  setting
1250           within the pattern.
1251    
1252             PCRE_UTF8
1253    
1254           This  option  causes PCRE to regard both the pattern and the subject as
1255           strings of UTF-8 characters instead of single-byte  character  strings.
1256           However,  it is available only when PCRE is built to include UTF-8 sup-
1257           port. If not, the use of this option provokes an error. Details of  how
1258           this  option  changes the behaviour of PCRE are given in the section on
1259           UTF-8 support in the main pcre page.
1260    
1261             PCRE_NO_UTF8_CHECK
1262    
1263           When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1264           automatically  checked.  There  is  a  discussion about the validity of
1265           UTF-8 strings in the main pcre page. If an invalid  UTF-8  sequence  of
1266           bytes  is  found,  pcre_compile() returns an error. If you already know
1267           that your pattern is valid, and you want to skip this check for perfor-
1268           mance  reasons,  you  can set the PCRE_NO_UTF8_CHECK option. When it is
1269           set, the effect of passing an invalid UTF-8  string  as  a  pattern  is
1270           undefined.  It  may  cause your program to crash. Note that this option
1271           can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress  the
1272           UTF-8 validity checking of subject strings.
1273    
1274    
1275    COMPILATION ERROR CODES
1276    
1277           The  following  table  lists  the  error  codes than may be returned by
1278           pcre_compile2(), along with the error messages that may be returned  by
1279           both  compiling functions. As PCRE has developed, some error codes have
1280           fallen out of use. To avoid confusion, they have not been re-used.
1281    
1282              0  no error
1283              1  \ at end of pattern
1284              2  \c at end of pattern
1285              3  unrecognized character follows \
1286              4  numbers out of order in {} quantifier
1287              5  number too big in {} quantifier
1288              6  missing terminating ] for character class
1289              7  invalid escape sequence in character class
1290              8  range out of order in character class
1291              9  nothing to repeat
1292             10  [this code is not in use]
1293             11  internal error: unexpected repeat
1294             12  unrecognized character after (?
1295             13  POSIX named classes are supported only within a class
1296             14  missing )
1297             15  reference to non-existent subpattern
1298             16  erroffset passed as NULL
1299             17  unknown option bit(s) set
1300             18  missing ) after comment
1301             19  [this code is not in use]
1302             20  regular expression too large
1303             21  failed to get memory
1304             22  unmatched parentheses
1305             23  internal error: code overflow
1306             24  unrecognized character after (?<
1307             25  lookbehind assertion is not fixed length
1308             26  malformed number or name after (?(
1309             27  conditional group contains more than two branches
1310             28  assertion expected after (?(
1311             29  (?R or (?[+-]digits must be followed by )
1312             30  unknown POSIX class name
1313             31  POSIX collating elements are not supported
1314             32  this version of PCRE is not compiled with PCRE_UTF8 support
1315             33  [this code is not in use]
1316             34  character value in \x{...} sequence is too large
1317             35  invalid condition (?(0)
1318             36  \C not allowed in lookbehind assertion
1319             37  PCRE does not support \L, \l, \N, \U, or \u
1320             38  number after (?C is > 255
1321             39  closing ) for (?C expected
1322             40  recursive call could loop indefinitely
1323             41  unrecognized character after (?P
1324             42  syntax error in subpattern name (missing terminator)
1325             43  two named subpatterns have the same name
1326             44  invalid UTF-8 string
1327             45  support for \P, \p, and \X has not been compiled
1328             46  malformed \P or \p sequence
1329             47  unknown property name after \P or \p
1330             48  subpattern name is too long (maximum 32 characters)
1331             49  too many named subpatterns (maximum 10,000)
1332             50  [this code is not in use]
1333             51  octal value is greater than \377 (not in UTF-8 mode)
1334             52  internal error: overran compiling workspace
1335             53  internal  error:  previously-checked  referenced  subpattern  not
1336           found
1337             54  DEFINE group contains more than one branch
1338             55  repeating a DEFINE group is not allowed
1339             56  inconsistent NEWLINE options"
1340             57  \g is not followed by a braced name or an optionally braced
1341                   non-zero number
1342             58  (?+ or (?- or (?(+ or (?(- must be followed by a non-zero number
1343    
1344    
1345  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  void  
      typedef)  containing  additional  information about the pat-  
      tern; 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.  
   
      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.  
1346    
1347           pcre_extra *pcre_study(const pcre *code, int options
1348                const char **errptr);
1349    
1350           If  a  compiled  pattern is going to be used several times, it is worth
1351           spending more time analyzing it in order to speed up the time taken for
1352           matching.  The function pcre_study() takes a pointer to a compiled pat-
1353           tern as its first argument. If studying the pattern produces additional
1354           information  that  will  help speed up matching, pcre_study() returns a
1355           pointer to a pcre_extra block, in which the study_data field points  to
1356           the results of the study.
1357    
1358           The  returned  value  from  pcre_study()  can  be  passed  directly  to
1359           pcre_exec(). However, a pcre_extra block  also  contains  other  fields
1360           that  can  be  set  by the caller before the block is passed; these are
1361           described below in the section on matching a pattern.
1362    
1363           If studying the pattern does not  produce  any  additional  information
1364           pcre_study() returns NULL. In that circumstance, if the calling program
1365           wants to pass any of the other fields to pcre_exec(), it  must  set  up
1366           its own pcre_extra block.
1367    
1368           The  second  argument of pcre_study() contains option bits. At present,
1369           no options are defined, and this argument should always be zero.
1370    
1371           The third argument for pcre_study() is a pointer for an error  message.
1372           If  studying  succeeds  (even  if no data is returned), the variable it
1373           points to is set to NULL. Otherwise it is set to  point  to  a  textual
1374           error message. This is a static string that is part of the library. You
1375           must not try to free it. You should test the  error  pointer  for  NULL
1376           after calling pcre_study(), to be sure that it has run successfully.
1377    
1378           This is a typical call to pcre_study():
1379    
1380             pcre_extra *pe;
1381             pe = pcre_study(
1382               re,             /* result of pcre_compile() */
1383               0,              /* no options exist */
1384               &error);        /* set to NULL or points to a message */
1385    
1386           At present, studying a pattern is useful only for non-anchored patterns
1387           that do not have a single fixed starting character. A bitmap of  possi-
1388           ble starting bytes is created.
1389    
1390    
1391  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.  
1392    
1393           PCRE  handles  caseless matching, and determines whether characters are
1394           letters, digits, or whatever, by reference to a set of tables,  indexed
1395           by  character  value.  When running in UTF-8 mode, this applies only to
1396           characters with codes less than 128. Higher-valued  codes  never  match
1397           escapes  such  as  \w or \d, but can be tested with \p if PCRE is built
1398           with Unicode character property support. The use of locales  with  Uni-
1399           code  is discouraged. If you are handling characters with codes greater
1400           than 128, you should either use UTF-8 and Unicode, or use locales,  but
1401           not try to mix the two.
1402    
1403           PCRE  contains  an  internal set of tables that are used when the final
1404           argument of pcre_compile() is  NULL.  These  are  sufficient  for  many
1405           applications.  Normally, the internal tables recognize only ASCII char-
1406           acters. However, when PCRE is built, it is possible to cause the inter-
1407           nal tables to be rebuilt in the default "C" locale of the local system,
1408           which may cause them to be different.
1409    
1410           The internal tables can always be overridden by tables supplied by  the
1411           application that calls PCRE. These may be created in a different locale
1412           from the default. As more and more applications change  to  using  Uni-
1413           code, the need for this locale support is expected to die away.
1414    
1415           External  tables  are  built by calling the pcre_maketables() function,
1416           which has no arguments, in the relevant locale. The result can then  be
1417           passed  to  pcre_compile()  or  pcre_exec()  as often as necessary. For
1418           example, to build and use tables that are appropriate  for  the  French
1419           locale  (where  accented  characters  with  values greater than 128 are
1420           treated as letters), the following code could be used:
1421    
1422             setlocale(LC_CTYPE, "fr_FR");
1423             tables = pcre_maketables();
1424             re = pcre_compile(..., tables);
1425    
1426           The locale name "fr_FR" is used on Linux and other  Unix-like  systems;
1427           if you are using Windows, the name for the French locale is "french".
1428    
1429           When  pcre_maketables()  runs,  the  tables are built in memory that is
1430           obtained via pcre_malloc. It is the caller's responsibility  to  ensure
1431           that  the memory containing the tables remains available for as long as
1432           it is needed.
1433    
1434           The pointer that is passed to pcre_compile() is saved with the compiled
1435           pattern,  and the same tables are used via this pointer by pcre_study()
1436           and normally also by pcre_exec(). Thus, by default, for any single pat-
1437           tern, compilation, studying and matching all happen in the same locale,
1438           but different patterns can be compiled in different locales.
1439    
1440           It is possible to pass a table pointer or NULL (indicating the  use  of
1441           the  internal  tables)  to  pcre_exec(). Although not intended for this
1442           purpose, this facility could be used to match a pattern in a  different
1443           locale from the one in which it was compiled. Passing table pointers at
1444           run time is discussed below in the section on matching a pattern.
1445    
1446    
1447  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  
   
      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 au 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  
      pattern.  The  fourth  argument should point to an int vari-  
      able. 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).  
1448    
1449           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1450                int what, void *where);
1451    
1452           The pcre_fullinfo() function returns information about a compiled  pat-
1453           tern. It replaces the obsolete pcre_info() function, which is neverthe-
1454           less retained for backwards compability (and is documented below).
1455    
1456           The first argument for pcre_fullinfo() is a  pointer  to  the  compiled
1457           pattern.  The second argument is the result of pcre_study(), or NULL if
1458           the pattern was not studied. The third argument specifies  which  piece
1459           of  information  is required, and the fourth argument is a pointer to a
1460           variable to receive the data. The yield of the  function  is  zero  for
1461           success, or one of the following negative numbers:
1462    
1463             PCRE_ERROR_NULL       the argument code was NULL
1464                                   the argument where was NULL
1465             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1466             PCRE_ERROR_BADOPTION  the value of what was invalid
1467    
1468           The  "magic  number" is placed at the start of each compiled pattern as
1469           an simple check against passing an arbitrary memory pointer. Here is  a
1470           typical  call  of pcre_fullinfo(), to obtain the length of the compiled
1471           pattern:
1472    
1473             int rc;
1474             size_t length;
1475             rc = pcre_fullinfo(
1476               re,               /* result of pcre_compile() */
1477               pe,               /* result of pcre_study(), or NULL */
1478               PCRE_INFO_SIZE,   /* what is required */
1479               &length);         /* where to put the data */
1480    
1481           The possible values for the third argument are defined in  pcre.h,  and
1482           are as follows:
1483    
1484             PCRE_INFO_BACKREFMAX
1485    
1486           Return  the  number  of  the highest back reference in the pattern. The
1487           fourth argument should point to an int variable. Zero  is  returned  if
1488           there are no back references.
1489    
1490             PCRE_INFO_CAPTURECOUNT
1491    
1492           Return  the  number of capturing subpatterns in the pattern. The fourth
1493           argument should point to an int variable.
1494    
1495             PCRE_INFO_DEFAULT_TABLES
1496    
1497           Return a pointer to the internal default character tables within  PCRE.
1498           The  fourth  argument should point to an unsigned char * variable. This
1499           information call is provided for internal use by the pcre_study() func-
1500           tion.  External  callers  can  cause PCRE to use its internal tables by
1501           passing a NULL table pointer.
1502    
1503             PCRE_INFO_FIRSTBYTE
1504    
1505           Return information about the first byte of any matched  string,  for  a
1506           non-anchored  pattern. The fourth argument should point to an int vari-
1507           able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old  name
1508           is still recognized for backwards compatibility.)
1509    
1510           If  there  is  a  fixed first byte, for example, from a pattern such as
1511           (cat|cow|coyote), its value is returned. Otherwise, if either
1512    
1513           (a) the pattern was compiled with the PCRE_MULTILINE option, and  every
1514           branch starts with "^", or
1515    
1516           (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1517           set (if it were set, the pattern would be anchored),
1518    
1519           -1 is returned, indicating that the pattern matches only at  the  start
1520           of  a  subject string or after any newline within the string. Otherwise
1521           -2 is returned. For anchored patterns, -2 is returned.
1522    
1523             PCRE_INFO_FIRSTTABLE
1524    
1525           If the pattern was studied, and this resulted in the construction of  a
1526           256-bit table indicating a fixed set of bytes for the first byte in any
1527           matching string, a pointer to the table is returned. Otherwise NULL  is
1528           returned.  The fourth argument should point to an unsigned char * vari-
1529           able.
1530    
1531             PCRE_INFO_HASCRORLF
1532    
1533           Return 1 if the pattern contains any explicit  matches  for  CR  or  LF
1534           characters,  otherwise  0.  The  fourth argument should point to an int
1535           variable.
1536    
1537             PCRE_INFO_JCHANGED
1538    
1539           Return 1 if the (?J) option setting is used in the  pattern,  otherwise
1540           0. The fourth argument should point to an int variable. The (?J) inter-
1541           nal option setting changes the local PCRE_DUPNAMES option.
1542    
1543             PCRE_INFO_LASTLITERAL
1544    
1545           Return the value of the rightmost literal byte that must exist  in  any
1546           matched  string,  other  than  at  its  start,  if such a byte has been
1547           recorded. The fourth argument should point to an int variable. If there
1548           is  no such byte, -1 is returned. For anchored patterns, a last literal
1549           byte is recorded only if it follows something of variable  length.  For
1550           example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1551           /^a\dz\d/ the returned value is -1.
1552    
1553             PCRE_INFO_NAMECOUNT
1554             PCRE_INFO_NAMEENTRYSIZE
1555             PCRE_INFO_NAMETABLE
1556    
1557           PCRE supports the use of named as well as numbered capturing  parenthe-
1558           ses.  The names are just an additional way of identifying the parenthe-
1559           ses, which still acquire numbers. Several convenience functions such as
1560           pcre_get_named_substring()  are  provided  for extracting captured sub-
1561           strings by name. It is also possible to extract the data  directly,  by
1562           first  converting  the  name to a number in order to access the correct
1563           pointers in the output vector (described with pcre_exec() below). To do
1564           the  conversion,  you  need  to  use  the  name-to-number map, which is
1565           described by these three values.
1566    
1567           The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1568           gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1569           of each entry; both of these  return  an  int  value.  The  entry  size
1570           depends  on the length of the longest name. PCRE_INFO_NAMETABLE returns
1571           a pointer to the first entry of the table  (a  pointer  to  char).  The
1572           first two bytes of each entry are the number of the capturing parenthe-
1573           sis, most significant byte first. The rest of the entry is  the  corre-
1574           sponding  name,  zero  terminated. The names are in alphabetical order.
1575           When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1576           theses  numbers.  For  example,  consider the following pattern (assume
1577           PCRE_EXTENDED is  set,  so  white  space  -  including  newlines  -  is
1578           ignored):
1579    
1580             (?<date> (?<year>(\d\d)?\d\d) -
1581             (?<month>\d\d) - (?<day>\d\d) )
1582    
1583           There  are  four  named subpatterns, so the table has four entries, and
1584           each entry in the table is eight bytes long. The table is  as  follows,
1585           with non-printing bytes shows in hexadecimal, and undefined bytes shown
1586           as ??:
1587    
1588             00 01 d  a  t  e  00 ??
1589             00 05 d  a  y  00 ?? ??
1590             00 04 m  o  n  t  h  00
1591             00 02 y  e  a  r  00 ??
1592    
1593           When writing code to extract data  from  named  subpatterns  using  the
1594           name-to-number  map,  remember that the length of the entries is likely
1595           to be different for each compiled pattern.
1596    
1597             PCRE_INFO_OKPARTIAL
1598    
1599           Return 1 if the pattern can be used for partial matching, otherwise  0.
1600           The  fourth  argument  should point to an int variable. The pcrepartial
1601           documentation lists the restrictions that apply to patterns  when  par-
1602           tial matching is used.
1603    
1604             PCRE_INFO_OPTIONS
1605    
1606           Return  a  copy of the options with which the pattern was compiled. The
1607           fourth argument should point to an unsigned long  int  variable.  These
1608           option bits are those specified in the call to pcre_compile(), modified
1609           by any top-level option settings at the start of the pattern itself. In
1610           other  words,  they are the options that will be in force when matching
1611           starts. For example, if the pattern /(?im)abc(?-i)d/ is  compiled  with
1612           the  PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
1613           and PCRE_EXTENDED.
1614    
1615           A pattern is automatically anchored by PCRE if  all  of  its  top-level
1616           alternatives begin with one of the following:
1617    
1618             ^     unless PCRE_MULTILINE is set
1619             \A    always
1620             \G    always
1621             .*    if PCRE_DOTALL is set and there are no back
1622                     references to the subpattern in which .* appears
1623    
1624           For such patterns, the PCRE_ANCHORED bit is set in the options returned
1625           by pcre_fullinfo().
1626    
1627             PCRE_INFO_SIZE
1628    
1629           Return the size of the compiled pattern, that is, the  value  that  was
1630           passed as the argument to pcre_malloc() when PCRE was getting memory in
1631           which to place the compiled data. The fourth argument should point to a
1632           size_t variable.
1633    
1634             PCRE_INFO_STUDYSIZE
1635    
1636           Return the size of the data block pointed to by the study_data field in
1637           a pcre_extra block. That is,  it  is  the  value  that  was  passed  to
1638           pcre_malloc() when PCRE was getting memory into which to place the data
1639           created by pcre_study(). The fourth argument should point to  a  size_t
1640           variable.
1641    
1642    
1643    OBSOLETE INFO FUNCTION
1644    
1645           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1646    
1647           The  pcre_info()  function is now obsolete because its interface is too
1648           restrictive to return all the available data about a compiled  pattern.
1649           New   programs   should  use  pcre_fullinfo()  instead.  The  yield  of
1650           pcre_info() is the number of capturing subpatterns, or one of the  fol-
1651           lowing negative numbers:
1652    
1653             PCRE_ERROR_NULL       the argument code was NULL
1654             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1655    
1656           If  the  optptr  argument is not NULL, a copy of the options with which
1657           the pattern was compiled is placed in the integer  it  points  to  (see
1658           PCRE_INFO_OPTIONS above).
1659    
1660           If  the  pattern  is  not anchored and the firstcharptr argument is not
1661           NULL, it is used to pass back information about the first character  of
1662           any matched string (see PCRE_INFO_FIRSTBYTE above).
1663    
1664    
1665    REFERENCE COUNTS
1666    
1667           int pcre_refcount(pcre *code, int adjust);
1668    
1669           The  pcre_refcount()  function is used to maintain a reference count in
1670           the data block that contains a compiled pattern. It is provided for the
1671           benefit  of  applications  that  operate  in an object-oriented manner,
1672           where different parts of the application may be using the same compiled
1673           pattern, but you want to free the block when they are all done.
1674    
1675           When a pattern is compiled, the reference count field is initialized to
1676           zero.  It is changed only by calling this function, whose action is  to
1677           add  the  adjust  value  (which may be positive or negative) to it. The
1678           yield of the function is the new value. However, the value of the count
1679           is  constrained to lie between 0 and 65535, inclusive. If the new value
1680           is outside these limits, it is forced to the appropriate limit value.
1681    
1682           Except when it is zero, the reference count is not correctly  preserved
1683           if  a  pattern  is  compiled on one host and then transferred to a host
1684           whose byte-order is different. (This seems a highly unlikely scenario.)
1685    
1686    
1687    MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1688    
1689           int pcre_exec(const pcre *code, const pcre_extra *extra,
1690                const char *subject, int length, int startoffset,
1691                int options, int *ovector, int ovecsize);
1692    
1693           The  function pcre_exec() is called to match a subject string against a
1694           compiled pattern, which is passed in the code argument. If the  pattern
1695           has been studied, the result of the study should be passed in the extra
1696           argument. This function is the main matching facility of  the  library,
1697           and it operates in a Perl-like manner. For specialist use there is also
1698           an alternative matching function, which is described below in the  sec-
1699           tion about the pcre_dfa_exec() function.
1700    
1701           In  most applications, the pattern will have been compiled (and option-
1702           ally studied) in the same process that calls pcre_exec().  However,  it
1703           is possible to save compiled patterns and study data, and then use them
1704           later in different processes, possibly even on different hosts.  For  a
1705           discussion about this, see the pcreprecompile documentation.
1706    
1707           Here is an example of a simple call to pcre_exec():
1708    
1709             int rc;
1710             int ovector[30];
1711             rc = pcre_exec(
1712               re,             /* result of pcre_compile() */
1713               NULL,           /* we didn't study the pattern */
1714               "some string",  /* the subject string */
1715               11,             /* the length of the subject string */
1716               0,              /* start at offset 0 in the subject */
1717               0,              /* default options */
1718               ovector,        /* vector of integers for substring information */
1719               30);            /* number of elements (NOT size in bytes) */
1720    
1721       Extra data for pcre_exec()
1722    
1723           If  the  extra argument is not NULL, it must point to a pcre_extra data
1724           block. The pcre_study() function returns such a block (when it  doesn't
1725           return  NULL), but you can also create one for yourself, and pass addi-
1726           tional information in it. The pcre_extra block contains  the  following
1727           fields (not necessarily in this order):
1728    
1729             unsigned long int flags;
1730             void *study_data;
1731             unsigned long int match_limit;
1732             unsigned long int match_limit_recursion;
1733             void *callout_data;
1734             const unsigned char *tables;
1735    
1736           The  flags  field  is a bitmap that specifies which of the other fields
1737           are set. The flag bits are:
1738    
1739             PCRE_EXTRA_STUDY_DATA
1740             PCRE_EXTRA_MATCH_LIMIT
1741             PCRE_EXTRA_MATCH_LIMIT_RECURSION
1742             PCRE_EXTRA_CALLOUT_DATA
1743             PCRE_EXTRA_TABLES
1744    
1745           Other flag bits should be set to zero. The study_data field is  set  in
1746           the  pcre_extra  block  that is returned by pcre_study(), together with
1747           the appropriate flag bit. You should not set this yourself, but you may
1748           add  to  the  block by setting the other fields and their corresponding
1749           flag bits.
1750    
1751           The match_limit field provides a means of preventing PCRE from using up
1752           a  vast amount of resources when running patterns that are not going to
1753           match, but which have a very large number  of  possibilities  in  their
1754           search  trees.  The  classic  example  is  the  use of nested unlimited
1755           repeats.
1756    
1757           Internally, PCRE uses a function called match() which it calls  repeat-
1758           edly  (sometimes  recursively). The limit set by match_limit is imposed
1759           on the number of times this function is called during  a  match,  which
1760           has  the  effect  of  limiting the amount of backtracking that can take
1761           place. For patterns that are not anchored, the count restarts from zero
1762           for each position in the subject string.
1763    
1764           The  default  value  for  the  limit can be set when PCRE is built; the
1765           default default is 10 million, which handles all but the  most  extreme
1766           cases.  You  can  override  the  default by suppling pcre_exec() with a
1767           pcre_extra    block    in    which    match_limit    is    set,     and
1768           PCRE_EXTRA_MATCH_LIMIT  is  set  in  the  flags  field. If the limit is
1769           exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1770    
1771           The match_limit_recursion field is similar to match_limit, but  instead
1772           of limiting the total number of times that match() is called, it limits
1773           the depth of recursion. The recursion depth is a  smaller  number  than
1774           the  total number of calls, because not all calls to match() are recur-
1775           sive.  This limit is of use only if it is set smaller than match_limit.
1776    
1777           Limiting  the  recursion  depth  limits the amount of stack that can be
1778           used, or, when PCRE has been compiled to use memory on the heap instead
1779           of the stack, the amount of heap memory that can be used.
1780    
1781           The  default  value  for  match_limit_recursion can be set when PCRE is
1782           built; the default default  is  the  same  value  as  the  default  for
1783           match_limit.  You can override the default by suppling pcre_exec() with
1784           a  pcre_extra  block  in  which  match_limit_recursion  is   set,   and
1785           PCRE_EXTRA_MATCH_LIMIT_RECURSION  is  set  in  the  flags field. If the
1786           limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1787    
1788           The pcre_callout field is used in conjunction with the  "callout"  fea-
1789           ture, which is described in the pcrecallout documentation.
1790    
1791           The  tables  field  is  used  to  pass  a  character  tables pointer to
1792           pcre_exec(); this overrides the value that is stored with the  compiled
1793           pattern.  A  non-NULL value is stored with the compiled pattern only if
1794           custom tables were supplied to pcre_compile() via  its  tableptr  argu-
1795           ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
1796           PCRE's internal tables to be used. This facility is  helpful  when  re-
1797           using  patterns  that  have been saved after compiling with an external
1798           set of tables, because the external tables  might  be  at  a  different
1799           address  when  pcre_exec() is called. See the pcreprecompile documenta-
1800           tion for a discussion of saving compiled patterns for later use.
1801    
1802       Option bits for pcre_exec()
1803    
1804           The unused bits of the options argument for pcre_exec() must  be  zero.
1805           The  only  bits  that  may  be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
1806           PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NO_UTF8_CHECK   and
1807           PCRE_PARTIAL.
1808    
1809             PCRE_ANCHORED
1810    
1811           The  PCRE_ANCHORED  option  limits pcre_exec() to matching at the first
1812           matching position. If a pattern was  compiled  with  PCRE_ANCHORED,  or
1813           turned  out to be anchored by virtue of its contents, it cannot be made
1814           unachored at matching time.
1815    
1816             PCRE_NEWLINE_CR
1817             PCRE_NEWLINE_LF
1818             PCRE_NEWLINE_CRLF
1819             PCRE_NEWLINE_ANYCRLF
1820             PCRE_NEWLINE_ANY
1821    
1822           These options override  the  newline  definition  that  was  chosen  or
1823           defaulted  when the pattern was compiled. For details, see the descrip-
1824           tion of pcre_compile()  above.  During  matching,  the  newline  choice
1825           affects  the  behaviour  of the dot, circumflex, and dollar metacharac-
1826           ters. It may also alter the way the match position is advanced after  a
1827           match failure for an unanchored pattern.
1828    
1829           When  PCRE_NEWLINE_CRLF,  PCRE_NEWLINE_ANYCRLF,  or PCRE_NEWLINE_ANY is
1830           set, and a match attempt for an unanchored pattern fails when the  cur-
1831           rent  position  is  at  a  CRLF  sequence,  and the pattern contains no
1832           explicit matches for  CR  or  NL  characters,  the  match  position  is
1833           advanced by two characters instead of one, in other words, to after the
1834           CRLF.
1835    
1836           The above rule is a compromise that makes the most common cases work as
1837           expected.  For  example,  if  the  pattern  is .+A (and the PCRE_DOTALL
1838           option is not set), it does not match the string "\r\nA" because, after
1839           failing  at the start, it skips both the CR and the LF before retrying.
1840           However, the pattern [\r\n]A does match that string,  because  it  con-
1841           tains an explicit CR or LF reference, and so advances only by one char-
1842           acter after the first failure.  Note than an explicit CR or  LF  refer-
1843           ence occurs for negated character classes such as [^X] because they can
1844           match CR or LF characters.
1845    
1846           Notwithstanding the above, anomalous effects may still occur when  CRLF
1847           is a valid newline sequence and explicit \r or \n escapes appear in the
1848           pattern.
1849    
1850             PCRE_NOTBOL
1851    
1852           This option specifies that first character of the subject string is not
1853           the  beginning  of  a  line, so the circumflex metacharacter should not
1854           match before it. Setting this without PCRE_MULTILINE (at compile  time)
1855           causes  circumflex  never to match. This option affects only the behav-
1856           iour of the circumflex metacharacter. It does not affect \A.
1857    
1858             PCRE_NOTEOL
1859    
1860           This option specifies that the end of the subject string is not the end
1861           of  a line, so the dollar metacharacter should not match it nor (except
1862           in multiline mode) a newline immediately before it. Setting this  with-
1863           out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1864           option affects only the behaviour of the dollar metacharacter. It  does
1865           not affect \Z or \z.
1866    
1867             PCRE_NOTEMPTY
1868    
1869           An empty string is not considered to be a valid match if this option is
1870           set. If there are alternatives in the pattern, they are tried.  If  all
1871           the  alternatives  match  the empty string, the entire match fails. For
1872           example, if the pattern
1873    
1874             a?b?
1875    
1876           is applied to a string not beginning with "a" or "b",  it  matches  the
1877           empty  string at the start of the subject. With PCRE_NOTEMPTY set, this
1878           match is not valid, so PCRE searches further into the string for occur-
1879           rences of "a" or "b".
1880    
1881           Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1882           cial case of a pattern match of the empty  string  within  its  split()
1883           function,  and  when  using  the /g modifier. It is possible to emulate
1884           Perl's behaviour after matching a null string by first trying the match
1885           again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1886           if that fails by advancing the starting offset (see below)  and  trying
1887           an ordinary match again. There is some code that demonstrates how to do
1888           this in the pcredemo.c sample program.
1889    
1890             PCRE_NO_UTF8_CHECK
1891    
1892           When PCRE_UTF8 is set at compile time, the validity of the subject as a
1893           UTF-8  string is automatically checked when pcre_exec() is subsequently
1894           called.  The value of startoffset is also checked  to  ensure  that  it
1895           points  to  the start of a UTF-8 character. There is a discussion about
1896           the validity of UTF-8 strings in the section on UTF-8  support  in  the
1897           main  pcre  page.  If  an  invalid  UTF-8  sequence  of bytes is found,
1898           pcre_exec() returns the error PCRE_ERROR_BADUTF8. If  startoffset  con-
1899           tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.
1900    
1901           If  you  already  know that your subject is valid, and you want to skip
1902           these   checks   for   performance   reasons,   you   can    set    the
1903           PCRE_NO_UTF8_CHECK  option  when calling pcre_exec(). You might want to
1904           do this for the second and subsequent calls to pcre_exec() if  you  are
1905           making  repeated  calls  to  find  all  the matches in a single subject
1906           string. However, you should be  sure  that  the  value  of  startoffset
1907           points  to  the  start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
1908           set, the effect of passing an invalid UTF-8 string as a subject,  or  a
1909           value  of startoffset that does not point to the start of a UTF-8 char-
1910           acter, is undefined. Your program may crash.
1911    
1912             PCRE_PARTIAL
1913    
1914           This option turns on the  partial  matching  feature.  If  the  subject
1915           string  fails to match the pattern, but at some point during the match-
1916           ing process the end of the subject was reached (that  is,  the  subject
1917           partially  matches  the  pattern and the failure to match occurred only
1918           because there were not enough subject characters), pcre_exec()  returns
1919           PCRE_ERROR_PARTIAL  instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
1920           used, there are restrictions on what may appear in the  pattern.  These
1921           are discussed in the pcrepartial documentation.
1922    
1923       The string to be matched by pcre_exec()
1924    
1925           The  subject string is passed to pcre_exec() as a pointer in subject, a
1926           length in length, and a starting byte offset in startoffset.  In  UTF-8
1927           mode,  the  byte  offset  must point to the start of a UTF-8 character.
1928           Unlike the pattern string, the subject may contain binary  zero  bytes.
1929           When  the starting offset is zero, the search for a match starts at the
1930           beginning of the subject, and this is by far the most common case.
1931    
1932           A non-zero starting offset is useful when searching for  another  match
1933           in  the same subject by calling pcre_exec() again after a previous suc-
1934           cess.  Setting startoffset differs from just passing over  a  shortened
1935           string  and  setting  PCRE_NOTBOL  in the case of a pattern that begins
1936           with any kind of lookbehind. For example, consider the pattern
1937    
1938             \Biss\B
1939    
1940           which finds occurrences of "iss" in the middle of  words.  (\B  matches
1941           only  if  the  current position in the subject is not a word boundary.)
1942           When applied to the string "Mississipi" the first call  to  pcre_exec()
1943           finds  the  first  occurrence. If pcre_exec() is called again with just
1944           the remainder of the subject,  namely  "issipi",  it  does  not  match,
1945           because \B is always false at the start of the subject, which is deemed
1946           to be a word boundary. However, if pcre_exec()  is  passed  the  entire
1947           string again, but with startoffset set to 4, it finds the second occur-
1948           rence of "iss" because it is able to look behind the starting point  to
1949           discover that it is preceded by a letter.
1950    
1951           If  a  non-zero starting offset is passed when the pattern is anchored,
1952           one attempt to match at the given offset is made. This can only succeed
1953           if  the  pattern  does  not require the match to be at the start of the
1954           subject.
1955    
1956       How pcre_exec() returns captured substrings
1957    
1958           In general, a pattern matches a certain portion of the subject, and  in
1959           addition,  further  substrings  from  the  subject may be picked out by
1960           parts of the pattern. Following the usage  in  Jeffrey  Friedl's  book,
1961           this  is  called "capturing" in what follows, and the phrase "capturing
1962           subpattern" is used for a fragment of a pattern that picks out  a  sub-
1963           string.  PCRE  supports several other kinds of parenthesized subpattern
1964           that do not cause substrings to be captured.
1965    
1966           Captured substrings are returned to the caller via a vector of  integer
1967           offsets  whose  address is passed in ovector. The number of elements in
1968           the vector is passed in ovecsize, which must be a non-negative  number.
1969           Note: this argument is NOT the size of ovector in bytes.
1970    
1971           The  first  two-thirds of the vector is used to pass back captured sub-
1972           strings, each substring using a pair of integers. The  remaining  third
1973           of  the  vector is used as workspace by pcre_exec() while matching cap-
1974           turing subpatterns, and is not available for passing back  information.
1975           The  length passed in ovecsize should always be a multiple of three. If
1976           it is not, it is rounded down.
1977    
1978           When a match is successful, information about  captured  substrings  is
1979           returned  in  pairs  of integers, starting at the beginning of ovector,
1980           and continuing up to two-thirds of its length at the  most.  The  first
1981           element of a pair is set to the offset of the first character in a sub-
1982           string, and the second is set to the  offset  of  the  first  character
1983           after  the  end  of  a  substring. The first pair, ovector[0] and ovec-
1984           tor[1], identify the portion of  the  subject  string  matched  by  the
1985           entire  pattern.  The next pair is used for the first capturing subpat-
1986           tern, and so on. The value returned by pcre_exec() is one more than the
1987           highest numbered pair that has been set. For example, if two substrings
1988           have been captured, the returned value is 3. If there are no  capturing
1989           subpatterns,  the return value from a successful match is 1, indicating
1990           that just the first pair of offsets has been set.
1991    
1992           If a capturing subpattern is matched repeatedly, it is the last portion
1993           of the string that it matched that is returned.
1994    
1995           If  the vector is too small to hold all the captured substring offsets,
1996           it is used as far as possible (up to two-thirds of its length), and the
1997           function  returns a value of zero. In particular, if the substring off-
1998           sets are not of interest, pcre_exec() may be called with ovector passed
1999           as  NULL  and  ovecsize  as zero. However, if the pattern contains back
2000           references and the ovector is not big enough to  remember  the  related
2001           substrings,  PCRE has to get additional memory for use during matching.
2002           Thus it is usually advisable to supply an ovector.
2003    
2004           The pcre_info() function can be used to find  out  how  many  capturing
2005           subpatterns  there  are  in  a  compiled pattern. The smallest size for
2006           ovector that will allow for n captured substrings, in addition  to  the
2007           offsets of the substring matched by the whole pattern, is (n+1)*3.
2008    
2009           It  is  possible for capturing subpattern number n+1 to match some part
2010           of the subject when subpattern n has not been used at all. For example,
2011           if  the  string  "abc"  is  matched against the pattern (a|(z))(bc) the
2012           return from the function is 4, and subpatterns 1 and 3 are matched, but
2013           2  is  not.  When  this happens, both values in the offset pairs corre-
2014           sponding to unused subpatterns are set to -1.
2015    
2016           Offset values that correspond to unused subpatterns at the end  of  the
2017           expression  are  also  set  to  -1. For example, if the string "abc" is
2018           matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are  not
2019           matched.  The  return  from the function is 2, because the highest used
2020           capturing subpattern number is 1. However, you can refer to the offsets
2021           for  the  second  and third capturing subpatterns if you wish (assuming
2022           the vector is large enough, of course).
2023    
2024           Some convenience functions are provided  for  extracting  the  captured
2025           substrings as separate strings. These are described below.
2026    
2027       Error return values from pcre_exec()
2028    
2029           If  pcre_exec()  fails, it returns a negative number. The following are
2030           defined in the header file:
2031    
2032             PCRE_ERROR_NOMATCH        (-1)
2033    
2034           The subject string did not match the pattern.
2035    
2036             PCRE_ERROR_NULL           (-2)
2037    
2038           Either code or subject was passed as NULL,  or  ovector  was  NULL  and
2039           ovecsize was not zero.
2040    
2041             PCRE_ERROR_BADOPTION      (-3)
2042    
2043           An unrecognized bit was set in the options argument.
2044    
2045             PCRE_ERROR_BADMAGIC       (-4)
2046    
2047           PCRE  stores a 4-byte "magic number" at the start of the compiled code,
2048           to catch the case when it is passed a junk pointer and to detect when a
2049           pattern that was compiled in an environment of one endianness is run in
2050           an environment with the other endianness. This is the error  that  PCRE
2051           gives when the magic number is not present.
2052    
2053             PCRE_ERROR_UNKNOWN_OPCODE (-5)
2054    
2055           While running the pattern match, an unknown item was encountered in the
2056           compiled pattern. This error could be caused by a bug  in  PCRE  or  by
2057           overwriting of the compiled pattern.
2058    
2059             PCRE_ERROR_NOMEMORY       (-6)
2060    
2061           If  a  pattern contains back references, but the ovector that is passed
2062           to pcre_exec() is not big enough to remember the referenced substrings,
2063           PCRE  gets  a  block of memory at the start of matching to use for this
2064           purpose. If the call via pcre_malloc() fails, this error is given.  The
2065           memory is automatically freed at the end of matching.
2066    
2067             PCRE_ERROR_NOSUBSTRING    (-7)
2068    
2069           This  error is used by the pcre_copy_substring(), pcre_get_substring(),
2070           and  pcre_get_substring_list()  functions  (see  below).  It  is  never
2071           returned by pcre_exec().
2072    
2073             PCRE_ERROR_MATCHLIMIT     (-8)
2074    
2075           The  backtracking  limit,  as  specified  by the match_limit field in a
2076           pcre_extra structure (or defaulted) was reached.  See  the  description
2077           above.
2078    
2079             PCRE_ERROR_CALLOUT        (-9)
2080    
2081           This error is never generated by pcre_exec() itself. It is provided for
2082           use by callout functions that want to yield a distinctive  error  code.
2083           See the pcrecallout documentation for details.
2084    
2085             PCRE_ERROR_BADUTF8        (-10)
2086    
2087           A  string  that contains an invalid UTF-8 byte sequence was passed as a
2088           subject.
2089    
2090             PCRE_ERROR_BADUTF8_OFFSET (-11)
2091    
2092           The UTF-8 byte sequence that was passed as a subject was valid, but the
2093           value  of startoffset did not point to the beginning of a UTF-8 charac-
2094           ter.
2095    
2096             PCRE_ERROR_PARTIAL        (-12)
2097    
2098           The subject string did not match, but it did match partially.  See  the
2099           pcrepartial documentation for details of partial matching.
2100    
2101             PCRE_ERROR_BADPARTIAL     (-13)
2102    
2103           The  PCRE_PARTIAL  option  was  used with a compiled pattern containing
2104           items that are not supported for partial matching. See the  pcrepartial
2105           documentation for details of partial matching.
2106    
2107             PCRE_ERROR_INTERNAL       (-14)
2108    
2109           An  unexpected  internal error has occurred. This error could be caused
2110           by a bug in PCRE or by overwriting of the compiled pattern.
2111    
2112             PCRE_ERROR_BADCOUNT       (-15)
2113    
2114           This error is given if the value of the ovecsize argument is  negative.
2115    
2116             PCRE_ERROR_RECURSIONLIMIT (-21)
2117    
2118           The internal recursion limit, as specified by the match_limit_recursion
2119           field in a pcre_extra structure (or defaulted)  was  reached.  See  the
2120           description above.
2121    
2122             PCRE_ERROR_BADNEWLINE     (-23)
2123    
2124           An invalid combination of PCRE_NEWLINE_xxx options was given.
2125    
2126           Error numbers -16 to -20 and -22 are not used by pcre_exec().
2127    
2128    
2129    EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2130    
2131           int pcre_copy_substring(const char *subject, int *ovector,
2132                int stringcount, int stringnumber, char *buffer,
2133                int buffersize);
2134    
2135           int pcre_get_substring(const char *subject, int *ovector,
2136                int stringcount, int stringnumber,
2137                const char **stringptr);
2138    
2139           int pcre_get_substring_list(const char *subject,
2140                int *ovector, int stringcount, const char ***listptr);
2141    
2142           Captured  substrings  can  be  accessed  directly  by using the offsets
2143           returned by pcre_exec() in  ovector.  For  convenience,  the  functions
2144           pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
2145           string_list() are provided for extracting captured substrings  as  new,
2146           separate,  zero-terminated strings. These functions identify substrings
2147           by number. The next section describes functions  for  extracting  named
2148           substrings.
2149    
2150           A  substring that contains a binary zero is correctly extracted and has
2151           a further zero added on the end, but the result is not, of course, a  C
2152           string.   However,  you  can  process such a string by referring to the
2153           length that is  returned  by  pcre_copy_substring()  and  pcre_get_sub-
2154           string().  Unfortunately, the interface to pcre_get_substring_list() is
2155           not adequate for handling strings containing binary zeros, because  the
2156           end of the final string is not independently indicated.
2157    
2158           The  first  three  arguments  are the same for all three of these func-
2159           tions: subject is the subject string that has  just  been  successfully
2160           matched, ovector is a pointer to the vector of integer offsets that was
2161           passed to pcre_exec(), and stringcount is the number of substrings that
2162           were  captured  by  the match, including the substring that matched the
2163           entire regular expression. This is the value returned by pcre_exec() if
2164           it  is greater than zero. If pcre_exec() returned zero, indicating that
2165           it ran out of space in ovector, the value passed as stringcount  should
2166           be the number of elements in the vector divided by three.
2167    
2168           The  functions pcre_copy_substring() and pcre_get_substring() extract a
2169           single substring, whose number is given as  stringnumber.  A  value  of
2170           zero  extracts  the  substring that matched the entire pattern, whereas
2171           higher values  extract  the  captured  substrings.  For  pcre_copy_sub-
2172           string(),  the  string  is  placed  in buffer, whose length is given by
2173           buffersize, while for pcre_get_substring() a new  block  of  memory  is
2174           obtained  via  pcre_malloc,  and its address is returned via stringptr.
2175           The yield of the function is the length of the  string,  not  including
2176           the terminating zero, or one of these error codes:
2177    
2178             PCRE_ERROR_NOMEMORY       (-6)
2179    
2180           The  buffer  was too small for pcre_copy_substring(), or the attempt to
2181           get memory failed for pcre_get_substring().
2182    
2183             PCRE_ERROR_NOSUBSTRING    (-7)
2184    
2185           There is no substring whose number is stringnumber.
2186    
2187           The pcre_get_substring_list()  function  extracts  all  available  sub-
2188           strings  and  builds  a list of pointers to them. All this is done in a
2189           single block of memory that is obtained via pcre_malloc. The address of
2190           the  memory  block  is returned via listptr, which is also the start of
2191           the list of string pointers. The end of the list is marked  by  a  NULL
2192           pointer.  The  yield  of  the function is zero if all went well, or the
2193           error code
2194    
2195             PCRE_ERROR_NOMEMORY       (-6)
2196    
2197           if the attempt to get the memory block failed.
2198    
2199           When any of these functions encounter a substring that is unset,  which
2200           can  happen  when  capturing subpattern number n+1 matches some part of
2201           the subject, but subpattern n has not been used at all, they return  an
2202           empty string. This can be distinguished from a genuine zero-length sub-
2203           string by inspecting the appropriate offset in ovector, which is  nega-
2204           tive for unset substrings.
2205    
2206           The  two convenience functions pcre_free_substring() and pcre_free_sub-
2207           string_list() can be used to free the memory  returned  by  a  previous
2208           call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
2209           tively. They do nothing more than  call  the  function  pointed  to  by
2210           pcre_free,  which  of course could be called directly from a C program.
2211           However, PCRE is used in some situations where it is linked via a  spe-
2212           cial   interface  to  another  programming  language  that  cannot  use
2213           pcre_free directly; it is for these cases that the functions  are  pro-
2214           vided.
2215    
2216    
2217    EXTRACTING CAPTURED SUBSTRINGS BY NAME
2218    
2219           int pcre_get_stringnumber(const pcre *code,
2220                const char *name);
2221    
2222           int pcre_copy_named_substring(const pcre *code,
2223                const char *subject, int *ovector,
2224                int stringcount, const char *stringname,
2225                char *buffer, int buffersize);
2226    
2227           int pcre_get_named_substring(const pcre *code,
2228                const char *subject, int *ovector,
2229                int stringcount, const char *stringname,
2230                const char **stringptr);
2231    
2232           To  extract a substring by name, you first have to find associated num-
2233           ber.  For example, for this pattern
2234    
2235             (a+)b(?<xxx>\d+)...
2236    
2237           the number of the subpattern called "xxx" is 2. If the name is known to
2238           be unique (PCRE_DUPNAMES was not set), you can find the number from the
2239           name by calling pcre_get_stringnumber(). The first argument is the com-
2240           piled pattern, and the second is the name. The yield of the function is
2241           the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if  there  is  no
2242           subpattern of that name.
2243    
2244           Given the number, you can extract the substring directly, or use one of
2245           the functions described in the previous section. For convenience, there
2246           are also two functions that do the whole job.
2247    
2248           Most    of    the    arguments   of   pcre_copy_named_substring()   and
2249           pcre_get_named_substring() are the same  as  those  for  the  similarly
2250           named  functions  that extract by number. As these are described in the
2251           previous section, they are not re-described here. There  are  just  two
2252           differences:
2253    
2254           First,  instead  of a substring number, a substring name is given. Sec-
2255           ond, there is an extra argument, given at the start, which is a pointer
2256           to  the compiled pattern. This is needed in order to gain access to the
2257           name-to-number translation table.
2258    
2259           These functions call pcre_get_stringnumber(), and if it succeeds,  they
2260           then  call  pcre_copy_substring() or pcre_get_substring(), as appropri-
2261           ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate  names,  the
2262           behaviour may not be what you want (see the next section).
2263    
2264    
2265    DUPLICATE SUBPATTERN NAMES
2266    
2267           int pcre_get_stringtable_entries(const pcre *code,
2268                const char *name, char **first, char **last);
2269    
2270           When  a  pattern  is  compiled with the PCRE_DUPNAMES option, names for
2271           subpatterns are not required to  be  unique.  Normally,  patterns  with
2272           duplicate  names  are such that in any one match, only one of the named
2273           subpatterns participates. An example is shown in the pcrepattern  docu-
2274           mentation.
2275    
2276           When    duplicates   are   present,   pcre_copy_named_substring()   and
2277           pcre_get_named_substring() return the first substring corresponding  to
2278           the  given  name  that  is set. If none are set, PCRE_ERROR_NOSUBSTRING
2279           (-7) is returned; no  data  is  returned.  The  pcre_get_stringnumber()
2280           function  returns one of the numbers that are associated with the name,
2281           but it is not defined which it is.
2282    
2283           If you want to get full details of all captured substrings for a  given
2284           name,  you  must  use  the pcre_get_stringtable_entries() function. The
2285           first argument is the compiled pattern, and the second is the name. The
2286           third  and  fourth  are  pointers to variables which are updated by the
2287           function. After it has run, they point to the first and last entries in
2288           the  name-to-number  table  for  the  given  name.  The function itself
2289           returns the length of each entry,  or  PCRE_ERROR_NOSUBSTRING  (-7)  if
2290           there  are none. The format of the table is described above in the sec-
2291           tion entitled Information about a  pattern.   Given  all  the  relevant
2292           entries  for the name, you can extract each of their numbers, and hence
2293           the captured data, if any.
2294    
2295    
2296    FINDING ALL POSSIBLE MATCHES
2297    
2298           The traditional matching function uses a  similar  algorithm  to  Perl,
2299           which stops when it finds the first match, starting at a given point in
2300           the subject. If you want to find all possible matches, or  the  longest
2301           possible  match,  consider using the alternative matching function (see
2302           below) instead. If you cannot use the alternative function,  but  still
2303           need  to  find all possible matches, you can kludge it up by making use
2304           of the callout facility, which is described in the pcrecallout documen-
2305           tation.
2306    
2307           What you have to do is to insert a callout right at the end of the pat-
2308           tern.  When your callout function is called, extract and save the  cur-
2309           rent  matched  substring.  Then  return  1, which forces pcre_exec() to
2310           backtrack and try other alternatives. Ultimately, when it runs  out  of
2311           matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2312    
2313    
2314    MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2315    
2316           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2317                const char *subject, int length, int startoffset,
2318                int options, int *ovector, int ovecsize,
2319                int *workspace, int wscount);
2320    
2321           The  function  pcre_dfa_exec()  is  called  to  match  a subject string
2322           against a compiled pattern, using a matching algorithm that  scans  the
2323           subject  string  just  once, and does not backtrack. This has different
2324           characteristics to the normal algorithm, and  is  not  compatible  with
2325           Perl.  Some  of the features of PCRE patterns are not supported. Never-
2326           theless, there are times when this kind of matching can be useful.  For
2327           a discussion of the two matching algorithms, see the pcrematching docu-
2328           mentation.
2329    
2330           The arguments for the pcre_dfa_exec() function  are  the  same  as  for
2331           pcre_exec(), plus two extras. The ovector argument is used in a differ-
2332           ent way, and this is described below. The other  common  arguments  are
2333           used  in  the  same way as for pcre_exec(), so their description is not
2334           repeated here.
2335    
2336           The two additional arguments provide workspace for  the  function.  The
2337           workspace  vector  should  contain at least 20 elements. It is used for
2338           keeping  track  of  multiple  paths  through  the  pattern  tree.  More
2339           workspace  will  be  needed for patterns and subjects where there are a
2340           lot of potential matches.
2341    
2342           Here is an example of a simple call to pcre_dfa_exec():
2343    
2344             int rc;
2345             int ovector[10];
2346             int wspace[20];
2347             rc = pcre_dfa_exec(
2348               re,             /* result of pcre_compile() */
2349               NULL,           /* we didn't study the pattern */
2350               "some string",  /* the subject string */
2351               11,             /* the length of the subject string */
2352               0,              /* start at offset 0 in the subject */
2353               0,              /* default options */
2354               ovector,        /* vector of integers for substring information */
2355               10,             /* number of elements (NOT size in bytes) */
2356               wspace,         /* working space vector */
2357               20);            /* number of elements (NOT size in bytes) */
2358    
2359       Option bits for pcre_dfa_exec()
2360    
2361           The unused bits of the options argument  for  pcre_dfa_exec()  must  be
2362           zero.  The  only  bits  that  may  be  set are PCRE_ANCHORED, PCRE_NEW-
2363           LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,  PCRE_NO_UTF8_CHECK,
2364           PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2365           three of these are the same as for pcre_exec(), so their description is
2366           not repeated here.
2367    
2368             PCRE_PARTIAL
2369    
2370           This  has  the  same general effect as it does for pcre_exec(), but the
2371           details  are  slightly  different.  When  PCRE_PARTIAL   is   set   for
2372           pcre_dfa_exec(),  the  return code PCRE_ERROR_NOMATCH is converted into
2373           PCRE_ERROR_PARTIAL if the end of the subject  is  reached,  there  have
2374           been no complete matches, but there is still at least one matching pos-
2375           sibility. The portion of the string that provided the partial match  is
2376           set as the first matching string.
2377    
2378             PCRE_DFA_SHORTEST
2379    
2380           Setting  the  PCRE_DFA_SHORTEST option causes the matching algorithm to
2381           stop as soon as it has found one match. Because of the way the alterna-
2382           tive  algorithm  works, this is necessarily the shortest possible match
2383           at the first possible matching point in the subject string.
2384    
2385             PCRE_DFA_RESTART
2386    
2387           When pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option,  and
2388           returns  a  partial  match, it is possible to call it again, with addi-
2389           tional subject characters, and have it continue with  the  same  match.
2390           The  PCRE_DFA_RESTART  option requests this action; when it is set, the
2391           workspace and wscount options must reference the same vector as  before
2392           because  data  about  the  match so far is left in them after a partial
2393           match. There is more discussion of this  facility  in  the  pcrepartial
2394           documentation.
2395    
2396       Successful returns from pcre_dfa_exec()
2397    
2398           When  pcre_dfa_exec()  succeeds, it may have matched more than one sub-
2399           string in the subject. Note, however, that all the matches from one run
2400           of  the  function  start  at the same point in the subject. The shorter
2401           matches are all initial substrings of the longer matches. For  example,
2402           if the pattern
2403    
2404             <.*>
2405    
2406           is matched against the string
2407    
2408             This is <something> <something else> <something further> no more
2409    
2410           the three matched strings are
2411    
2412             <something>
2413             <something> <something else>
2414             <something> <something else> <something further>
2415    
2416           On  success,  the  yield of the function is a number greater than zero,
2417           which is the number of matched substrings.  The  substrings  themselves
2418           are  returned  in  ovector. Each string uses two elements; the first is
2419           the offset to the start, and the second is the offset to  the  end.  In
2420           fact,  all  the  strings  have the same start offset. (Space could have
2421           been saved by giving this only once, but it was decided to retain  some
2422           compatibility  with  the  way pcre_exec() returns data, even though the
2423           meaning of the strings is different.)
2424    
2425           The strings are returned in reverse order of length; that is, the long-
2426           est  matching  string is given first. If there were too many matches to
2427           fit into ovector, the yield of the function is zero, and the vector  is
2428           filled with the longest matches.
2429    
2430       Error returns from pcre_dfa_exec()
2431    
2432           The  pcre_dfa_exec()  function returns a negative number when it fails.
2433           Many of the errors are the same  as  for  pcre_exec(),  and  these  are
2434           described  above.   There are in addition the following errors that are
2435           specific to pcre_dfa_exec():
2436    
2437             PCRE_ERROR_DFA_UITEM      (-16)
2438    
2439           This return is given if pcre_dfa_exec() encounters an item in the  pat-
2440           tern  that  it  does not support, for instance, the use of \C or a back
2441           reference.
2442    
2443             PCRE_ERROR_DFA_UCOND      (-17)
2444    
2445           This return is given if pcre_dfa_exec()  encounters  a  condition  item
2446           that  uses  a back reference for the condition, or a test for recursion
2447           in a specific group. These are not supported.
2448    
2449             PCRE_ERROR_DFA_UMLIMIT    (-18)
2450    
2451           This return is given if pcre_dfa_exec() is called with an  extra  block
2452           that contains a setting of the match_limit field. This is not supported
2453           (it is meaningless).
2454    
2455             PCRE_ERROR_DFA_WSSIZE     (-19)
2456    
2457           This return is given if  pcre_dfa_exec()  runs  out  of  space  in  the
2458           workspace vector.
2459    
2460             PCRE_ERROR_DFA_RECURSE    (-20)
2461    
2462           When  a  recursive subpattern is processed, the matching function calls
2463           itself recursively, using private vectors for  ovector  and  workspace.
2464           This  error  is  given  if  the output vector is not large enough. This
2465           should be extremely rare, as a vector of size 1000 is used.
2466    
 MATCHING A PATTERN  
      The function pcre_exec() is called to match a subject string  
      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.  
   
      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, it may contain binary zero  char-  
      acters.  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  
   
   
   
   
   
 SunOS 5.8                 Last change:                         12  
   
   
   
      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.  
2467    
2468    SEE ALSO
2469    
2470           pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3),  pcrepar-
2471           tial(3),  pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2472    
 LIMITATIONS  
      There are some size limitations in PCRE but it is hoped that  
      they will never in practice be relevant.  The maximum length  
      of a compiled pattern is 65539 (sic) bytes.  All  values  in  
      repeating  quantifiers must be less than 65536.  The maximum  
      number of capturing subpatterns is 99.  The  maximum  number  
      of  all  parenthesized subpatterns, including capturing sub-  
      patterns, assertions, and other types of subpattern, is 200.  
   
      The maximum length of a subject string is the largest  posi-  
      tive number that an integer variable can hold. However, PCRE  
      uses recursion to handle subpatterns and indefinite  repeti-  
      tion.  This  means  that the available stack space may limit  
      the size of a subject string that can be processed  by  cer-  
      tain patterns.  
   
   
   
 DIFFERENCES FROM PERL  
      The differences described here  are  with  respect  to  Perl  
      5.005.  
   
      1. By default, a whitespace character is any character  that  
      the  C  library  function isspace() recognizes, though it is  
      possible to compile PCRE  with  alternative  character  type  
      tables. Normally isspace() matches space, formfeed, newline,  
      carriage return, horizontal tab, and vertical tab. Perl 5 no  
      longer  includes vertical tab in its set of whitespace char-  
      acters. The \v escape that was in the Perl documentation for  
      a long time was never in fact recognized. However, the char-  
      acter itself was treated as whitespace at least up to 5.002.  
      In 5.004 and 5.005 it does not match \s.  
   
      2. PCRE does  not  allow  repeat  quantifiers  on  lookahead  
      assertions. Perl permits them, but they do not mean what you  
      might think. For example, (?!a){3} does not assert that  the  
      next  three characters are not "a". It just asserts that the  
      next character is not "a" three times.  
   
      3. Capturing subpatterns that occur inside  negative  looka-  
      head  assertions  are  counted,  but  their  entries  in the  
      offsets vector are never set. Perl sets its numerical  vari-  
      ables  from  any  such  patterns that are matched before the  
      assertion fails to match something (thereby succeeding), but  
      only  if  the negative lookahead assertion contains just one  
      branch.  
   
      4. Though binary zero characters are supported in  the  sub-  
      ject  string,  they  are  not  allowed  in  a pattern string  
      because it is passed as a normal  C  string,  terminated  by  
      zero. The escape sequence "\0" can be used in the pattern to  
      represent a binary zero.  
   
      5. The following Perl escape sequences  are  not  supported:  
      \l,  \u,  \L,  \U,  \E, \Q. In fact these are implemented by  
      Perl's general string-handling and are not part of its  pat-  
      tern matching engine.  
   
      6. The Perl \G assertion is  not  supported  as  it  is  not  
      relevant to single pattern matches.  
   
      7. Fairly obviously, PCRE does not support the (?{code}) and  
      (?p{code})  constructions. However, there is some experimen-  
      tal support for recursive patterns using the  non-Perl  item  
      (?R).  
   
      8. There are at the time of writing some  oddities  in  Perl  
      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  
2473    
2474       The following sections describe  the  use  of  each  of  the  AUTHOR
      meta-characters.  
2475    
2476           Philip Hazel
2477           University Computing Service
2478           Cambridge CB2 3QH, England.
2479    
2480    
2481  BACKSLASH  REVISION
      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.  
2482    
2483           Last updated: 21 August 2007
2484           Copyright (c) 1997-2007 University of Cambridge.
2485    ------------------------------------------------------------------------------
2486    
2487    
2488  CIRCUMFLEX AND DOLLAR  PCRECALLOUT(3)                                                  PCRECALLOUT(3)
      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 is it  always  anchored,  
      whether PCRE_MULTILINE is set or not.  
2489    
2490    
2491    NAME
2492           PCRE - Perl-compatible regular expressions
2493    
 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.  
2494    
2495    PCRE CALLOUTS
2496    
2497           int (*pcre_callout)(pcre_callout_block *);
2498    
2499  POSIX CHARACTER CLASSES         PCRE provides a feature called "callout", which is a means of temporar-
2500       Perl 5.6 (not yet released at the time of writing) is  going         ily passing control to the caller of PCRE  in  the  middle  of  pattern
2501       to  support  the POSIX notation for character classes, which         matching.  The  caller of PCRE provides an external function by putting
2502       uses names enclosed by  [:  and  :]   within  the  enclosing         its entry point in the global variable pcre_callout. By  default,  this
2503       square brackets. PCRE supports this notation. For example,         variable contains NULL, which disables all calling out.
2504    
2505         [01[:alpha:]%]         Within  a  regular  expression,  (?C) indicates the points at which the
2506           external function is to be called.  Different  callout  points  can  be
2507       matches "0", "1", any alphabetic character, or "%". The sup-         identified  by  putting  a number less than 256 after the letter C. The
2508       ported class names are         default value is zero.  For  example,  this  pattern  has  two  callout
2509           points:
2510         alnum    letters and digits  
2511         alpha    letters           (?C1)abc(?C2)def
2512         ascii    character codes 0 - 127  
2513         cntrl    control characters         If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is
2514         digit    decimal digits (same as \d)         called, PCRE automatically  inserts  callouts,  all  with  number  255,
2515         graph    printing characters, excluding space         before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2516         lower    lower case letters         used with the pattern
2517         print    printing characters, including space  
2518         punct    printing characters, excluding letters and digits           A(\d{2}|--)
2519         space    white space (same as \s)  
2520         upper    upper case letters         it is processed as if it were
2521         word     "word" characters (same as \w)  
2522         xdigit   hexadecimal digits         (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2523    
2524       The names "ascii" and "word" are  Perl  extensions.  Another         Notice that there is a callout before and after  each  parenthesis  and
2525       Perl  extension is negation, which is indicated by a ^ char-         alternation  bar.  Automatic  callouts  can  be  used  for tracking the
2526       acter after the colon. For example,         progress of pattern matching. The pcretest command has an  option  that
2527           sets  automatic callouts; when it is used, the output indicates how the
2528         [12[:^digit:]]         pattern is matched. This is useful information when you are  trying  to
2529           optimize the performance of a particular pattern.
2530       matches "1", "2", or any non-digit.  PCRE  (and  Perl)  also  
2531       recogize  the POSIX syntax [.ch.] and [=ch=] where "ch" is a  
2532       "collating element", but these are  not  supported,  and  an  MISSING CALLOUTS
2533       error is given if they are encountered.  
2534           You  should  be  aware  that,  because of optimizations in the way PCRE
2535           matches patterns, callouts sometimes do not happen. For example, if the
2536           pattern is
2537    
2538             ab(?C4)cd
2539    
2540           PCRE knows that any matching string must contain the letter "d". If the
2541           subject string is "abyz", the lack of "d" means that  matching  doesn't
2542           ever  start,  and  the  callout is never reached. However, with "abyd",
2543           though the result is still no match, the callout is obeyed.
2544    
2545    
2546    THE CALLOUT INTERFACE
2547    
2548           During matching, when PCRE reaches a callout point, the external  func-
2549           tion  defined by pcre_callout is called (if it is set). This applies to
2550           both the pcre_exec() and the pcre_dfa_exec()  matching  functions.  The
2551           only  argument  to  the callout function is a pointer to a pcre_callout
2552           block. This structure contains the following fields:
2553    
2554             int          version;
2555             int          callout_number;
2556             int         *offset_vector;
2557             const char  *subject;
2558             int          subject_length;
2559             int          start_match;
2560             int          current_position;
2561             int          capture_top;
2562             int          capture_last;
2563             void        *callout_data;
2564             int          pattern_position;
2565             int          next_item_length;
2566    
2567           The version field is an integer containing the version  number  of  the
2568           block  format. The initial version was 0; the current version is 1. The
2569           version number will change again in future  if  additional  fields  are
2570           added, but the intention is never to remove any of the existing fields.
2571    
2572           The callout_number field contains the number of the  callout,  as  com-
2573           piled  into  the pattern (that is, the number after ?C for manual call-
2574           outs, and 255 for automatically generated callouts).
2575    
2576           The offset_vector field is a pointer to the vector of offsets that  was
2577           passed   by   the   caller  to  pcre_exec()  or  pcre_dfa_exec().  When
2578           pcre_exec() is used, the contents can be inspected in order to  extract
2579           substrings  that  have  been  matched  so  far,  in the same way as for
2580           extracting substrings after a match has completed. For  pcre_dfa_exec()
2581           this field is not useful.
2582    
2583           The subject and subject_length fields contain copies of the values that
2584           were passed to pcre_exec().
2585    
2586           The start_match field normally contains the offset within  the  subject
2587           at  which  the  current  match  attempt started. However, if the escape
2588           sequence \K has been encountered, this value is changed to reflect  the
2589           modified  starting  point.  If the pattern is not anchored, the callout
2590           function may be called several times from the same point in the pattern
2591           for different starting points in the subject.
2592    
2593           The  current_position  field  contains the offset within the subject of
2594           the current match pointer.
2595    
2596           When the pcre_exec() function is used, the capture_top  field  contains
2597           one  more than the number of the highest numbered captured substring so
2598           far. If no substrings have been captured, the value of  capture_top  is
2599           one.  This  is always the case when pcre_dfa_exec() is used, because it
2600           does not support captured substrings.
2601    
2602           The capture_last field contains the number of the  most  recently  cap-
2603           tured  substring. If no substrings have been captured, its value is -1.
2604           This is always the case when pcre_dfa_exec() is used.
2605    
2606           The callout_data field contains a value that is passed  to  pcre_exec()
2607           or  pcre_dfa_exec() specifically so that it can be passed back in call-
2608           outs. It is passed in the pcre_callout field  of  the  pcre_extra  data
2609           structure.  If  no such data was passed, the value of callout_data in a
2610           pcre_callout block is NULL. There is a description  of  the  pcre_extra
2611           structure in the pcreapi documentation.
2612    
2613           The  pattern_position field is present from version 1 of the pcre_call-
2614           out structure. It contains the offset to the next item to be matched in
2615           the pattern string.
2616    
2617           The  next_item_length field is present from version 1 of the pcre_call-
2618           out structure. It contains the length of the next item to be matched in
2619           the  pattern  string. When the callout immediately precedes an alterna-
2620           tion bar, a closing parenthesis, or the end of the pattern, the  length
2621           is  zero.  When the callout precedes an opening parenthesis, the length
2622           is that of the entire subpattern.
2623    
2624           The pattern_position and next_item_length fields are intended  to  help
2625           in  distinguishing between different automatic callouts, which all have
2626           the same callout number. However, they are set for all callouts.
2627    
2628    
2629    RETURN VALUES
2630    
2631           The external callout function returns an integer to PCRE. If the  value
2632           is  zero,  matching  proceeds  as  normal. If the value is greater than
2633           zero, matching fails at the current point, but  the  testing  of  other
2634           matching possibilities goes ahead, just as if a lookahead assertion had
2635           failed. If the value is less than zero, the  match  is  abandoned,  and
2636           pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2637    
2638           Negative   values   should   normally   be   chosen  from  the  set  of
2639           PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2640           dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
2641           reserved for use by callout functions; it will never be  used  by  PCRE
2642           itself.
2643    
2644    
2645    AUTHOR
2646    
2647  VERTICAL BAR         Philip Hazel
2648       Vertical bar characters are  used  to  separate  alternative         University Computing Service
2649       patterns. For example, the pattern         Cambridge CB2 3QH, England.
2650    
        gilbert|sullivan  
2651    
2652       matches either "gilbert" or "sullivan". Any number of alter-  REVISION
      natives  may  appear,  and an empty alternative is permitted  
      (matching the empty string).   The  matching  process  tries  
      each  alternative in turn, from left to right, and the first  
      one that succeeds is used. If the alternatives are within  a  
      subpattern  (defined  below),  "succeeds" means matching the  
      rest of the main pattern as well as the alternative  in  the  
      subpattern.  
2653    
2654           Last updated: 29 May 2007
2655           Copyright (c) 1997-2007 University of Cambridge.
2656    ------------------------------------------------------------------------------
2657    
2658    
2659  INTERNAL OPTION SETTING  PCRECOMPAT(3)                                                    PCRECOMPAT(3)
      The settings of PCRE_CASELESS, PCRE_MULTILINE,  PCRE_DOTALL,  
      and  PCRE_EXTENDED can be changed from within the pattern by  
      a sequence of Perl option letters enclosed between "(?"  and  
      ")". The option letters are  
   
        i  for PCRE_CASELESS  
        m  for PCRE_MULTILINE  
        s  for PCRE_DOTALL  
        x  for PCRE_EXTENDED  
   
      For example, (?im) sets caseless, multiline matching. It  is  
      also possible to unset these options by preceding the letter  
      with a hyphen, and a combined setting and unsetting such  as  
      (?im-sx),  which sets PCRE_CASELESS and PCRE_MULTILINE while  
      unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  permitted.  
      If  a  letter  appears both before and after the hyphen, the  
      option is unset.  
   
      The scope of these option changes depends on  where  in  the  
      pattern  the  setting  occurs. For settings that are outside  
      any subpattern (defined below), the effect is the same as if  
      the  options were set or unset at the start of matching. The  
      following patterns all behave in exactly the same way:  
   
        (?i)abc  
        a(?i)bc  
        ab(?i)c  
        abc(?i)  
   
      which in turn is the same as compiling the pattern abc  with  
      PCRE_CASELESS  set.   In  other words, such "top level" set-  
      tings apply to the whole pattern  (unless  there  are  other  
      changes  inside subpatterns). If there is more than one set-  
      ting of the same option at top level, the rightmost  setting  
      is used.  
   
      If an option change occurs inside a subpattern,  the  effect  
      is  different.  This is a change of behaviour in Perl 5.005.  
      An option change inside a subpattern affects only that  part  
      of the subpattern that follows it, so  
   
        (a(?i)b)c  
   
      matches  abc  and  aBc  and  no  other   strings   (assuming  
      PCRE_CASELESS  is  not used).  By this means, options can be  
      made to have different settings in different  parts  of  the  
      pattern.  Any  changes  made  in one alternative do carry on  
      into subsequent branches within  the  same  subpattern.  For  
      example,  
   
        (a(?i)b|c)  
   
      matches "ab", "aB", "c", and "C", even though when  matching  
      "C" the first branch is abandoned before the option setting.  
      This is because the effects of  option  settings  happen  at  
      compile  time. There would be some very weird behaviour oth-  
      erwise.  
   
      The PCRE-specific options PCRE_UNGREEDY and  PCRE_EXTRA  can  
      be changed in the same way as the Perl-compatible options by  
      using the characters U and X  respectively.  The  (?X)  flag  
      setting  is  special in that it must always occur earlier in  
      the pattern than any of the additional features it turns on,  
      even when it is at top level. It is best put at the start.  
2660    
2661    
2662    NAME
2663           PCRE - Perl-compatible regular expressions
2664    
 SUBPATTERNS  
      Subpatterns are delimited by parentheses  (round  brackets),  
      which can be nested.  Marking part of a pattern as a subpat-  
      tern does two things:  
   
      1. It localizes a set of alternatives. For example, the pat-  
      tern  
   
        cat(aract|erpillar|)  
   
      matches one of the words "cat",  "cataract",  or  "caterpil-  
      lar".  Without  the  parentheses, it would match "cataract",  
      "erpillar" or the empty string.  
   
      2. It sets up the subpattern as a capturing  subpattern  (as  
      defined  above).   When the whole pattern matches, that por-  
      tion of the subject string that matched  the  subpattern  is  
      passed  back  to  the  caller  via  the  ovector argument of  
      pcre_exec(). Opening parentheses are counted  from  left  to  
      right (starting from 1) to obtain the numbers of the captur-  
      ing subpatterns.  
   
      For example, if the string "the red king" is matched against  
      the pattern  
   
        the ((red|white) (king|queen))  
   
      the captured substrings are "red king", "red",  and  "king",  
      and are numbered 1, 2, and 3.  
   
      The fact that plain parentheses fulfil two functions is  not  
      always  helpful.  There are often times when a grouping sub-  
      pattern is required without a capturing requirement.  If  an  
      opening parenthesis is followed by "?:", the subpattern does  
      not do any capturing, and is not counted when computing  the  
      number of any subsequent capturing subpatterns. For example,  
      if the string "the white queen" is matched against the  pat-  
      tern  
   
        the ((?:red|white) (king|queen))  
   
      the captured substrings are "white queen" and  "queen",  and  
      are  numbered  1  and 2. The maximum number of captured sub-  
      strings is 99, and the maximum number  of  all  subpatterns,  
      both capturing and non-capturing, is 200.  
   
      As a  convenient  shorthand,  if  any  option  settings  are  
      required  at  the  start  of a non-capturing subpattern, the  
      option letters may appear between the "?" and the ":".  Thus  
      the two patterns  
   
        (?i:saturday|sunday)  
        (?:(?i)saturday|sunday)  
   
      match exactly the same set of strings.  Because  alternative  
      branches  are  tried from left to right, and options are not  
      reset until the end of the subpattern is reached, an  option  
      setting  in  one  branch does affect subsequent branches, so  
      the above patterns match "SUNDAY" as well as "Saturday".  
2665    
2666    DIFFERENCES BETWEEN PCRE AND PERL
2667    
2668           This  document describes the differences in the ways that PCRE and Perl
2669           handle regular expressions. The differences described here  are  mainly
2670           with  respect  to  Perl 5.8, though PCRE versions 7.0 and later contain
2671           some features that are expected to be in the forthcoming Perl 5.10.
2672    
2673           1. PCRE has only a subset of Perl's UTF-8 and Unicode support.  Details
2674           of  what  it does have are given in the section on UTF-8 support in the