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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           The following comments apply when PCRE is running in UTF-8 mode:
159    
160           1. When you set the PCRE_UTF8 flag, the strings passed as patterns  and
161           subjects  are  checked for validity on entry to the relevant functions.
162           If an invalid UTF-8 string is passed, an error return is given. In some
163           situations,  you  may  already  know  that  your strings are valid, and
164           therefore want to skip these checks in order to improve performance. If
165           you  set  the  PCRE_NO_UTF8_CHECK  flag at compile time or at run time,
166           PCRE assumes that the pattern or subject  it  is  given  (respectively)
167           contains  only valid UTF-8 codes. In this case, it does not diagnose an
168           invalid UTF-8 string. If you pass an invalid UTF-8 string to PCRE  when
169           PCRE_NO_UTF8_CHECK  is set, the results are undefined. Your program may
170           crash.
171    
172           2. An unbraced hexadecimal escape sequence (such  as  \xb3)  matches  a
173           two-byte UTF-8 character if the value is greater than 127.
174    
175           3.  Octal  numbers  up to \777 are recognized, and match two-byte UTF-8
176           characters for values greater than \177.
177    
178           4. Repeat quantifiers apply to complete UTF-8 characters, not to  indi-
179           vidual bytes, for example: \x{100}{3}.
180    
181           5.  The dot metacharacter matches one UTF-8 character instead of a sin-
182           gle byte.
183    
184           6. The escape sequence \C can be used to match a single byte  in  UTF-8
185           mode,  but  its  use can lead to some strange effects. This facility is
186           not available in the alternative matching function, pcre_dfa_exec().
187    
188           7. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  correctly
189           test  characters of any code value, but the characters that PCRE recog-
190           nizes as digits, spaces, or word characters  remain  the  same  set  as
191           before, all with values less than 256. This remains true even when PCRE
192           includes Unicode property support, because to do otherwise  would  slow
193           down  PCRE in many common cases. If you really want to test for a wider
194           sense of, say, "digit", you must use Unicode  property  tests  such  as
195           \p{Nd}.
196    
197           8.  Similarly,  characters that match the POSIX named character classes
198           are all low-valued characters.
199    
200           9. However, the Perl 5.10 horizontal and vertical  whitespace  matching
201           escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
202           acters.
203    
204           10. Case-insensitive matching applies only to characters  whose  values
205           are  less than 128, unless PCRE is built with Unicode property support.
206           Even when Unicode property support is available, PCRE  still  uses  its
207           own  character  tables when checking the case of low-valued characters,
208           so as not to degrade performance.  The Unicode property information  is
209           used only for characters with higher values. Even when Unicode property
210           support is available, PCRE supports case-insensitive matching only when
211           there  is  a  one-to-one  mapping between a letter's cases. There are a
212           small number of many-to-one mappings in Unicode;  these  are  not  sup-
213           ported by PCRE.
214    
      int pcre_get_substring_list(const char *subject,  
           int *ovector, int stringcount, const char ***listptr);  
215    
216       void pcre_free_substring(const char *stringptr);  AUTHOR
217    
218       void pcre_free_substring_list(const char **stringptr);         Philip Hazel
219           University Computing Service
220           Cambridge CB2 3QH, England.
221    
222       const unsigned char *pcre_maketables(void);         Putting  an actual email address here seems to have been a spam magnet,
223           so I've taken it away. If you want to email me, use  my  two  initials,
224           followed by the two digits 10, at the domain cam.ac.uk.
225    
      int pcre_fullinfo(const pcre *code, const pcre_extra *extra,  
           int what, void *where);  
226    
227       int pcre_info(const pcre *code, int *optptr, *firstcharptr);  REVISION
228    
229       char *pcre_version(void);         Last updated: 06 August 2007
230           Copyright (c) 1997-2007 University of Cambridge.
231    ------------------------------------------------------------------------------
232    
      void *(*pcre_malloc)(size_t);  
233    
234       void (*pcre_free)(void *);  PCREBUILD(3)                                                      PCREBUILD(3)
235    
236    
237    NAME
238           PCRE - Perl-compatible regular expressions
239    
240    
241  DESCRIPTION  PCRE BUILD-TIME OPTIONS
242       The PCRE library is a set of functions that implement  regu-  
243       lar  expression  pattern  matching using the same syntax and         This  document  describes  the  optional  features  of PCRE that can be
244       semantics as Perl  5,  with  just  a  few  differences  (see         selected when the library is compiled. They are all selected, or  dese-
245           lected, by providing options to the configure script that is run before
246           the make command. The complete list of  options  for  configure  (which
247           includes  the  standard  ones such as the selection of the installation
248           directory) can be obtained by running
249    
250             ./configure --help
251    
252           The following sections include  descriptions  of  options  whose  names
253           begin with --enable or --disable. These settings specify changes to the
254           defaults for the configure command. Because of the way  that  configure
255           works,  --enable  and --disable always come in pairs, so the complemen-
256           tary option always exists as well, but as it specifies the default,  it
257           is not described.
258    
259    
260    C++ SUPPORT
261    
262           By default, the configure script will search for a C++ compiler and C++
263           header files. If it finds them, it automatically builds the C++ wrapper
264           library for PCRE. You can disable this by adding
265    
266             --disable-cpp
267    
268           to the configure command.
269    
270    
271    UTF-8 SUPPORT
272    
273           To build PCRE with support for UTF-8 character strings, add
274    
275             --enable-utf8
276    
277           to  the  configure  command.  Of  itself, this does not make PCRE treat
278           strings as UTF-8. As well as compiling PCRE with this option, you  also
279           have  have to set the PCRE_UTF8 option when you call the pcre_compile()
280           function.
281    
282    
283    UNICODE CHARACTER PROPERTY SUPPORT
284    
285           UTF-8 support allows PCRE to process character values greater than  255
286           in  the  strings that it handles. On its own, however, it does not pro-
287           vide any facilities for accessing the properties of such characters. If
288           you  want  to  be able to use the pattern escapes \P, \p, and \X, which
289           refer to Unicode character properties, you must add
290    
291             --enable-unicode-properties
292    
293           to the configure command. This implies UTF-8 support, even if you  have
294           not explicitly requested it.
295    
296           Including  Unicode  property  support  adds around 30K of tables to the
297           PCRE library. Only the general category properties such as  Lu  and  Nd
298           are supported. Details are given in the pcrepattern documentation.
299    
300    
301    CODE VALUE OF NEWLINE
302    
303           By  default,  PCRE interprets character 10 (linefeed, LF) as indicating
304           the end of a line. This is the normal newline  character  on  Unix-like
305           systems. You can compile PCRE to use character 13 (carriage return, CR)
306           instead, by adding
307    
308             --enable-newline-is-cr
309    
310           to the  configure  command.  There  is  also  a  --enable-newline-is-lf
311           option, which explicitly specifies linefeed as the newline character.
312    
313           Alternatively, you can specify that line endings are to be indicated by
314           the two character sequence CRLF. If you want this, add
315    
316             --enable-newline-is-crlf
317    
318           to the configure command. There is a fourth option, specified by
319    
320             --enable-newline-is-anycrlf
321    
322           which causes PCRE to recognize any of the three sequences  CR,  LF,  or
323           CRLF as indicating a line ending. Finally, a fifth option, specified by
324    
325             --enable-newline-is-any
326    
327           causes PCRE to recognize any Unicode newline sequence.
328    
329           Whatever line ending convention is selected when PCRE is built  can  be
330           overridden  when  the library functions are called. At build time it is
331           conventional to use the standard for your operating system.
332    
333    
334    BUILDING SHARED AND STATIC LIBRARIES
335    
336           The PCRE building process uses libtool to build both shared and  static
337           Unix  libraries by default. You can suppress one of these by adding one
338           of
339    
340             --disable-shared
341             --disable-static
342    
343           to the configure command, as required.
344    
345    
346    POSIX MALLOC USAGE
347    
348           When PCRE is called through the POSIX interface (see the pcreposix doc-
349           umentation),  additional  working  storage  is required for holding the
350           pointers to capturing substrings, because PCRE requires three  integers
351           per  substring,  whereas  the POSIX interface provides only two. If the
352           number of expected substrings is small, the wrapper function uses space
353           on the stack, because this is faster than using malloc() for each call.
354           The default threshold above which the stack is no longer used is 10; it
355           can be changed by adding a setting such as
356    
357             --with-posix-malloc-threshold=20
358    
359           to the configure command.
360    
361    
362    HANDLING VERY LARGE PATTERNS
363    
364           Within  a  compiled  pattern,  offset values are used to point from one
365           part to another (for example, from an opening parenthesis to an  alter-
366           nation  metacharacter).  By default, two-byte values are used for these
367           offsets, leading to a maximum size for a  compiled  pattern  of  around
368           64K.  This  is sufficient to handle all but the most gigantic patterns.
369           Nevertheless, some people do want to process enormous patterns,  so  it
370           is  possible  to compile PCRE to use three-byte or four-byte offsets by
371           adding a setting such as
372    
373             --with-link-size=3
374    
375           to the configure command. The value given must be 2,  3,  or  4.  Using
376           longer  offsets slows down the operation of PCRE because it has to load
377           additional bytes when handling them.
378    
379    
380    AVOIDING EXCESSIVE STACK USAGE
381    
382           When matching with the pcre_exec() function, PCRE implements backtrack-
383           ing  by  making recursive calls to an internal function called match().
384           In environments where the size of the stack is limited,  this  can  se-
385           verely  limit  PCRE's operation. (The Unix environment does not usually
386           suffer from this problem, but it may sometimes be necessary to increase
387           the  maximum  stack size.  There is a discussion in the pcrestack docu-
388           mentation.) An alternative approach to recursion that uses memory  from
389           the  heap  to remember data, instead of using recursive function calls,
390           has been implemented to work round the problem of limited  stack  size.
391           If you want to build a version of PCRE that works this way, add
392    
393             --disable-stack-for-recursion
394    
395           to  the  configure  command. With this configuration, PCRE will use the
396           pcre_stack_malloc and pcre_stack_free variables to call memory  manage-
397           ment  functions. By default these point to malloc() and free(), but you
398           can replace the pointers so that your own functions are used.
399    
400           Separate functions are  provided  rather  than  using  pcre_malloc  and
401           pcre_free  because  the  usage  is  very  predictable:  the block sizes
402           requested are always the same, and  the  blocks  are  always  freed  in
403           reverse  order.  A calling program might be able to implement optimized
404           functions that perform better  than  malloc()  and  free().  PCRE  runs
405           noticeably more slowly when built in this way. This option affects only
406           the  pcre_exec()  function;  it   is   not   relevant   for   the   the
407           pcre_dfa_exec() function.
408    
409    
410    LIMITING PCRE RESOURCE USAGE
411    
412           Internally,  PCRE has a function called match(), which it calls repeat-
413           edly  (sometimes  recursively)  when  matching  a  pattern   with   the
414           pcre_exec()  function.  By controlling the maximum number of times this
415           function may be called during a single matching operation, a limit  can
416           be  placed  on  the resources used by a single call to pcre_exec(). The
417           limit can be changed at run time, as described in the pcreapi  documen-
418           tation.  The default is 10 million, but this can be changed by adding a
419           setting such as
420    
421             --with-match-limit=500000
422    
423           to  the  configure  command.  This  setting  has  no  effect   on   the
424           pcre_dfa_exec() matching function.
425    
426           In  some  environments  it is desirable to limit the depth of recursive
427           calls of match() more strictly than the total number of calls, in order
428           to  restrict  the maximum amount of stack (or heap, if --disable-stack-
429           for-recursion is specified) that is used. A second limit controls this;
430           it  defaults  to  the  value  that is set for --with-match-limit, which
431           imposes no additional constraints. However, you can set a  lower  limit
432           by adding, for example,
433    
434             --with-match-limit-recursion=10000
435    
436           to  the  configure  command.  This  value can also be overridden at run
437           time.
438    
439    
440    CREATING CHARACTER TABLES AT BUILD TIME
441    
442           PCRE uses fixed tables for processing characters whose code values  are
443           less  than 256. By default, PCRE is built with a set of tables that are
444           distributed in the file pcre_chartables.c.dist. These  tables  are  for
445           ASCII codes only. If you add
446    
447             --enable-rebuild-chartables
448    
449           to  the  configure  command, the distributed tables are no longer used.
450           Instead, a program called dftables is compiled and  run.  This  outputs
451           the source for new set of tables, created in the default locale of your
452           C runtime system. (This method of replacing the tables does not work if
453           you  are cross compiling, because dftables is run on the local host. If
454           you need to create alternative tables when cross  compiling,  you  will
455           have to do so "by hand".)
456    
457    
458    USING EBCDIC CODE
459    
460           PCRE  assumes  by  default that it will run in an environment where the
461           character code is ASCII (or Unicode, which is  a  superset  of  ASCII).
462           This  is  the  case for most computer operating systems. PCRE can, how-
463           ever, be compiled to run in an EBCDIC environment by adding
464    
465             --enable-ebcdic
466    
467           to the configure command. This setting implies --enable-rebuild-charta-
468           bles.  You  should  only  use  it if you know that you are in an EBCDIC
469           environment (for example, an IBM mainframe operating system).
470    
471    
472    SEE ALSO
473    
474           pcreapi(3), pcre_config(3).
475    
476    
477    AUTHOR
478    
479           Philip Hazel
480           University Computing Service
481           Cambridge CB2 3QH, England.
482    
483    
484    REVISION
485    
486           Last updated: 30 July 2007
487           Copyright (c) 1997-2007 University of Cambridge.
488    ------------------------------------------------------------------------------
489    
490    
491    PCREMATCHING(3)                                                PCREMATCHING(3)
492    
493    
494    NAME
495           PCRE - Perl-compatible regular expressions
496    
497    
498    PCRE MATCHING ALGORITHMS
499    
500           This document describes the two different algorithms that are available
501           in PCRE for matching a compiled regular expression against a given sub-
502           ject  string.  The  "standard"  algorithm  is  the  one provided by the
503           pcre_exec() function.  This works in the same was  as  Perl's  matching
504           function, and provides a Perl-compatible matching operation.
505    
506           An  alternative  algorithm is provided by the pcre_dfa_exec() function;
507           this operates in a different way, and is not  Perl-compatible.  It  has
508           advantages  and disadvantages compared with the standard algorithm, and
509           these are described below.
510    
511           When there is only one possible way in which a given subject string can
512           match  a pattern, the two algorithms give the same answer. A difference
513           arises, however, when there are multiple possibilities. For example, if
514           the pattern
515    
516             ^<.*>
517    
518           is matched against the string
519    
520             <something> <something else> <something further>
521    
522           there are three possible answers. The standard algorithm finds only one
523           of them, whereas the alternative algorithm finds all three.
524    
525    
526    REGULAR EXPRESSIONS AS TREES
527    
528           The set of strings that are matched by a regular expression can be rep-
529           resented  as  a  tree structure. An unlimited repetition in the pattern
530           makes the tree of infinite size, but it is still a tree.  Matching  the
531           pattern  to a given subject string (from a given starting point) can be
532           thought of as a search of the tree.  There are two  ways  to  search  a
533           tree:  depth-first  and  breadth-first, and these correspond to the two
534           matching algorithms provided by PCRE.
535    
536    
537    THE STANDARD MATCHING ALGORITHM
538    
539           In the terminology of Jeffrey Friedl's book "Mastering Regular  Expres-
540           sions",  the  standard  algorithm  is an "NFA algorithm". It conducts a
541           depth-first search of the pattern tree. That is, it  proceeds  along  a
542           single path through the tree, checking that the subject matches what is
543           required. When there is a mismatch, the algorithm  tries  any  alterna-
544           tives  at  the  current point, and if they all fail, it backs up to the
545           previous branch point in the  tree,  and  tries  the  next  alternative
546           branch  at  that  level.  This often involves backing up (moving to the
547           left) in the subject string as well.  The  order  in  which  repetition
548           branches  are  tried  is controlled by the greedy or ungreedy nature of
549           the quantifier.
550    
551           If a leaf node is reached, a matching string has  been  found,  and  at
552           that  point the algorithm stops. Thus, if there is more than one possi-
553           ble match, this algorithm returns the first one that it finds.  Whether
554           this  is the shortest, the longest, or some intermediate length depends
555           on the way the greedy and ungreedy repetition quantifiers are specified
556           in the pattern.
557    
558           Because  it  ends  up  with a single path through the tree, it is rela-
559           tively straightforward for this algorithm to keep  track  of  the  sub-
560           strings  that  are  matched  by portions of the pattern in parentheses.
561           This provides support for capturing parentheses and back references.
562    
563    
564    THE ALTERNATIVE MATCHING ALGORITHM
565    
566           This algorithm conducts a breadth-first search of  the  tree.  Starting
567           from  the  first  matching  point  in the subject, it scans the subject
568           string from left to right, once, character by character, and as it does
569           this,  it remembers all the paths through the tree that represent valid
570           matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
571           though  it is not implemented as a traditional finite state machine (it
572           keeps multiple states active simultaneously).
573    
574           The scan continues until either the end of the subject is  reached,  or
575           there  are  no more unterminated paths. At this point, terminated paths
576           represent the different matching possibilities (if there are none,  the
577           match  has  failed).   Thus,  if there is more than one possible match,
578           this algorithm finds all of them, and in particular, it finds the long-
579           est.  In PCRE, there is an option to stop the algorithm after the first
580           match (which is necessarily the shortest) has been found.
581    
582           Note that all the matches that are found start at the same point in the
583           subject. If the pattern
584    
585             cat(er(pillar)?)
586    
587           is  matched  against the string "the caterpillar catchment", the result
588           will be the three strings "cat", "cater", and "caterpillar" that  start
589           at the fourth character of the subject. The algorithm does not automat-
590           ically move on to find matches that start at later positions.
591    
592           There are a number of features of PCRE regular expressions that are not
593           supported by the alternative matching algorithm. They are as follows:
594    
595           1.  Because  the  algorithm  finds  all possible matches, the greedy or
596           ungreedy nature of repetition quantifiers is not relevant.  Greedy  and
597           ungreedy quantifiers are treated in exactly the same way. However, pos-
598           sessive quantifiers can make a difference when what follows could  also
599           match what is quantified, for example in a pattern like this:
600    
601             ^a++\w!
602    
603           This  pattern matches "aaab!" but not "aaa!", which would be matched by
604           a non-possessive quantifier. Similarly, if an atomic group is  present,
605           it  is matched as if it were a standalone pattern at the current point,
606           and the longest match is then "locked in" for the rest of  the  overall
607           pattern.
608    
609           2. When dealing with multiple paths through the tree simultaneously, it
610           is not straightforward to keep track of  captured  substrings  for  the
611           different  matching  possibilities,  and  PCRE's implementation of this
612           algorithm does not attempt to do this. This means that no captured sub-
613           strings are available.
614    
615           3.  Because no substrings are captured, back references within the pat-
616           tern are not supported, and cause errors if encountered.
617    
618           4. For the same reason, conditional expressions that use  a  backrefer-
619           ence  as  the  condition or test for a specific group recursion are not
620           supported.
621    
622           5. Because many paths through the tree may be  active,  the  \K  escape
623           sequence, which resets the start of the match when encountered (but may
624           be on some paths and not on others), is not  supported.  It  causes  an
625           error if encountered.
626    
627           6.  Callouts  are  supported, but the value of the capture_top field is
628           always 1, and the value of the capture_last field is always -1.
629    
630       below).  The  current  implementation  corresponds  to  Perl         7.  The \C escape sequence, which (in the standard algorithm) matches a
631       5.005, with some additional features  from  later  versions.         single  byte, even in UTF-8 mode, is not supported because the alterna-
632       This  includes  some  experimental,  incomplete  support for         tive algorithm moves through the subject  string  one  character  at  a
633       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.  
634    
      PCRE has its own native API,  which  is  described  in  this  
      document.  There  is  also  a  set of wrapper functions that  
      correspond to the POSIX regular expression API.   These  are  
      described in the pcreposix documentation.  
635    
636       The native API function prototypes are defined in the header  ADVANTAGES OF THE ALTERNATIVE ALGORITHM
      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.  
637    
638       The functions pcre_compile(), pcre_study(), and  pcre_exec()         Using  the alternative matching algorithm provides the following advan-
639       are used for compiling and matching regular expressions.         tages:
640    
641       The functions  pcre_copy_substring(),  pcre_get_substring(),         1. All possible matches (at a single point in the subject) are automat-
642       and  pcre_get_substring_list() are convenience functions for         ically  found,  and  in particular, the longest match is found. To find
643       extracting  captured  substrings  from  a  matched   subject         more than one match using the standard algorithm, you have to do kludgy
644       string; pcre_free_substring() and pcre_free_substring_list()         things with callouts.
      are also provided, to free the  memory  used  for  extracted  
      strings.  
645    
646       The function pcre_maketables() is used (optionally) to build         2.  There is much better support for partial matching. The restrictions
647       a  set of character tables in the current locale for passing         on the content of the pattern that apply when using the standard  algo-
648       to pcre_compile().         rithm  for  partial matching do not apply to the alternative algorithm.
649           For non-anchored patterns, the starting position of a partial match  is
650           available.
651    
652       The function pcre_fullinfo() is used to find out information         3.  Because  the  alternative  algorithm  scans the subject string just
653       about a compiled pattern; pcre_info() is an obsolete version         once, and never needs to backtrack, it is possible to  pass  very  long
654       which returns only some of the available information, but is         subject  strings  to  the matching function in several pieces, checking
655       retained   for   backwards   compatibility.    The  function         for partial matching each time.
      pcre_version() returns a pointer to a string containing  the  
      version of PCRE and its date of release.  
656    
      The global variables  pcre_malloc  and  pcre_free  initially  
      contain the entry points of the standard malloc() and free()  
      functions respectively. PCRE  calls  the  memory  management  
      functions  via  these  variables,  so  a calling program can  
      replace them if it  wishes  to  intercept  the  calls.  This  
      should be done before calling any PCRE functions.  
657    
658    DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
659    
660           The alternative algorithm suffers from a number of disadvantages:
661    
662  MULTI-THREADING         1. It is substantially slower than  the  standard  algorithm.  This  is
663       The  PCRE  functions  can   be   used   in   multi-threading         partly  because  it has to search for all possible matches, but is also
664           because it is less susceptible to optimization.
665    
666           2. Capturing parentheses and back references are not supported.
667    
668           3. Although atomic groups are supported, their use does not provide the
669           performance advantage that it does for the standard algorithm.
670    
671    
672    AUTHOR
673    
674           Philip Hazel
675           University Computing Service
676           Cambridge CB2 3QH, England.
677    
678    
679    REVISION
680    
681           Last updated: 29 May 2007
682           Copyright (c) 1997-2007 University of Cambridge.
683    ------------------------------------------------------------------------------
684    
685    
686    PCREAPI(3)                                                          PCREAPI(3)
687    
688    
689    NAME
690           PCRE - Perl-compatible regular expressions
691    
692    
693    PCRE NATIVE API
694    
695           #include <pcre.h>
696    
697           pcre *pcre_compile(const char *pattern, int options,
698                const char **errptr, int *erroffset,
699                const unsigned char *tableptr);
700    
701           pcre *pcre_compile2(const char *pattern, int options,
702                int *errorcodeptr,
703                const char **errptr, int *erroffset,
704                const unsigned char *tableptr);
705    
706           pcre_extra *pcre_study(const pcre *code, int options,
707                const char **errptr);
708    
709           int pcre_exec(const pcre *code, const pcre_extra *extra,
710                const char *subject, int length, int startoffset,
711                int options, int *ovector, int ovecsize);
712    
713           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
714                const char *subject, int length, int startoffset,
715                int options, int *ovector, int ovecsize,
716                int *workspace, int wscount);
717    
718           int pcre_copy_named_substring(const pcre *code,
719                const char *subject, int *ovector,
720                int stringcount, const char *stringname,
721                char *buffer, int buffersize);
722    
723           int pcre_copy_substring(const char *subject, int *ovector,
724                int stringcount, int stringnumber, char *buffer,
725                int buffersize);
726    
727           int pcre_get_named_substring(const pcre *code,
728                const char *subject, int *ovector,
729                int stringcount, const char *stringname,
730                const char **stringptr);
731    
732           int pcre_get_stringnumber(const pcre *code,
733                const char *name);
734    
735           int pcre_get_stringtable_entries(const pcre *code,
736                const char *name, char **first, char **last);
737    
738           int pcre_get_substring(const char *subject, int *ovector,
739                int stringcount, int stringnumber,
740                const char **stringptr);
741    
742           int pcre_get_substring_list(const char *subject,
743                int *ovector, int stringcount, const char ***listptr);
744    
745           void pcre_free_substring(const char *stringptr);
746    
747           void pcre_free_substring_list(const char **stringptr);
748    
749           const unsigned char *pcre_maketables(void);
750    
751           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
752                int what, void *where);
753    
754           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
755    
756           int pcre_refcount(pcre *code, int adjust);
757    
758           int pcre_config(int what, void *where);
759    
760           char *pcre_version(void);
761    
762           void *(*pcre_malloc)(size_t);
763    
764           void (*pcre_free)(void *);
765    
766           void *(*pcre_stack_malloc)(size_t);
767    
768           void (*pcre_stack_free)(void *);
769    
770           int (*pcre_callout)(pcre_callout_block *);
771    
772    
773    PCRE API OVERVIEW
774    
775           PCRE has its own native API, which is described in this document. There
776           are also some wrapper functions that correspond to  the  POSIX  regular
777           expression  API.  These  are  described in the pcreposix documentation.
778           Both of these APIs define a set of C function calls. A C++  wrapper  is
779           distributed with PCRE. It is documented in the pcrecpp page.
780    
781           The  native  API  C  function prototypes are defined in the header file
782           pcre.h, and on Unix systems the library itself is called  libpcre.   It
783           can normally be accessed by adding -lpcre to the command for linking an
784           application  that  uses  PCRE.  The  header  file  defines  the  macros
785           PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
786           bers for the library.  Applications can use these  to  include  support
787           for different releases of PCRE.
788    
789           The   functions   pcre_compile(),  pcre_compile2(),  pcre_study(),  and
790           pcre_exec() are used for compiling and matching regular expressions  in
791           a  Perl-compatible  manner. A sample program that demonstrates the sim-
792           plest way of using them is provided in the file  called  pcredemo.c  in
793           the  source distribution. The pcresample documentation describes how to
794           run it.
795    
796           A second matching function, pcre_dfa_exec(), which is not Perl-compati-
797           ble,  is  also provided. This uses a different algorithm for the match-
798           ing. The alternative algorithm finds all possible matches (at  a  given
799           point  in  the subject), and scans the subject just once. However, this
800           algorithm does not return captured substrings. A description of the two
801           matching  algorithms and their advantages and disadvantages is given in
802           the pcrematching documentation.
803    
804           In addition to the main compiling and  matching  functions,  there  are
805           convenience functions for extracting captured substrings from a subject
806           string that is matched by pcre_exec(). They are:
807    
808             pcre_copy_substring()
809             pcre_copy_named_substring()
810             pcre_get_substring()
811             pcre_get_named_substring()
812             pcre_get_substring_list()
813             pcre_get_stringnumber()
814             pcre_get_stringtable_entries()
815    
816           pcre_free_substring() and pcre_free_substring_list() are also provided,
817           to free the memory used for extracted strings.
818    
819           The  function  pcre_maketables()  is  used  to build a set of character
820           tables  in  the  current  locale   for   passing   to   pcre_compile(),
821           pcre_exec(),  or  pcre_dfa_exec(). This is an optional facility that is
822           provided for specialist use.  Most  commonly,  no  special  tables  are
823           passed,  in  which case internal tables that are generated when PCRE is
824           built are used.
825    
826           The function pcre_fullinfo() is used to find out  information  about  a
827           compiled  pattern; pcre_info() is an obsolete version that returns only
828           some of the available information, but is retained for  backwards  com-
829           patibility.   The function pcre_version() returns a pointer to a string
830           containing the version of PCRE and its date of release.
831    
832           The function pcre_refcount() maintains a  reference  count  in  a  data
833           block  containing  a compiled pattern. This is provided for the benefit
834           of object-oriented applications.
835    
836           The global variables pcre_malloc and pcre_free  initially  contain  the
837           entry  points  of  the  standard malloc() and free() functions, respec-
838           tively. PCRE calls the memory management functions via these variables,
839           so  a  calling  program  can replace them if it wishes to intercept the
840           calls. This should be done before calling any PCRE functions.
841    
842           The global variables pcre_stack_malloc  and  pcre_stack_free  are  also
843           indirections  to  memory  management functions. These special functions
844           are used only when PCRE is compiled to use  the  heap  for  remembering
845           data, instead of recursive function calls, when running the pcre_exec()
846           function. See the pcrebuild documentation for  details  of  how  to  do
847           this.  It  is  a non-standard way of building PCRE, for use in environ-
848           ments that have limited stacks. Because of the greater  use  of  memory
849           management,  it  runs  more  slowly. Separate functions are provided so
850           that special-purpose external code can be  used  for  this  case.  When
851           used,  these  functions  are always called in a stack-like manner (last
852           obtained, first freed), and always for memory blocks of the same  size.
853           There  is  a discussion about PCRE's stack usage in the pcrestack docu-
854           mentation.
855    
856           The global variable pcre_callout initially contains NULL. It can be set
857           by  the  caller  to  a "callout" function, which PCRE will then call at
858           specified points during a matching operation. Details are given in  the
859           pcrecallout documentation.
860    
861    
862    NEWLINES
863    
864           PCRE  supports five different conventions for indicating line breaks in
865           strings: a single CR (carriage return) character, a  single  LF  (line-
866           feed) character, the two-character sequence CRLF, any of the three pre-
867           ceding, or any Unicode newline sequence. The Unicode newline  sequences
868           are  the  three just mentioned, plus the single characters VT (vertical
869           tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS  (line
870           separator, U+2028), and PS (paragraph separator, U+2029).
871    
872           Each  of  the first three conventions is used by at least one operating
873           system as its standard newline sequence. When PCRE is built, a  default
874           can  be  specified.  The default default is LF, which is the Unix stan-
875           dard. When PCRE is run, the default can be overridden,  either  when  a
876           pattern is compiled, or when it is matched.
877    
878           In the PCRE documentation the word "newline" is used to mean "the char-
879           acter or pair of characters that indicate a line break". The choice  of
880           newline  convention  affects  the  handling of the dot, circumflex, and
881           dollar metacharacters, the handling of #-comments in /x mode, and, when
882           CRLF  is a recognized line ending sequence, the match position advance-
883           ment for a non-anchored pattern. The choice of newline convention  does
884           not affect the interpretation of the \n or \r escape sequences.
885    
886    
887    MULTITHREADING
888    
889           The  PCRE  functions  can be used in multi-threading applications, with
890           the  proviso  that  the  memory  management  functions  pointed  to  by
891           pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
892           callout function pointed to by pcre_callout, are shared by all threads.
893    
894           The  compiled form of a regular expression is not altered during match-
895           ing, so the same compiled pattern can safely be used by several threads
896           at once.
897    
898    
899    SAVING PRECOMPILED PATTERNS FOR LATER USE
900    
901  SunOS 5.8                 Last change:                          2         The compiled form of a regular expression can be saved and re-used at a
902           later time, possibly by a different program, and even on a  host  other
903           than  the  one  on  which  it  was  compiled.  Details are given in the
904           pcreprecompile documentation. However, compiling a  regular  expression
905           with  one version of PCRE for use with a different version is not guar-
906           anteed to work and may cause crashes.
907    
908    
909    CHECKING BUILD-TIME OPTIONS
910    
911       applications,  with  the  proviso that the memory management         int pcre_config(int what, void *where);
      functions pointed to by pcre_malloc and pcre_free are shared  
      by all threads.  
912    
913       The compiled form of a regular  expression  is  not  altered         The function pcre_config() makes it possible for a PCRE client to  dis-
914       during  matching, so the same compiled pattern can safely be         cover which optional features have been compiled into the PCRE library.
915       used by several threads at once.         The pcrebuild documentation has more details about these optional  fea-
916           tures.
917    
918           The  first  argument  for pcre_config() is an integer, specifying which
919           information is required; the second argument is a pointer to a variable
920           into  which  the  information  is  placed. The following information is
921           available:
922    
923             PCRE_CONFIG_UTF8
924    
925           The output is an integer that is set to one if UTF-8 support is  avail-
926           able; otherwise it is set to zero.
927    
928             PCRE_CONFIG_UNICODE_PROPERTIES
929    
930           The  output  is  an  integer  that is set to one if support for Unicode
931           character properties is available; otherwise it is set to zero.
932    
933             PCRE_CONFIG_NEWLINE
934    
935           The output is an integer whose value specifies  the  default  character
936           sequence  that is recognized as meaning "newline". The four values that
937           are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
938           and  -1  for  ANY. The default should normally be the standard sequence
939           for your operating system.
940    
941             PCRE_CONFIG_LINK_SIZE
942    
943           The output is an integer that contains the number  of  bytes  used  for
944           internal linkage in compiled regular expressions. The value is 2, 3, or
945           4. Larger values allow larger regular expressions to  be  compiled,  at
946           the  expense  of  slower matching. The default value of 2 is sufficient
947           for all but the most massive patterns, since  it  allows  the  compiled
948           pattern to be up to 64K in size.
949    
950             PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
951    
952           The  output  is  an integer that contains the threshold above which the
953           POSIX interface uses malloc() for output vectors. Further  details  are
954           given in the pcreposix documentation.
955    
956             PCRE_CONFIG_MATCH_LIMIT
957    
958           The output is an integer that gives the default limit for the number of
959           internal matching function calls in a  pcre_exec()  execution.  Further
960           details are given with pcre_exec() below.
961    
962             PCRE_CONFIG_MATCH_LIMIT_RECURSION
963    
964           The  output is an integer that gives the default limit for the depth of
965           recursion when calling the internal matching function in a  pcre_exec()
966           execution. Further details are given with pcre_exec() below.
967    
968             PCRE_CONFIG_STACKRECURSE
969    
970           The  output is an integer that is set to one if internal recursion when
971           running pcre_exec() is implemented by recursive function calls that use
972           the  stack  to remember their state. This is the usual way that PCRE is
973           compiled. The output is zero if PCRE was compiled to use blocks of data
974           on  the  heap  instead  of  recursive  function  calls.  In  this case,
975           pcre_stack_malloc and  pcre_stack_free  are  called  to  manage  memory
976           blocks on the heap, thus avoiding the use of the stack.
977    
978    
979  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.  
980    
981           pcre *pcre_compile(const char *pattern, int options,
982                const char **errptr, int *erroffset,
983                const unsigned char *tableptr);
984    
985           pcre *pcre_compile2(const char *pattern, int options,
986                int *errorcodeptr,
987                const char **errptr, int *erroffset,
988                const unsigned char *tableptr);
989    
990           Either of the functions pcre_compile() or pcre_compile2() can be called
991           to compile a pattern into an internal form. The only difference between
992           the  two interfaces is that pcre_compile2() has an additional argument,
993           errorcodeptr, via which a numerical error code can be returned.
994    
995           The pattern is a C string terminated by a binary zero, and is passed in
996           the  pattern  argument.  A  pointer to a single block of memory that is
997           obtained via pcre_malloc is returned. This contains the  compiled  code
998           and related data. The pcre type is defined for the returned block; this
999           is a typedef for a structure whose contents are not externally defined.
1000           It is up to the caller to free the memory (via pcre_free) when it is no
1001           longer required.
1002    
1003           Although the compiled code of a PCRE regex is relocatable, that is,  it
1004           does not depend on memory location, the complete pcre data block is not
1005           fully relocatable, because it may contain a copy of the tableptr  argu-
1006           ment, which is an address (see below).
1007    
1008           The options argument contains various bit settings that affect the com-
1009           pilation. It should be zero if no options are required.  The  available
1010           options  are  described  below. Some of them, in particular, those that
1011           are compatible with Perl, can also be set and  unset  from  within  the
1012           pattern  (see  the  detailed  description in the pcrepattern documenta-
1013           tion). For these options, the contents of the options  argument  speci-
1014           fies  their initial settings at the start of compilation and execution.
1015           The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at  the  time
1016           of matching as well as at compile time.
1017    
1018           If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
1019           if compilation of a pattern fails,  pcre_compile()  returns  NULL,  and
1020           sets the variable pointed to by errptr to point to a textual error mes-
1021           sage. This is a static string that is part of the library. You must not
1022           try to free it. The offset from the start of the pattern to the charac-
1023           ter where the error was discovered is placed in the variable pointed to
1024           by  erroffset,  which must not be NULL. If it is, an immediate error is
1025           given.
1026    
1027           If pcre_compile2() is used instead of pcre_compile(),  and  the  error-
1028           codeptr  argument is not NULL, a non-zero error code number is returned
1029           via this argument in the event of an error. This is in addition to  the
1030           textual error message. Error codes and messages are listed below.
1031    
1032           If  the  final  argument, tableptr, is NULL, PCRE uses a default set of
1033           character tables that are  built  when  PCRE  is  compiled,  using  the
1034           default  C  locale.  Otherwise, tableptr must be an address that is the
1035           result of a call to pcre_maketables(). This value is  stored  with  the
1036           compiled  pattern,  and used again by pcre_exec(), unless another table
1037           pointer is passed to it. For more discussion, see the section on locale
1038           support below.
1039    
1040           This  code  fragment  shows a typical straightforward call to pcre_com-
1041           pile():
1042    
1043             pcre *re;
1044             const char *error;
1045             int erroffset;
1046             re = pcre_compile(
1047               "^A.*Z",          /* the pattern */
1048               0,                /* default options */
1049               &error,           /* for error message */
1050               &erroffset,       /* for error offset */
1051               NULL);            /* use default character tables */
1052    
1053           The following names for option bits are defined in  the  pcre.h  header
1054           file:
1055    
1056             PCRE_ANCHORED
1057    
1058           If this bit is set, the pattern is forced to be "anchored", that is, it
1059           is constrained to match only at the first matching point in the  string
1060           that  is being searched (the "subject string"). This effect can also be
1061           achieved by appropriate constructs in the pattern itself, which is  the
1062           only way to do it in Perl.
1063    
1064             PCRE_AUTO_CALLOUT
1065    
1066           If this bit is set, pcre_compile() automatically inserts callout items,
1067           all with number 255, before each pattern item. For  discussion  of  the
1068           callout facility, see the pcrecallout documentation.
1069    
1070             PCRE_CASELESS
1071    
1072           If  this  bit is set, letters in the pattern match both upper and lower
1073           case letters. It is equivalent to Perl's  /i  option,  and  it  can  be
1074           changed  within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1075           always understands the concept of case for characters whose values  are
1076           less  than 128, so caseless matching is always possible. For characters
1077           with higher values, the concept of case is supported if  PCRE  is  com-
1078           piled  with Unicode property support, but not otherwise. If you want to
1079           use caseless matching for characters 128 and  above,  you  must  ensure
1080           that  PCRE  is  compiled  with Unicode property support as well as with
1081           UTF-8 support.
1082    
1083             PCRE_DOLLAR_ENDONLY
1084    
1085           If this bit is set, a dollar metacharacter in the pattern matches  only
1086           at  the  end  of the subject string. Without this option, a dollar also
1087           matches immediately before a newline at the end of the string (but  not
1088           before  any  other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1089           if PCRE_MULTILINE is set.  There is no equivalent  to  this  option  in
1090           Perl, and no way to set it within a pattern.
1091    
1092             PCRE_DOTALL
1093    
1094           If this bit is set, a dot metacharater in the pattern matches all char-
1095           acters, including those that indicate newline. Without it, a  dot  does
1096           not  match  when  the  current position is at a newline. This option is
1097           equivalent to Perl's /s option, and it can be changed within a  pattern
1098           by  a (?s) option setting. A negative class such as [^a] always matches
1099           newline characters, independent of the setting of this option.
1100    
1101             PCRE_DUPNAMES
1102    
1103           If this bit is set, names used to identify capturing  subpatterns  need
1104           not be unique. This can be helpful for certain types of pattern when it
1105           is known that only one instance of the named  subpattern  can  ever  be
1106           matched.  There  are  more details of named subpatterns below; see also
1107           the pcrepattern documentation.
1108    
1109             PCRE_EXTENDED
1110    
1111           If this bit is set, whitespace  data  characters  in  the  pattern  are
1112           totally ignored except when escaped or inside a character class. White-
1113           space does not include the VT character (code 11). In addition, charac-
1114           ters between an unescaped # outside a character class and the next new-
1115           line, inclusive, are also ignored. This  is  equivalent  to  Perl's  /x
1116           option,  and  it  can be changed within a pattern by a (?x) option set-
1117           ting.
1118    
1119           This option makes it possible to include  comments  inside  complicated
1120           patterns.   Note,  however,  that this applies only to data characters.
1121           Whitespace  characters  may  never  appear  within  special   character
1122           sequences  in  a  pattern,  for  example  within the sequence (?( which
1123           introduces a conditional subpattern.
1124    
1125             PCRE_EXTRA
1126    
1127           This option was invented in order to turn on  additional  functionality
1128           of  PCRE  that  is  incompatible with Perl, but it is currently of very
1129           little use. When set, any backslash in a pattern that is followed by  a
1130           letter  that  has  no  special  meaning causes an error, thus reserving
1131           these combinations for future expansion. By  default,  as  in  Perl,  a
1132           backslash  followed by a letter with no special meaning is treated as a
1133           literal. (Perl can, however, be persuaded to give a warning for  this.)
1134           There  are  at  present no other features controlled by this option. It
1135           can also be set by a (?X) option setting within a pattern.
1136    
1137             PCRE_FIRSTLINE
1138    
1139           If this option is set, an  unanchored  pattern  is  required  to  match
1140           before  or  at  the  first  newline  in  the subject string, though the
1141           matched text may continue over the newline.
1142    
1143             PCRE_MULTILINE
1144    
1145           By default, PCRE treats the subject string as consisting  of  a  single
1146           line  of characters (even if it actually contains newlines). The "start
1147           of line" metacharacter (^) matches only at the  start  of  the  string,
1148           while  the  "end  of line" metacharacter ($) matches only at the end of
1149           the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1150           is set). This is the same as Perl.
1151    
1152           When  PCRE_MULTILINE  it  is set, the "start of line" and "end of line"
1153           constructs match immediately following or immediately  before  internal
1154           newlines  in  the  subject string, respectively, as well as at the very
1155           start and end. This is equivalent to Perl's /m option, and  it  can  be
1156           changed within a pattern by a (?m) option setting. If there are no new-
1157           lines in a subject string, or no occurrences of ^ or $  in  a  pattern,
1158           setting PCRE_MULTILINE has no effect.
1159    
1160             PCRE_NEWLINE_CR
1161             PCRE_NEWLINE_LF
1162             PCRE_NEWLINE_CRLF
1163             PCRE_NEWLINE_ANYCRLF
1164             PCRE_NEWLINE_ANY
1165    
1166           These  options  override the default newline definition that was chosen
1167           when PCRE was built. Setting the first or the second specifies  that  a
1168           newline  is  indicated  by a single character (CR or LF, respectively).
1169           Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by  the
1170           two-character  CRLF  sequence.  Setting  PCRE_NEWLINE_ANYCRLF specifies
1171           that any of the three preceding sequences should be recognized. Setting
1172           PCRE_NEWLINE_ANY  specifies that any Unicode newline sequence should be
1173           recognized. The Unicode newline sequences are the three just mentioned,
1174           plus  the  single  characters  VT (vertical tab, U+000B), FF (formfeed,
1175           U+000C), NEL (next line, U+0085), LS (line separator, U+2028),  and  PS
1176           (paragraph  separator,  U+2029).  The  last  two are recognized only in
1177           UTF-8 mode.
1178    
1179           The newline setting in the  options  word  uses  three  bits  that  are
1180           treated as a number, giving eight possibilities. Currently only six are
1181           used (default plus the five values above). This means that if  you  set
1182           more  than one newline option, the combination may or may not be sensi-
1183           ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1184           PCRE_NEWLINE_CRLF,  but other combinations may yield unused numbers and
1185           cause an error.
1186    
1187           The only time that a line break is specially recognized when  compiling
1188           a  pattern  is  if  PCRE_EXTENDED  is set, and an unescaped # outside a
1189           character class is encountered. This indicates  a  comment  that  lasts
1190           until  after the next line break sequence. In other circumstances, line
1191           break  sequences  are  treated  as  literal  data,   except   that   in
1192           PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1193           and are therefore ignored.
1194    
1195           The newline option that is set at compile time becomes the default that
1196           is  used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1197    
1198             PCRE_NO_AUTO_CAPTURE
1199    
1200           If this option is set, it disables the use of numbered capturing paren-
1201           theses  in the pattern. Any opening parenthesis that is not followed by
1202           ? behaves as if it were followed by ?: but named parentheses can  still
1203           be  used  for  capturing  (and  they acquire numbers in the usual way).
1204           There is no equivalent of this option in Perl.
1205    
1206             PCRE_UNGREEDY
1207    
1208           This option inverts the "greediness" of the quantifiers  so  that  they
1209           are  not greedy by default, but become greedy if followed by "?". It is
1210           not compatible with Perl. It can also be set by a (?U)  option  setting
1211           within the pattern.
1212    
1213             PCRE_UTF8
1214    
1215           This  option  causes PCRE to regard both the pattern and the subject as
1216           strings of UTF-8 characters instead of single-byte  character  strings.
1217           However,  it is available only when PCRE is built to include UTF-8 sup-
1218           port. If not, the use of this option provokes an error. Details of  how
1219           this  option  changes the behaviour of PCRE are given in the section on
1220           UTF-8 support in the main pcre page.
1221    
1222             PCRE_NO_UTF8_CHECK
1223    
1224           When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1225           automatically  checked. If an invalid UTF-8 sequence of bytes is found,
1226           pcre_compile() returns an error. If you already know that your  pattern
1227           is  valid, and you want to skip this check for performance reasons, you
1228           can set the PCRE_NO_UTF8_CHECK option. When it is set,  the  effect  of
1229           passing an invalid UTF-8 string as a pattern is undefined. It may cause
1230           your program to crash.  Note that this option can  also  be  passed  to
1231           pcre_exec()  and pcre_dfa_exec(), to suppress the UTF-8 validity check-
1232           ing of subject strings.
1233    
1234    
1235    COMPILATION ERROR CODES
1236    
1237           The following table lists the error  codes  than  may  be  returned  by
1238           pcre_compile2(),  along with the error messages that may be returned by
1239           both compiling functions. As PCRE has developed, some error codes  have
1240           fallen out of use. To avoid confusion, they have not been re-used.
1241    
1242              0  no error
1243              1  \ at end of pattern
1244              2  \c at end of pattern
1245              3  unrecognized character follows \
1246              4  numbers out of order in {} quantifier
1247              5  number too big in {} quantifier
1248              6  missing terminating ] for character class
1249              7  invalid escape sequence in character class
1250              8  range out of order in character class
1251              9  nothing to repeat
1252             10  [this code is not in use]
1253             11  internal error: unexpected repeat
1254             12  unrecognized character after (?
1255             13  POSIX named classes are supported only within a class
1256             14  missing )
1257             15  reference to non-existent subpattern
1258             16  erroffset passed as NULL
1259             17  unknown option bit(s) set
1260             18  missing ) after comment
1261             19  [this code is not in use]
1262             20  regular expression too large
1263             21  failed to get memory
1264             22  unmatched parentheses
1265             23  internal error: code overflow
1266             24  unrecognized character after (?<
1267             25  lookbehind assertion is not fixed length
1268             26  malformed number or name after (?(
1269             27  conditional group contains more than two branches
1270             28  assertion expected after (?(
1271             29  (?R or (?[+-]digits must be followed by )
1272             30  unknown POSIX class name
1273             31  POSIX collating elements are not supported
1274             32  this version of PCRE is not compiled with PCRE_UTF8 support
1275             33  [this code is not in use]
1276             34  character value in \x{...} sequence is too large
1277             35  invalid condition (?(0)
1278             36  \C not allowed in lookbehind assertion
1279             37  PCRE does not support \L, \l, \N, \U, or \u
1280             38  number after (?C is > 255
1281             39  closing ) for (?C expected
1282             40  recursive call could loop indefinitely
1283             41  unrecognized character after (?P
1284             42  syntax error in subpattern name (missing terminator)
1285             43  two named subpatterns have the same name
1286             44  invalid UTF-8 string
1287             45  support for \P, \p, and \X has not been compiled
1288             46  malformed \P or \p sequence
1289             47  unknown property name after \P or \p
1290             48  subpattern name is too long (maximum 32 characters)
1291             49  too many named subpatterns (maximum 10,000)
1292             50  [this code is not in use]
1293             51  octal value is greater than \377 (not in UTF-8 mode)
1294             52  internal error: overran compiling workspace
1295             53   internal  error:  previously-checked  referenced  subpattern not
1296           found
1297             54  DEFINE group contains more than one branch
1298             55  repeating a DEFINE group is not allowed
1299             56  inconsistent NEWLINE options"
1300             57  \g is not followed by a braced name or an optionally braced
1301                   non-zero number
1302             58  (?+ or (?- or (?(+ or (?(- must be followed by a non-zero number
1303    
1304    
1305  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.  
1306    
1307           pcre_extra *pcre_study(const pcre *code, int options
1308                const char **errptr);
1309    
1310           If a compiled pattern is going to be used several times,  it  is  worth
1311           spending more time analyzing it in order to speed up the time taken for
1312           matching. The function pcre_study() takes a pointer to a compiled  pat-
1313           tern as its first argument. If studying the pattern produces additional
1314           information that will help speed up matching,  pcre_study()  returns  a
1315           pointer  to a pcre_extra block, in which the study_data field points to
1316           the results of the study.
1317    
1318           The  returned  value  from  pcre_study()  can  be  passed  directly  to
1319           pcre_exec().  However,  a  pcre_extra  block also contains other fields
1320           that can be set by the caller before the block  is  passed;  these  are
1321           described below in the section on matching a pattern.
1322    
1323           If  studying  the  pattern  does not produce any additional information
1324           pcre_study() returns NULL. In that circumstance, if the calling program
1325           wants  to  pass  any of the other fields to pcre_exec(), it must set up
1326           its own pcre_extra block.
1327    
1328           The second argument of pcre_study() contains option bits.  At  present,
1329           no options are defined, and this argument should always be zero.
1330    
1331           The  third argument for pcre_study() is a pointer for an error message.
1332           If studying succeeds (even if no data is  returned),  the  variable  it
1333           points  to  is  set  to NULL. Otherwise it is set to point to a textual
1334           error message. This is a static string that is part of the library. You
1335           must  not  try  to  free it. You should test the error pointer for NULL
1336           after calling pcre_study(), to be sure that it has run successfully.
1337    
1338           This is a typical call to pcre_study():
1339    
1340             pcre_extra *pe;
1341             pe = pcre_study(
1342               re,             /* result of pcre_compile() */
1343               0,              /* no options exist */
1344               &error);        /* set to NULL or points to a message */
1345    
1346           At present, studying a pattern is useful only for non-anchored patterns
1347           that  do not have a single fixed starting character. A bitmap of possi-
1348           ble starting bytes is created.
1349    
1350    
1351  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.  
1352    
1353           PCRE handles caseless matching, and determines whether  characters  are
1354           letters,  digits, or whatever, by reference to a set of tables, indexed
1355           by character value. When running in UTF-8 mode, this  applies  only  to
1356           characters  with  codes  less than 128. Higher-valued codes never match
1357           escapes such as \w or \d, but can be tested with \p if  PCRE  is  built
1358           with  Unicode  character property support. The use of locales with Uni-
1359           code is discouraged. If you are handling characters with codes  greater
1360           than  128, you should either use UTF-8 and Unicode, or use locales, but
1361           not try to mix the two.
1362    
1363           PCRE contains an internal set of tables that are used  when  the  final
1364           argument  of  pcre_compile()  is  NULL.  These  are sufficient for many
1365           applications.  Normally, the internal tables recognize only ASCII char-
1366           acters. However, when PCRE is built, it is possible to cause the inter-
1367           nal tables to be rebuilt in the default "C" locale of the local system,
1368           which may cause them to be different.
1369    
1370           The  internal tables can always be overridden by tables supplied by the
1371           application that calls PCRE. These may be created in a different locale
1372           from  the  default.  As more and more applications change to using Uni-
1373           code, the need for this locale support is expected to die away.
1374    
1375           External tables are built by calling  the  pcre_maketables()  function,
1376           which  has no arguments, in the relevant locale. The result can then be
1377           passed to pcre_compile() or pcre_exec()  as  often  as  necessary.  For
1378           example,  to  build  and use tables that are appropriate for the French
1379           locale (where accented characters with  values  greater  than  128  are
1380           treated as letters), the following code could be used:
1381    
1382             setlocale(LC_CTYPE, "fr_FR");
1383             tables = pcre_maketables();
1384             re = pcre_compile(..., tables);
1385    
1386           The  locale  name "fr_FR" is used on Linux and other Unix-like systems;
1387           if you are using Windows, the name for the French locale is "french".
1388    
1389           When pcre_maketables() runs, the tables are built  in  memory  that  is
1390           obtained  via  pcre_malloc. It is the caller's responsibility to ensure
1391           that the memory containing the tables remains available for as long  as
1392           it is needed.
1393    
1394           The pointer that is passed to pcre_compile() is saved with the compiled
1395           pattern, and the same tables are used via this pointer by  pcre_study()
1396           and normally also by pcre_exec(). Thus, by default, for any single pat-
1397           tern, compilation, studying and matching all happen in the same locale,
1398           but different patterns can be compiled in different locales.
1399    
1400           It  is  possible to pass a table pointer or NULL (indicating the use of
1401           the internal tables) to pcre_exec(). Although  not  intended  for  this
1402           purpose,  this facility could be used to match a pattern in a different
1403           locale from the one in which it was compiled. Passing table pointers at
1404           run time is discussed below in the section on matching a pattern.
1405    
1406    
1407  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).  
1408    
1409           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1410                int what, void *where);
1411    
1412           The  pcre_fullinfo() function returns information about a compiled pat-
1413           tern. It replaces the obsolete pcre_info() function, which is neverthe-
1414           less retained for backwards compability (and is documented below).
1415    
1416           The  first  argument  for  pcre_fullinfo() is a pointer to the compiled
1417           pattern. The second argument is the result of pcre_study(), or NULL  if
1418           the  pattern  was not studied. The third argument specifies which piece
1419           of information is required, and the fourth argument is a pointer  to  a
1420           variable  to  receive  the  data. The yield of the function is zero for
1421           success, or one of the following negative numbers:
1422    
1423             PCRE_ERROR_NULL       the argument code was NULL
1424                                   the argument where was NULL
1425             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1426             PCRE_ERROR_BADOPTION  the value of what was invalid
1427    
1428           The "magic number" is placed at the start of each compiled  pattern  as
1429           an  simple check against passing an arbitrary memory pointer. Here is a
1430           typical call of pcre_fullinfo(), to obtain the length of  the  compiled
1431           pattern:
1432    
1433             int rc;
1434             size_t length;
1435             rc = pcre_fullinfo(
1436               re,               /* result of pcre_compile() */
1437               pe,               /* result of pcre_study(), or NULL */
1438               PCRE_INFO_SIZE,   /* what is required */
1439               &length);         /* where to put the data */
1440    
1441           The  possible  values for the third argument are defined in pcre.h, and
1442           are as follows:
1443    
1444             PCRE_INFO_BACKREFMAX
1445    
1446           Return the number of the highest back reference  in  the  pattern.  The
1447           fourth  argument  should  point to an int variable. Zero is returned if
1448           there are no back references.
1449    
1450             PCRE_INFO_CAPTURECOUNT
1451    
1452           Return the number of capturing subpatterns in the pattern.  The  fourth
1453           argument should point to an int variable.
1454    
1455             PCRE_INFO_DEFAULT_TABLES
1456    
1457           Return  a pointer to the internal default character tables within PCRE.
1458           The fourth argument should point to an unsigned char *  variable.  This
1459           information call is provided for internal use by the pcre_study() func-
1460           tion. External callers can cause PCRE to use  its  internal  tables  by
1461           passing a NULL table pointer.
1462    
1463             PCRE_INFO_FIRSTBYTE
1464    
1465           Return  information  about  the first byte of any matched string, for a
1466           non-anchored pattern. The fourth argument should point to an int  vari-
1467           able.  (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1468           is still recognized for backwards compatibility.)
1469    
1470           If there is a fixed first byte, for example, from  a  pattern  such  as
1471           (cat|cow|coyote), its value is returned. Otherwise, if either
1472    
1473           (a)  the pattern was compiled with the PCRE_MULTILINE option, and every
1474           branch starts with "^", or
1475    
1476           (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1477           set (if it were set, the pattern would be anchored),
1478    
1479           -1  is  returned, indicating that the pattern matches only at the start
1480           of a subject string or after any newline within the  string.  Otherwise
1481           -2 is returned. For anchored patterns, -2 is returned.
1482    
1483             PCRE_INFO_FIRSTTABLE
1484    
1485           If  the pattern was studied, and this resulted in the construction of a
1486           256-bit table indicating a fixed set of bytes for the first byte in any
1487           matching  string, a pointer to the table is returned. Otherwise NULL is
1488           returned. The fourth argument should point to an unsigned char *  vari-
1489           able.
1490    
1491             PCRE_INFO_JCHANGED
1492    
1493           Return  1  if the (?J) option setting is used in the pattern, otherwise
1494           0. The fourth argument should point to an int variable. The (?J) inter-
1495           nal option setting changes the local PCRE_DUPNAMES option.
1496    
1497             PCRE_INFO_LASTLITERAL
1498    
1499           Return  the  value of the rightmost literal byte that must exist in any
1500           matched string, other than at its  start,  if  such  a  byte  has  been
1501           recorded. The fourth argument should point to an int variable. If there
1502           is no such byte, -1 is returned. For anchored patterns, a last  literal
1503           byte  is  recorded only if it follows something of variable length. For
1504           example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1505           /^a\dz\d/ the returned value is -1.
1506    
1507             PCRE_INFO_NAMECOUNT
1508             PCRE_INFO_NAMEENTRYSIZE
1509             PCRE_INFO_NAMETABLE
1510    
1511           PCRE  supports the use of named as well as numbered capturing parenthe-
1512           ses. The names are just an additional way of identifying the  parenthe-
1513           ses, which still acquire numbers. Several convenience functions such as
1514           pcre_get_named_substring() are provided for  extracting  captured  sub-
1515           strings  by  name. It is also possible to extract the data directly, by
1516           first converting the name to a number in order to  access  the  correct
1517           pointers in the output vector (described with pcre_exec() below). To do
1518           the conversion, you need  to  use  the  name-to-number  map,  which  is
1519           described by these three values.
1520    
1521           The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1522           gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1523           of  each  entry;  both  of  these  return  an int value. The entry size
1524           depends on the length of the longest name. PCRE_INFO_NAMETABLE  returns
1525           a  pointer  to  the  first  entry of the table (a pointer to char). The
1526           first two bytes of each entry are the number of the capturing parenthe-
1527           sis,  most  significant byte first. The rest of the entry is the corre-
1528           sponding name, zero terminated. The names are  in  alphabetical  order.
1529           When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1530           theses numbers. For example, consider  the  following  pattern  (assume
1531           PCRE_EXTENDED  is  set,  so  white  space  -  including  newlines  - is
1532           ignored):
1533    
1534             (?<date> (?<year>(\d\d)?\d\d) -
1535             (?<month>\d\d) - (?<day>\d\d) )
1536    
1537           There are four named subpatterns, so the table has  four  entries,  and
1538           each  entry  in the table is eight bytes long. The table is as follows,
1539           with non-printing bytes shows in hexadecimal, and undefined bytes shown
1540           as ??:
1541    
1542             00 01 d  a  t  e  00 ??
1543             00 05 d  a  y  00 ?? ??
1544             00 04 m  o  n  t  h  00
1545             00 02 y  e  a  r  00 ??
1546    
1547           When  writing  code  to  extract  data from named subpatterns using the
1548           name-to-number map, remember that the length of the entries  is  likely
1549           to be different for each compiled pattern.
1550    
1551             PCRE_INFO_OKPARTIAL
1552    
1553           Return  1 if the pattern can be used for partial matching, otherwise 0.
1554           The fourth argument should point to an int  variable.  The  pcrepartial
1555           documentation  lists  the restrictions that apply to patterns when par-
1556           tial matching is used.
1557    
1558             PCRE_INFO_OPTIONS
1559    
1560           Return a copy of the options with which the pattern was  compiled.  The
1561           fourth  argument  should  point to an unsigned long int variable. These
1562           option bits are those specified in the call to pcre_compile(), modified
1563           by any top-level option settings at the start of the pattern itself. In
1564           other words, they are the options that will be in force  when  matching
1565           starts.  For  example, if the pattern /(?im)abc(?-i)d/ is compiled with
1566           the PCRE_EXTENDED option, the result is PCRE_CASELESS,  PCRE_MULTILINE,
1567           and PCRE_EXTENDED.
1568    
1569           A  pattern  is  automatically  anchored by PCRE if all of its top-level
1570           alternatives begin with one of the following:
1571    
1572             ^     unless PCRE_MULTILINE is set
1573             \A    always
1574             \G    always
1575             .*    if PCRE_DOTALL is set and there are no back
1576                     references to the subpattern in which .* appears
1577    
1578           For such patterns, the PCRE_ANCHORED bit is set in the options returned
1579           by pcre_fullinfo().
1580    
1581             PCRE_INFO_SIZE
1582    
1583           Return  the  size  of the compiled pattern, that is, the value that was
1584           passed as the argument to pcre_malloc() when PCRE was getting memory in
1585           which to place the compiled data. The fourth argument should point to a
1586           size_t variable.
1587    
1588             PCRE_INFO_STUDYSIZE
1589    
1590           Return the size of the data block pointed to by the study_data field in
1591           a  pcre_extra  block.  That  is,  it  is  the  value that was passed to
1592           pcre_malloc() when PCRE was getting memory into which to place the data
1593           created  by  pcre_study(). The fourth argument should point to a size_t
1594           variable.
1595    
1596    
1597    OBSOLETE INFO FUNCTION
1598    
1599           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1600    
1601           The pcre_info() function is now obsolete because its interface  is  too
1602           restrictive  to return all the available data about a compiled pattern.
1603           New  programs  should  use  pcre_fullinfo()  instead.  The   yield   of
1604           pcre_info()  is the number of capturing subpatterns, or one of the fol-
1605           lowing negative numbers:
1606    
1607             PCRE_ERROR_NULL       the argument code was NULL
1608             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1609    
1610           If the optptr argument is not NULL, a copy of the  options  with  which
1611           the  pattern  was  compiled  is placed in the integer it points to (see
1612           PCRE_INFO_OPTIONS above).
1613    
1614           If the pattern is not anchored and the  firstcharptr  argument  is  not
1615           NULL,  it is used to pass back information about the first character of
1616           any matched string (see PCRE_INFO_FIRSTBYTE above).
1617    
1618    
1619    REFERENCE COUNTS
1620    
1621           int pcre_refcount(pcre *code, int adjust);
1622    
1623           The pcre_refcount() function is used to maintain a reference  count  in
1624           the data block that contains a compiled pattern. It is provided for the
1625           benefit of applications that  operate  in  an  object-oriented  manner,
1626           where different parts of the application may be using the same compiled
1627           pattern, but you want to free the block when they are all done.
1628    
1629           When a pattern is compiled, the reference count field is initialized to
1630           zero.   It is changed only by calling this function, whose action is to
1631           add the adjust value (which may be positive or  negative)  to  it.  The
1632           yield of the function is the new value. However, the value of the count
1633           is constrained to lie between 0 and 65535, inclusive. If the new  value
1634           is outside these limits, it is forced to the appropriate limit value.
1635    
1636           Except  when it is zero, the reference count is not correctly preserved
1637           if a pattern is compiled on one host and then  transferred  to  a  host
1638           whose byte-order is different. (This seems a highly unlikely scenario.)
1639    
1640    
1641    MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1642    
1643           int pcre_exec(const pcre *code, const pcre_extra *extra,
1644                const char *subject, int length, int startoffset,
1645                int options, int *ovector, int ovecsize);
1646    
1647           The function pcre_exec() is called to match a subject string against  a
1648           compiled  pattern, which is passed in the code argument. If the pattern
1649           has been studied, the result of the study should be passed in the extra
1650           argument.  This  function is the main matching facility of the library,
1651           and it operates in a Perl-like manner. For specialist use there is also
1652           an  alternative matching function, which is described below in the sec-
1653           tion about the pcre_dfa_exec() function.
1654    
1655           In most applications, the pattern will have been compiled (and  option-
1656           ally  studied)  in the same process that calls pcre_exec(). However, it
1657           is possible to save compiled patterns and study data, and then use them
1658           later  in  different processes, possibly even on different hosts. For a
1659           discussion about this, see the pcreprecompile documentation.
1660    
1661           Here is an example of a simple call to pcre_exec():
1662    
1663             int rc;
1664             int ovector[30];
1665             rc = pcre_exec(
1666               re,             /* result of pcre_compile() */
1667               NULL,           /* we didn't study the pattern */
1668               "some string",  /* the subject string */
1669               11,             /* the length of the subject string */
1670               0,              /* start at offset 0 in the subject */
1671               0,              /* default options */
1672               ovector,        /* vector of integers for substring information */
1673               30);            /* number of elements (NOT size in bytes) */
1674    
1675       Extra data for pcre_exec()
1676    
1677           If the extra argument is not NULL, it must point to a  pcre_extra  data
1678           block.  The pcre_study() function returns such a block (when it doesn't
1679           return NULL), but you can also create one for yourself, and pass  addi-
1680           tional  information  in it. The pcre_extra block contains the following
1681           fields (not necessarily in this order):
1682    
1683             unsigned long int flags;
1684             void *study_data;
1685             unsigned long int match_limit;
1686             unsigned long int match_limit_recursion;
1687             void *callout_data;
1688             const unsigned char *tables;
1689    
1690           The flags field is a bitmap that specifies which of  the  other  fields
1691           are set. The flag bits are:
1692    
1693             PCRE_EXTRA_STUDY_DATA
1694             PCRE_EXTRA_MATCH_LIMIT
1695             PCRE_EXTRA_MATCH_LIMIT_RECURSION
1696             PCRE_EXTRA_CALLOUT_DATA
1697             PCRE_EXTRA_TABLES
1698    
1699           Other  flag  bits should be set to zero. The study_data field is set in
1700           the pcre_extra block that is returned by  pcre_study(),  together  with
1701           the appropriate flag bit. You should not set this yourself, but you may
1702           add to the block by setting the other fields  and  their  corresponding
1703           flag bits.
1704    
1705           The match_limit field provides a means of preventing PCRE from using up
1706           a vast amount of resources when running patterns that are not going  to
1707           match,  but  which  have  a very large number of possibilities in their
1708           search trees. The classic  example  is  the  use  of  nested  unlimited
1709           repeats.
1710    
1711           Internally,  PCRE uses a function called match() which it calls repeat-
1712           edly (sometimes recursively). The limit set by match_limit  is  imposed
1713           on  the  number  of times this function is called during a match, which
1714           has the effect of limiting the amount of  backtracking  that  can  take
1715           place. For patterns that are not anchored, the count restarts from zero
1716           for each position in the subject string.
1717    
1718           The default value for the limit can be set  when  PCRE  is  built;  the
1719           default  default  is 10 million, which handles all but the most extreme
1720           cases. You can override the default  by  suppling  pcre_exec()  with  a
1721           pcre_extra     block    in    which    match_limit    is    set,    and
1722           PCRE_EXTRA_MATCH_LIMIT is set in the  flags  field.  If  the  limit  is
1723           exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1724    
1725           The  match_limit_recursion field is similar to match_limit, but instead
1726           of limiting the total number of times that match() is called, it limits
1727           the  depth  of  recursion. The recursion depth is a smaller number than
1728           the total number of calls, because not all calls to match() are  recur-
1729           sive.  This limit is of use only if it is set smaller than match_limit.
1730    
1731           Limiting the recursion depth limits the amount of  stack  that  can  be
1732           used, or, when PCRE has been compiled to use memory on the heap instead
1733           of the stack, the amount of heap memory that can be used.
1734    
1735           The default value for match_limit_recursion can be  set  when  PCRE  is
1736           built;  the  default  default  is  the  same  value  as the default for
1737           match_limit. You can override the default by suppling pcre_exec()  with
1738           a   pcre_extra   block  in  which  match_limit_recursion  is  set,  and
1739           PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in  the  flags  field.  If  the
1740           limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1741    
1742           The  pcre_callout  field is used in conjunction with the "callout" fea-
1743           ture, which is described in the pcrecallout documentation.
1744    
1745           The tables field  is  used  to  pass  a  character  tables  pointer  to
1746           pcre_exec();  this overrides the value that is stored with the compiled
1747           pattern. A non-NULL value is stored with the compiled pattern  only  if
1748           custom  tables  were  supplied to pcre_compile() via its tableptr argu-
1749           ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
1750           PCRE's  internal  tables  to be used. This facility is helpful when re-
1751           using patterns that have been saved after compiling  with  an  external
1752           set  of  tables,  because  the  external tables might be at a different
1753           address when pcre_exec() is called. See the  pcreprecompile  documenta-
1754           tion for a discussion of saving compiled patterns for later use.
1755    
1756       Option bits for pcre_exec()
1757    
1758           The  unused  bits of the options argument for pcre_exec() must be zero.
1759           The only bits that may  be  set  are  PCRE_ANCHORED,  PCRE_NEWLINE_xxx,
1760           PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NO_UTF8_CHECK   and
1761           PCRE_PARTIAL.
1762    
1763             PCRE_ANCHORED
1764    
1765           The PCRE_ANCHORED option limits pcre_exec() to matching  at  the  first
1766           matching  position.  If  a  pattern was compiled with PCRE_ANCHORED, or
1767           turned out to be anchored by virtue of its contents, it cannot be  made
1768           unachored at matching time.
1769    
1770             PCRE_NEWLINE_CR
1771             PCRE_NEWLINE_LF
1772             PCRE_NEWLINE_CRLF
1773             PCRE_NEWLINE_ANYCRLF
1774             PCRE_NEWLINE_ANY
1775    
1776           These  options  override  the  newline  definition  that  was chosen or
1777           defaulted when the pattern was compiled. For details, see the  descrip-
1778           tion  of  pcre_compile()  above.  During  matching,  the newline choice
1779           affects the behaviour of the dot, circumflex,  and  dollar  metacharac-
1780           ters.  It may also alter the way the match position is advanced after a
1781           match  failure  for  an  unanchored  pattern.  When  PCRE_NEWLINE_CRLF,
1782           PCRE_NEWLINE_ANYCRLF,  or  PCRE_NEWLINE_ANY is set, and a match attempt
1783           fails when the current position is at a CRLF sequence, the match  posi-
1784           tion  is  advanced by two characters instead of one, in other words, to
1785           after the CRLF.
1786    
1787             PCRE_NOTBOL
1788    
1789           This option specifies that first character of the subject string is not
1790           the  beginning  of  a  line, so the circumflex metacharacter should not
1791           match before it. Setting this without PCRE_MULTILINE (at compile  time)
1792           causes  circumflex  never to match. This option affects only the behav-
1793           iour of the circumflex metacharacter. It does not affect \A.
1794    
1795             PCRE_NOTEOL
1796    
1797           This option specifies that the end of the subject string is not the end
1798           of  a line, so the dollar metacharacter should not match it nor (except
1799           in multiline mode) a newline immediately before it. Setting this  with-
1800           out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1801           option affects only the behaviour of the dollar metacharacter. It  does
1802           not affect \Z or \z.
1803    
1804             PCRE_NOTEMPTY
1805    
1806           An empty string is not considered to be a valid match if this option is
1807           set. If there are alternatives in the pattern, they are tried.  If  all
1808           the  alternatives  match  the empty string, the entire match fails. For
1809           example, if the pattern
1810    
1811             a?b?
1812    
1813           is applied to a string not beginning with "a" or "b",  it  matches  the
1814           empty  string at the start of the subject. With PCRE_NOTEMPTY set, this
1815           match is not valid, so PCRE searches further into the string for occur-
1816           rences of "a" or "b".
1817    
1818           Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1819           cial case of a pattern match of the empty  string  within  its  split()
1820           function,  and  when  using  the /g modifier. It is possible to emulate
1821           Perl's behaviour after matching a null string by first trying the match
1822           again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1823           if that fails by advancing the starting offset (see below)  and  trying
1824           an ordinary match again. There is some code that demonstrates how to do
1825           this in the pcredemo.c sample program.
1826    
1827             PCRE_NO_UTF8_CHECK
1828    
1829           When PCRE_UTF8 is set at compile time, the validity of the subject as a
1830           UTF-8  string is automatically checked when pcre_exec() is subsequently
1831           called.  The value of startoffset is also checked  to  ensure  that  it
1832           points  to the start of a UTF-8 character. If an invalid UTF-8 sequence
1833           of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If
1834           startoffset  contains  an  invalid  value, PCRE_ERROR_BADUTF8_OFFSET is
1835           returned.
1836    
1837           If you already know that your subject is valid, and you  want  to  skip
1838           these    checks    for   performance   reasons,   you   can   set   the
1839           PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might  want  to
1840           do  this  for the second and subsequent calls to pcre_exec() if you are
1841           making repeated calls to find all  the  matches  in  a  single  subject
1842           string.  However,  you  should  be  sure  that the value of startoffset
1843           points to the start of a UTF-8 character.  When  PCRE_NO_UTF8_CHECK  is
1844           set,  the  effect of passing an invalid UTF-8 string as a subject, or a
1845           value of startoffset that does not point to the start of a UTF-8  char-
1846           acter, is undefined. Your program may crash.
1847    
1848             PCRE_PARTIAL
1849    
1850           This  option  turns  on  the  partial  matching feature. If the subject
1851           string fails to match the pattern, but at some point during the  match-
1852           ing  process  the  end of the subject was reached (that is, the subject
1853           partially matches the pattern and the failure to  match  occurred  only
1854           because  there were not enough subject characters), pcre_exec() returns
1855           PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL  is
1856           used,  there  are restrictions on what may appear in the pattern. These
1857           are discussed in the pcrepartial documentation.
1858    
1859       The string to be matched by pcre_exec()
1860    
1861           The subject string is passed to pcre_exec() as a pointer in subject,  a
1862           length  in  length, and a starting byte offset in startoffset. In UTF-8
1863           mode, the byte offset must point to the start  of  a  UTF-8  character.
1864           Unlike  the  pattern string, the subject may contain binary zero bytes.
1865           When the starting offset is zero, the search for a match starts at  the
1866           beginning of the subject, and this is by far the most common case.
1867    
1868           A  non-zero  starting offset is useful when searching for another match
1869           in the same subject by calling pcre_exec() again after a previous  suc-
1870           cess.   Setting  startoffset differs from just passing over a shortened
1871           string and setting PCRE_NOTBOL in the case of  a  pattern  that  begins
1872           with any kind of lookbehind. For example, consider the pattern
1873    
1874             \Biss\B
1875    
1876           which  finds  occurrences  of "iss" in the middle of words. (\B matches
1877           only if the current position in the subject is not  a  word  boundary.)
1878           When  applied  to the string "Mississipi" the first call to pcre_exec()
1879           finds the first occurrence. If pcre_exec() is called  again  with  just
1880           the  remainder  of  the  subject,  namely  "issipi", it does not match,
1881           because \B is always false at the start of the subject, which is deemed
1882           to  be  a  word  boundary. However, if pcre_exec() is passed the entire
1883           string again, but with startoffset set to 4, it finds the second occur-
1884           rence  of "iss" because it is able to look behind the starting point to
1885           discover that it is preceded by a letter.
1886    
1887           If a non-zero starting offset is passed when the pattern  is  anchored,
1888           one attempt to match at the given offset is made. This can only succeed
1889           if the pattern does not require the match to be at  the  start  of  the
1890           subject.
1891    
1892       How pcre_exec() returns captured substrings
1893    
1894           In  general, a pattern matches a certain portion of the subject, and in
1895           addition, further substrings from the subject  may  be  picked  out  by
1896           parts  of  the  pattern.  Following the usage in Jeffrey Friedl's book,
1897           this is called "capturing" in what follows, and the  phrase  "capturing
1898           subpattern"  is  used for a fragment of a pattern that picks out a sub-
1899           string. PCRE supports several other kinds of  parenthesized  subpattern
1900           that do not cause substrings to be captured.
1901    
1902           Captured  substrings are returned to the caller via a vector of integer
1903           offsets whose address is passed in ovector. The number of  elements  in
1904           the  vector is passed in ovecsize, which must be a non-negative number.
1905           Note: this argument is NOT the size of ovector in bytes.
1906    
1907           The first two-thirds of the vector is used to pass back  captured  sub-
1908           strings,  each  substring using a pair of integers. The remaining third
1909           of the vector is used as workspace by pcre_exec() while  matching  cap-
1910           turing  subpatterns, and is not available for passing back information.
1911           The length passed in ovecsize should always be a multiple of three.  If
1912           it is not, it is rounded down.
1913    
1914           When  a  match  is successful, information about captured substrings is
1915           returned in pairs of integers, starting at the  beginning  of  ovector,
1916           and  continuing  up  to two-thirds of its length at the most. The first
1917           element of a pair is set to the offset of the first character in a sub-
1918           string,  and  the  second  is  set to the offset of the first character
1919           after the end of a substring. The  first  pair,  ovector[0]  and  ovec-
1920           tor[1],  identify  the  portion  of  the  subject string matched by the
1921           entire pattern. The next pair is used for the first  capturing  subpat-
1922           tern, and so on. The value returned by pcre_exec() is one more than the
1923           highest numbered pair that has been set. For example, if two substrings
1924           have  been captured, the returned value is 3. If there are no capturing
1925           subpatterns, the return value from a successful match is 1,  indicating
1926           that just the first pair of offsets has been set.
1927    
1928           If a capturing subpattern is matched repeatedly, it is the last portion
1929           of the string that it matched that is returned.
1930    
1931           If the vector is too small to hold all the captured substring  offsets,
1932           it is used as far as possible (up to two-thirds of its length), and the
1933           function returns a value of zero. In particular, if the substring  off-
1934           sets are not of interest, pcre_exec() may be called with ovector passed
1935           as NULL and ovecsize as zero. However, if  the  pattern  contains  back
1936           references  and  the  ovector is not big enough to remember the related
1937           substrings, PCRE has to get additional memory for use during  matching.
1938           Thus it is usually advisable to supply an ovector.
1939    
1940           The  pcre_info()  function  can  be used to find out how many capturing
1941           subpatterns there are in a compiled  pattern.  The  smallest  size  for
1942           ovector  that  will allow for n captured substrings, in addition to the
1943           offsets of the substring matched by the whole pattern, is (n+1)*3.
1944    
1945           It is possible for capturing subpattern number n+1 to match  some  part
1946           of the subject when subpattern n has not been used at all. For example,
1947           if the string "abc" is matched  against  the  pattern  (a|(z))(bc)  the
1948           return from the function is 4, and subpatterns 1 and 3 are matched, but
1949           2 is not. When this happens, both values in  the  offset  pairs  corre-
1950           sponding to unused subpatterns are set to -1.
1951    
1952           Offset  values  that correspond to unused subpatterns at the end of the
1953           expression are also set to -1. For example,  if  the  string  "abc"  is
1954           matched  against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
1955           matched. The return from the function is 2, because  the  highest  used
1956           capturing subpattern number is 1. However, you can refer to the offsets
1957           for the second and third capturing subpatterns if  you  wish  (assuming
1958           the vector is large enough, of course).
1959    
1960           Some  convenience  functions  are  provided for extracting the captured
1961           substrings as separate strings. These are described below.
1962    
1963       Error return values from pcre_exec()
1964    
1965           If pcre_exec() fails, it returns a negative number. The  following  are
1966           defined in the header file:
1967    
1968             PCRE_ERROR_NOMATCH        (-1)
1969    
1970           The subject string did not match the pattern.
1971    
1972             PCRE_ERROR_NULL           (-2)
1973    
1974           Either  code  or  subject  was  passed as NULL, or ovector was NULL and
1975           ovecsize was not zero.
1976    
1977             PCRE_ERROR_BADOPTION      (-3)
1978    
1979           An unrecognized bit was set in the options argument.
1980    
1981             PCRE_ERROR_BADMAGIC       (-4)
1982    
1983           PCRE stores a 4-byte "magic number" at the start of the compiled  code,
1984           to catch the case when it is passed a junk pointer and to detect when a
1985           pattern that was compiled in an environment of one endianness is run in
1986           an  environment  with the other endianness. This is the error that PCRE
1987           gives when the magic number is not present.
1988    
1989             PCRE_ERROR_UNKNOWN_OPCODE (-5)
1990    
1991           While running the pattern match, an unknown item was encountered in the
1992           compiled  pattern.  This  error  could be caused by a bug in PCRE or by
1993           overwriting of the compiled pattern.
1994    
1995             PCRE_ERROR_NOMEMORY       (-6)
1996    
1997           If a pattern contains back references, but the ovector that  is  passed
1998           to pcre_exec() is not big enough to remember the referenced substrings,
1999           PCRE gets a block of memory at the start of matching to  use  for  this
2000           purpose.  If the call via pcre_malloc() fails, this error is given. The
2001           memory is automatically freed at the end of matching.
2002    
2003             PCRE_ERROR_NOSUBSTRING    (-7)
2004    
2005           This error is used by the pcre_copy_substring(),  pcre_get_substring(),
2006           and  pcre_get_substring_list()  functions  (see  below).  It  is  never
2007           returned by pcre_exec().
2008    
2009             PCRE_ERROR_MATCHLIMIT     (-8)
2010    
2011           The backtracking limit, as specified by  the  match_limit  field  in  a
2012           pcre_extra  structure  (or  defaulted) was reached. See the description
2013           above.
2014    
2015             PCRE_ERROR_CALLOUT        (-9)
2016    
2017           This error is never generated by pcre_exec() itself. It is provided for
2018           use  by  callout functions that want to yield a distinctive error code.
2019           See the pcrecallout documentation for details.
2020    
2021             PCRE_ERROR_BADUTF8        (-10)
2022    
2023           A string that contains an invalid UTF-8 byte sequence was passed  as  a
2024           subject.
2025    
2026             PCRE_ERROR_BADUTF8_OFFSET (-11)
2027    
2028           The UTF-8 byte sequence that was passed as a subject was valid, but the
2029           value of startoffset did not point to the beginning of a UTF-8  charac-
2030           ter.
2031    
2032             PCRE_ERROR_PARTIAL        (-12)
2033    
2034           The  subject  string did not match, but it did match partially. See the
2035           pcrepartial documentation for details of partial matching.
2036    
2037             PCRE_ERROR_BADPARTIAL     (-13)
2038    
2039           The PCRE_PARTIAL option was used with  a  compiled  pattern  containing
2040           items  that are not supported for partial matching. See the pcrepartial
2041           documentation for details of partial matching.
2042    
2043             PCRE_ERROR_INTERNAL       (-14)
2044    
2045           An unexpected internal error has occurred. This error could  be  caused
2046           by a bug in PCRE or by overwriting of the compiled pattern.
2047    
2048             PCRE_ERROR_BADCOUNT       (-15)
2049    
2050           This  error is given if the value of the ovecsize argument is negative.
2051    
2052             PCRE_ERROR_RECURSIONLIMIT (-21)
2053    
2054           The internal recursion limit, as specified by the match_limit_recursion
2055           field  in  a  pcre_extra  structure (or defaulted) was reached. See the
2056           description above.
2057    
2058             PCRE_ERROR_BADNEWLINE     (-23)
2059    
2060           An invalid combination of PCRE_NEWLINE_xxx options was given.
2061    
2062           Error numbers -16 to -20 and -22 are not used by pcre_exec().
2063    
2064    
2065    EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2066    
2067           int pcre_copy_substring(const char *subject, int *ovector,
2068                int stringcount, int stringnumber, char *buffer,
2069                int buffersize);
2070    
2071           int pcre_get_substring(const char *subject, int *ovector,
2072                int stringcount, int stringnumber,
2073                const char **stringptr);
2074    
2075           int pcre_get_substring_list(const char *subject,
2076                int *ovector, int stringcount, const char ***listptr);
2077    
2078           Captured substrings can be  accessed  directly  by  using  the  offsets
2079           returned  by  pcre_exec()  in  ovector.  For convenience, the functions
2080           pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
2081           string_list()  are  provided for extracting captured substrings as new,
2082           separate, zero-terminated strings. These functions identify  substrings
2083           by  number.  The  next section describes functions for extracting named
2084           substrings.
2085    
2086           A substring that contains a binary zero is correctly extracted and  has
2087           a  further zero added on the end, but the result is not, of course, a C
2088           string.  However, you can process such a string  by  referring  to  the
2089           length  that  is  returned  by  pcre_copy_substring() and pcre_get_sub-
2090           string().  Unfortunately, the interface to pcre_get_substring_list() is
2091           not  adequate for handling strings containing binary zeros, because the
2092           end of the final string is not independently indicated.
2093    
2094           The first three arguments are the same for all  three  of  these  func-
2095           tions:  subject  is  the subject string that has just been successfully
2096           matched, ovector is a pointer to the vector of integer offsets that was
2097           passed to pcre_exec(), and stringcount is the number of substrings that
2098           were captured by the match, including the substring  that  matched  the
2099           entire regular expression. This is the value returned by pcre_exec() if
2100           it is greater than zero. If pcre_exec() returned zero, indicating  that
2101           it  ran out of space in ovector, the value passed as stringcount should
2102           be the number of elements in the vector divided by three.
2103    
2104           The functions pcre_copy_substring() and pcre_get_substring() extract  a
2105           single  substring,  whose  number  is given as stringnumber. A value of
2106           zero extracts the substring that matched the  entire  pattern,  whereas
2107           higher  values  extract  the  captured  substrings.  For pcre_copy_sub-
2108           string(), the string is placed in buffer,  whose  length  is  given  by
2109           buffersize,  while  for  pcre_get_substring()  a new block of memory is
2110           obtained via pcre_malloc, and its address is  returned  via  stringptr.
2111           The  yield  of  the function is the length of the string, not including
2112           the terminating zero, or one of these error codes:
2113    
2114             PCRE_ERROR_NOMEMORY       (-6)
2115    
2116           The buffer was too small for pcre_copy_substring(), or the  attempt  to
2117           get memory failed for pcre_get_substring().
2118    
2119             PCRE_ERROR_NOSUBSTRING    (-7)
2120    
2121           There is no substring whose number is stringnumber.
2122    
2123           The  pcre_get_substring_list()  function  extracts  all  available sub-
2124           strings and builds a list of pointers to them. All this is  done  in  a
2125           single block of memory that is obtained via pcre_malloc. The address of
2126           the memory block is returned via listptr, which is also  the  start  of
2127           the  list  of  string pointers. The end of the list is marked by a NULL
2128           pointer. The yield of the function is zero if all  went  well,  or  the
2129           error code
2130    
2131             PCRE_ERROR_NOMEMORY       (-6)
2132    
2133           if the attempt to get the memory block failed.
2134    
2135           When  any of these functions encounter a substring that is unset, which
2136           can happen when capturing subpattern number n+1 matches  some  part  of
2137           the  subject, but subpattern n has not been used at all, they return an
2138           empty string. This can be distinguished from a genuine zero-length sub-
2139           string  by inspecting the appropriate offset in ovector, which is nega-
2140           tive for unset substrings.
2141    
2142           The two convenience functions pcre_free_substring() and  pcre_free_sub-
2143           string_list()  can  be  used  to free the memory returned by a previous
2144           call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
2145           tively.  They  do  nothing  more  than  call the function pointed to by
2146           pcre_free, which of course could be called directly from a  C  program.
2147           However,  PCRE is used in some situations where it is linked via a spe-
2148           cial  interface  to  another  programming  language  that  cannot   use
2149           pcre_free  directly;  it is for these cases that the functions are pro-
2150           vided.
2151    
2152    
2153    EXTRACTING CAPTURED SUBSTRINGS BY NAME
2154    
2155           int pcre_get_stringnumber(const pcre *code,
2156                const char *name);
2157    
2158           int pcre_copy_named_substring(const pcre *code,
2159                const char *subject, int *ovector,
2160                int stringcount, const char *stringname,
2161                char *buffer, int buffersize);
2162    
2163           int pcre_get_named_substring(const pcre *code,
2164                const char *subject, int *ovector,
2165                int stringcount, const char *stringname,
2166                const char **stringptr);
2167    
2168           To extract a substring by name, you first have to find associated  num-
2169           ber.  For example, for this pattern
2170    
2171             (a+)b(?<xxx>\d+)...
2172    
2173           the number of the subpattern called "xxx" is 2. If the name is known to
2174           be unique (PCRE_DUPNAMES was not set), you can find the number from the
2175           name by calling pcre_get_stringnumber(). The first argument is the com-
2176           piled pattern, and the second is the name. The yield of the function is
2177           the  subpattern  number,  or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2178           subpattern of that name.
2179    
2180           Given the number, you can extract the substring directly, or use one of
2181           the functions described in the previous section. For convenience, there
2182           are also two functions that do the whole job.
2183    
2184           Most   of   the   arguments    of    pcre_copy_named_substring()    and
2185           pcre_get_named_substring()  are  the  same  as  those for the similarly
2186           named functions that extract by number. As these are described  in  the
2187           previous  section,  they  are not re-described here. There are just two
2188           differences:
2189    
2190           First, instead of a substring number, a substring name is  given.  Sec-
2191           ond, there is an extra argument, given at the start, which is a pointer
2192           to the compiled pattern. This is needed in order to gain access to  the
2193           name-to-number translation table.
2194    
2195           These  functions call pcre_get_stringnumber(), and if it succeeds, they
2196           then call pcre_copy_substring() or pcre_get_substring(),  as  appropri-
2197           ate.  NOTE:  If PCRE_DUPNAMES is set and there are duplicate names, the
2198           behaviour may not be what you want (see the next section).
2199    
2200    
2201    DUPLICATE SUBPATTERN NAMES
2202    
2203           int pcre_get_stringtable_entries(const pcre *code,
2204                const char *name, char **first, char **last);
2205    
2206           When a pattern is compiled with the  PCRE_DUPNAMES  option,  names  for
2207           subpatterns  are  not  required  to  be unique. Normally, patterns with
2208           duplicate names are such that in any one match, only one of  the  named
2209           subpatterns  participates. An example is shown in the pcrepattern docu-
2210           mentation.
2211    
2212           When   duplicates   are   present,   pcre_copy_named_substring()    and
2213           pcre_get_named_substring()  return the first substring corresponding to
2214           the given name that is set. If  none  are  set,  PCRE_ERROR_NOSUBSTRING
2215           (-7)  is  returned;  no  data  is returned. The pcre_get_stringnumber()
2216           function returns one of the numbers that are associated with the  name,
2217           but it is not defined which it is.
2218    
2219           If  you want to get full details of all captured substrings for a given
2220           name, you must use  the  pcre_get_stringtable_entries()  function.  The
2221           first argument is the compiled pattern, and the second is the name. The
2222           third and fourth are pointers to variables which  are  updated  by  the
2223           function. After it has run, they point to the first and last entries in
2224           the name-to-number table  for  the  given  name.  The  function  itself
2225           returns  the  length  of  each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2226           there are none. The format of the table is described above in the  sec-
2227           tion  entitled  Information  about  a  pattern.  Given all the relevant
2228           entries for the name, you can extract each of their numbers, and  hence
2229           the captured data, if any.
2230    
2231    
2232    FINDING ALL POSSIBLE MATCHES
2233    
2234           The  traditional  matching  function  uses a similar algorithm to Perl,
2235           which stops when it finds the first match, starting at a given point in
2236           the  subject.  If you want to find all possible matches, or the longest
2237           possible match, consider using the alternative matching  function  (see
2238           below)  instead.  If you cannot use the alternative function, but still
2239           need to find all possible matches, you can kludge it up by  making  use
2240           of the callout facility, which is described in the pcrecallout documen-
2241           tation.
2242    
2243           What you have to do is to insert a callout right at the end of the pat-
2244           tern.   When your callout function is called, extract and save the cur-
2245           rent matched substring. Then return  1,  which  forces  pcre_exec()  to
2246           backtrack  and  try other alternatives. Ultimately, when it runs out of
2247           matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2248    
2249    
2250    MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2251    
2252           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2253                const char *subject, int length, int startoffset,
2254                int options, int *ovector, int ovecsize,
2255                int *workspace, int wscount);
2256    
2257           The function pcre_dfa_exec()  is  called  to  match  a  subject  string
2258           against  a  compiled pattern, using a matching algorithm that scans the
2259           subject string just once, and does not backtrack.  This  has  different
2260           characteristics  to  the  normal  algorithm, and is not compatible with
2261           Perl. Some of the features of PCRE patterns are not  supported.  Never-
2262           theless,  there are times when this kind of matching can be useful. For
2263           a discussion of the two matching algorithms, see the pcrematching docu-
2264           mentation.
2265    
2266           The  arguments  for  the  pcre_dfa_exec()  function are the same as for
2267           pcre_exec(), plus two extras. The ovector argument is used in a differ-
2268           ent  way,  and  this is described below. The other common arguments are
2269           used in the same way as for pcre_exec(), so their  description  is  not
2270           repeated here.
2271    
2272           The  two  additional  arguments provide workspace for the function. The
2273           workspace vector should contain at least 20 elements. It  is  used  for
2274           keeping  track  of  multiple  paths  through  the  pattern  tree.  More
2275           workspace will be needed for patterns and subjects where  there  are  a
2276           lot of potential matches.
2277    
2278           Here is an example of a simple call to pcre_dfa_exec():
2279    
2280             int rc;
2281             int ovector[10];
2282             int wspace[20];
2283             rc = pcre_dfa_exec(
2284               re,             /* result of pcre_compile() */
2285               NULL,           /* we didn't study the pattern */
2286               "some string",  /* the subject string */
2287               11,             /* the length of the subject string */
2288               0,              /* start at offset 0 in the subject */
2289               0,              /* default options */
2290               ovector,        /* vector of integers for substring information */
2291               10,             /* number of elements (NOT size in bytes) */
2292               wspace,         /* working space vector */
2293               20);            /* number of elements (NOT size in bytes) */
2294    
2295       Option bits for pcre_dfa_exec()
2296    
2297           The  unused  bits  of  the options argument for pcre_dfa_exec() must be
2298           zero. The only bits  that  may  be  set  are  PCRE_ANCHORED,  PCRE_NEW-
2299           LINE_xxx,  PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
2300           PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2301           three of these are the same as for pcre_exec(), so their description is
2302           not repeated here.
2303    
2304             PCRE_PARTIAL
2305    
2306           This has the same general effect as it does for  pcre_exec(),  but  the
2307           details   are   slightly   different.  When  PCRE_PARTIAL  is  set  for
2308           pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is  converted  into
2309           PCRE_ERROR_PARTIAL  if  the  end  of the subject is reached, there have
2310           been no complete matches, but there is still at least one matching pos-
2311           sibility.  The portion of the string that provided the partial match is
2312           set as the first matching string.
2313    
2314             PCRE_DFA_SHORTEST
2315    
2316           Setting the PCRE_DFA_SHORTEST option causes the matching  algorithm  to
2317           stop as soon as it has found one match. Because of the way the alterna-
2318           tive algorithm works, this is necessarily the shortest  possible  match
2319           at the first possible matching point in the subject string.
2320    
2321             PCRE_DFA_RESTART
2322    
2323           When  pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option, and
2324           returns a partial match, it is possible to call it  again,  with  addi-
2325           tional  subject  characters,  and have it continue with the same match.
2326           The PCRE_DFA_RESTART option requests this action; when it is  set,  the
2327           workspace  and wscount options must reference the same vector as before
2328           because data about the match so far is left in  them  after  a  partial
2329           match.  There  is  more  discussion of this facility in the pcrepartial
2330           documentation.
2331    
2332       Successful returns from pcre_dfa_exec()
2333    
2334           When pcre_dfa_exec() succeeds, it may have matched more than  one  sub-
2335           string in the subject. Note, however, that all the matches from one run
2336           of the function start at the same point in  the  subject.  The  shorter
2337           matches  are all initial substrings of the longer matches. For example,
2338           if the pattern
2339    
2340             <.*>
2341    
2342           is matched against the string
2343    
2344             This is <something> <something else> <something further> no more
2345    
2346           the three matched strings are
2347    
2348             <something>
2349             <something> <something else>
2350             <something> <something else> <something further>
2351    
2352           On success, the yield of the function is a number  greater  than  zero,
2353           which  is  the  number of matched substrings. The substrings themselves
2354           are returned in ovector. Each string uses two elements;  the  first  is
2355           the  offset  to  the start, and the second is the offset to the end. In
2356           fact, all the strings have the same start  offset.  (Space  could  have
2357           been  saved by giving this only once, but it was decided to retain some
2358           compatibility with the way pcre_exec() returns data,  even  though  the
2359           meaning of the strings is different.)
2360    
2361           The strings are returned in reverse order of length; that is, the long-
2362           est matching string is given first. If there were too many  matches  to
2363           fit  into ovector, the yield of the function is zero, and the vector is
2364           filled with the longest matches.
2365    
2366       Error returns from pcre_dfa_exec()
2367    
2368           The pcre_dfa_exec() function returns a negative number when  it  fails.
2369           Many  of  the  errors  are  the  same as for pcre_exec(), and these are
2370           described above.  There are in addition the following errors  that  are
2371           specific to pcre_dfa_exec():
2372    
2373             PCRE_ERROR_DFA_UITEM      (-16)
2374    
2375           This  return is given if pcre_dfa_exec() encounters an item in the pat-
2376           tern that it does not support, for instance, the use of \C  or  a  back
2377           reference.
2378    
2379             PCRE_ERROR_DFA_UCOND      (-17)
2380    
2381           This  return  is  given  if pcre_dfa_exec() encounters a condition item
2382           that uses a back reference for the condition, or a test  for  recursion
2383           in a specific group. These are not supported.
2384    
2385             PCRE_ERROR_DFA_UMLIMIT    (-18)
2386    
2387           This  return  is given if pcre_dfa_exec() is called with an extra block
2388           that contains a setting of the match_limit field. This is not supported
2389           (it is meaningless).
2390    
2391             PCRE_ERROR_DFA_WSSIZE     (-19)
2392    
2393           This  return  is  given  if  pcre_dfa_exec()  runs  out of space in the
2394           workspace vector.
2395    
2396             PCRE_ERROR_DFA_RECURSE    (-20)
2397    
2398           When a recursive subpattern is processed, the matching  function  calls
2399           itself  recursively,  using  private vectors for ovector and workspace.
2400           This error is given if the output vector  is  not  large  enough.  This
2401           should be extremely rare, as a vector of size 1000 is used.
2402    
 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.  
2403    
2404    SEE ALSO
2405    
2406           pcrebuild(3),  pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2407           tial(3), pcreposix(3), pcreprecompile(3), pcresample(3),  pcrestack(3).
2408    
 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  
2409    
2410       The following sections describe  the  use  of  each  of  the  AUTHOR
      meta-characters.  
2411    
2412           Philip Hazel
2413           University Computing Service
2414           Cambridge CB2 3QH, England.
2415    
2416    
2417  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.  
2418    
2419           Last updated: 30 July 2007
2420           Copyright (c) 1997-2007 University of Cambridge.
2421    ------------------------------------------------------------------------------
2422    
2423    
2424  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.  
2425    
2426    
2427    NAME
2428           PCRE - Perl-compatible regular expressions
2429    
 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.  
2430    
2431    PCRE CALLOUTS
2432    
2433           int (*pcre_callout)(pcre_callout_block *);
2434    
2435  POSIX CHARACTER CLASSES         PCRE provides a feature called "callout", which is a means of temporar-
2436       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
2437       to  support  the POSIX notation for character classes, which         matching.  The  caller of PCRE provides an external function by putting
2438       uses names enclosed by  [:  and  :]   within  the  enclosing         its entry point in the global variable pcre_callout. By  default,  this
2439       square brackets. PCRE supports this notation. For example,         variable contains NULL, which disables all calling out.
2440    
2441         [01[:alpha:]%]         Within  a  regular  expression,  (?C) indicates the points at which the
2442           external function is to be called.  Different  callout  points  can  be
2443       matches "0", "1", any alphabetic character, or "%". The sup-         identified  by  putting  a number less than 256 after the letter C. The
2444       ported class names are         default value is zero.  For  example,  this  pattern  has  two  callout
2445           points:
2446         alnum    letters and digits  
2447         alpha    letters           (?C1)abc(?C2)def
2448         ascii    character codes 0 - 127  
2449         cntrl    control characters         If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is
2450         digit    decimal digits (same as \d)         called, PCRE automatically  inserts  callouts,  all  with  number  255,
2451         graph    printing characters, excluding space         before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2452         lower    lower case letters         used with the pattern
2453         print    printing characters, including space  
2454         punct    printing characters, excluding letters and digits           A(\d{2}|--)
2455         space    white space (same as \s)  
2456         upper    upper case letters         it is processed as if it were
2457         word     "word" characters (same as \w)  
2458         xdigit   hexadecimal digits         (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2459    
2460       The names "ascii" and "word" are  Perl  extensions.  Another         Notice that there is a callout before and after  each  parenthesis  and
2461       Perl  extension is negation, which is indicated by a ^ char-         alternation  bar.  Automatic  callouts  can  be  used  for tracking the
2462       acter after the colon. For example,         progress of pattern matching. The pcretest command has an  option  that
2463           sets  automatic callouts; when it is used, the output indicates how the
2464         [12[:^digit:]]         pattern is matched. This is useful information when you are  trying  to
2465           optimize the performance of a particular pattern.
2466       matches "1", "2", or any non-digit.  PCRE  (and  Perl)  also  
2467       recogize  the POSIX syntax [.ch.] and [=ch=] where "ch" is a  
2468       "collating element", but these are  not  supported,  and  an  MISSING CALLOUTS
2469       error is given if they are encountered.  
2470           You  should  be  aware  that,  because of optimizations in the way PCRE
2471           matches patterns, callouts sometimes do not happen. For example, if the
2472           pattern is
2473    
2474             ab(?C4)cd
2475    
2476           PCRE knows that any matching string must contain the letter "d". If the
2477           subject string is "abyz", the lack of "d" means that  matching  doesn't
2478           ever  start,  and  the  callout is never reached. However, with "abyd",
2479           though the result is still no match, the callout is obeyed.
2480    
2481    
2482    THE CALLOUT INTERFACE
2483    
2484           During matching, when PCRE reaches a callout point, the external  func-
2485           tion  defined by pcre_callout is called (if it is set). This applies to
2486           both the pcre_exec() and the pcre_dfa_exec()  matching  functions.  The
2487           only  argument  to  the callout function is a pointer to a pcre_callout
2488           block. This structure contains the following fields:
2489    
2490             int          version;
2491             int          callout_number;
2492             int         *offset_vector;
2493             const char  *subject;
2494             int          subject_length;
2495             int          start_match;
2496             int          current_position;
2497             int          capture_top;
2498             int          capture_last;
2499             void        *callout_data;
2500             int          pattern_position;
2501             int          next_item_length;
2502    
2503           The version field is an integer containing the version  number  of  the
2504           block  format. The initial version was 0; the current version is 1. The
2505           version number will change again in future  if  additional  fields  are
2506           added, but the intention is never to remove any of the existing fields.
2507    
2508           The callout_number field contains the number of the  callout,  as  com-
2509           piled  into  the pattern (that is, the number after ?C for manual call-
2510           outs, and 255 for automatically generated callouts).
2511    
2512           The offset_vector field is a pointer to the vector of offsets that  was
2513           passed   by   the   caller  to  pcre_exec()  or  pcre_dfa_exec().  When
2514           pcre_exec() is used, the contents can be inspected in order to  extract
2515           substrings  that  have  been  matched  so  far,  in the same way as for
2516           extracting substrings after a match has completed. For  pcre_dfa_exec()
2517           this field is not useful.
2518    
2519           The subject and subject_length fields contain copies of the values that
2520           were passed to pcre_exec().
2521    
2522           The start_match field normally contains the offset within  the  subject
2523           at  which  the  current  match  attempt started. However, if the escape
2524           sequence \K has been encountered, this value is changed to reflect  the
2525           modified  starting  point.  If the pattern is not anchored, the callout
2526           function may be called several times from the same point in the pattern
2527           for different starting points in the subject.
2528    
2529           The  current_position  field  contains the offset within the subject of
2530           the current match pointer.
2531    
2532           When the pcre_exec() function is used, the capture_top  field  contains
2533           one  more than the number of the highest numbered captured substring so
2534           far. If no substrings have been captured, the value of  capture_top  is
2535           one.  This  is always the case when pcre_dfa_exec() is used, because it
2536           does not support captured substrings.
2537    
2538           The capture_last field contains the number of the  most  recently  cap-
2539           tured  substring. If no substrings have been captured, its value is -1.
2540           This is always the case when pcre_dfa_exec() is used.
2541    
2542           The callout_data field contains a value that is passed  to  pcre_exec()
2543           or  pcre_dfa_exec() specifically so that it can be passed back in call-
2544           outs. It is passed in the pcre_callout field  of  the  pcre_extra  data
2545           structure.  If  no such data was passed, the value of callout_data in a
2546           pcre_callout block is NULL. There is a description  of  the  pcre_extra
2547           structure in the pcreapi documentation.
2548    
2549           The  pattern_position field is present from version 1 of the pcre_call-
2550           out structure. It contains the offset to the next item to be matched in
2551           the pattern string.
2552    
2553           The  next_item_length field is present from version 1 of the pcre_call-
2554           out structure. It contains the length of the next item to be matched in
2555           the  pattern  string. When the callout immediately precedes an alterna-
2556           tion bar, a closing parenthesis, or the end of the pattern, the  length
2557           is  zero.  When the callout precedes an opening parenthesis, the length
2558           is that of the entire subpattern.
2559    
2560           The pattern_position and next_item_length fields are intended  to  help
2561           in  distinguishing between different automatic callouts, which all have
2562           the same callout number. However, they are set for all callouts.
2563    
2564    
2565    RETURN VALUES
2566    
2567           The external callout function returns an integer to PCRE. If the  value
2568           is  zero,  matching  proceeds  as  normal. If the value is greater than
2569           zero, matching fails at the current point, but  the  testing  of  other
2570           matching possibilities goes ahead, just as if a lookahead assertion had
2571           failed. If the value is less than zero, the  match  is  abandoned,  and
2572           pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2573    
2574           Negative   values   should   normally   be   chosen  from  the  set  of
2575           PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2576           dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
2577           reserved for use by callout functions; it will never be  used  by  PCRE
2578           itself.
2579    
2580    
2581    AUTHOR
2582    
2583  VERTICAL BAR         Philip Hazel
2584       Vertical bar characters are  used  to  separate  alternative         University Computing Service
2585       patterns. For example, the pattern         Cambridge CB2 3QH, England.
2586    
        gilbert|sullivan  
2587    
2588       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.  
2589    
2590           Last updated: 29 May 2007
2591           Copyright (c) 1997-2007 University of Cambridge.
2592    ------------------------------------------------------------------------------
2593    
2594    
2595  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.  
2596    
2597    
2598    NAME
2599           PCRE - Perl-compatible regular expressions
2600    
 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".  
2601    
2602    DIFFERENCES BETWEEN PCRE AND PERL
2603    
2604           This  document describes the differences in the ways that PCRE and Perl
2605           handle regular expressions. The differences described here  are  mainly
2606           with  respect  to  Perl 5.8, though PCRE versions 7.0 and later contain
2607           some features that are expected to be in the forthcoming Perl 5.10.
2608    
2609           1. PCRE has only a subset of Perl's UTF-8 and Unicode support.  Details
2610           of  what  it does have are given in the section on UTF-8 support in the
2611           main pcre page.
2612    
2613           2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2614           permits  them,  but they do not mean what you might think. For example,
2615           (?!a){3} does not assert that the next three characters are not "a". It
2616           just asserts that the next character is not "a" three times.
2617    
2618           3.  Capturing  subpatterns  that occur inside negative lookahead asser-
2619           tions are counted, but their entries in the offsets  vector  are  never
2620           set.  Perl sets its numerical variables from any such patterns that are
2621           matched before the assertion fails to match something (thereby succeed-
2622           ing),  but  only  if the negative lookahead assertion contains just one
2623           branch.
2624    
2625           4. Though binary zero characters are supported in the  subject  string,
2626           they are not allowed in a pattern string because it is passed as a nor-
2627           mal C string, terminated by zero. The escape sequence \0 can be used in
2628           the pattern to represent a binary zero.
2629    
2630           5.  The  following Perl escape sequences are not supported: \l, \u, \L,
2631           \U, and \N. In fact these are implemented by Perl's general string-han-
2632           dling  and are not part of its pattern matching engine. If any of these
2633           are encountered by PCRE, an error is generated.
2634    
2635           6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE
2636           is  built  with Unicode character property support. The properties that
2637           can be tested with \p and \P are limited to the general category  prop-
2638           erties  such  as  Lu and Nd, script names such as Greek or Han, and the
2639           derived properties Any and L&.
2640    
2641           7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2642           ters  in  between  are  treated as literals. This is slightly different
2643           from Perl in that $ and @ are  also  handled  as  literals  inside  the
2644           quotes.  In Perl, they cause variable interpolation (but of course PCRE
2645           does not have variables). Note the following examples:
2646    
2647               Pattern            PCRE matches      Perl matches
2648    
2649               \Qabc$xyz\E        abc$xyz           abc followed by the
2650                                                      contents of $xyz
2651               \Qabc\$xyz\E       abc\$xyz          abc\$xyz
2652               \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
2653    
2654           The \Q...\E sequence is recognized both inside  and  outside  character
2655           classes.
2656    
2657           8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2658           constructions. However, there is support for recursive  patterns.  This
2659           is  not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
2660           "callout" feature allows an external function to be called during  pat-
2661           tern matching. See the pcrecallout documentation for details.
2662    
2663           9.  Subpatterns  that  are  called  recursively or as "subroutines" are
2664           always treated as atomic groups in  PCRE.  This  is  like  Python,  but
2665           unlike Perl.
2666