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

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

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

revision 47 by nigel, Sat Feb 24 21:39:29 2007 UTC revision 128 by ph10, Tue Mar 20 11:46:50 2007 UTC
# Line 1  Line 1 
1    -----------------------------------------------------------------------------
2    This file contains a concatenation of the PCRE man pages, converted to plain
3    text format for ease of searching with a text editor, or for use on systems
4    that do not have a man page processor. The small individual files that give
5    synopses of each function in the library have not been included. There are
6    separate text files for the pcregrep and pcretest commands.
7    -----------------------------------------------------------------------------
8    
9    
10    PCRE(3)                                                                PCRE(3)
11    
12    
13  NAME  NAME
14       pcre - Perl-compatible regular expressions.         PCRE - Perl-compatible regular expressions
15    
16    
17    INTRODUCTION
18    
19  SYNOPSIS         The  PCRE  library is a set of functions that implement regular expres-
20       #include <pcre.h>         sion pattern matching using the same syntax and semantics as Perl, with
21           just  a  few differences. (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.
49    
50           Some features of PCRE can be included, excluded, or  changed  when  the
51           library  is  built.  The pcre_config() function makes it possible for a
52           client to discover which features are  available.  The  features  them-
53           selves  are described in the pcrebuild page. Documentation about build-
54           ing PCRE for various operating systems can be found in the README  file
55           in the source distribution.
56    
57           The  library  contains  a number of undocumented internal functions and
58           data tables that are used by more than one  of  the  exported  external
59           functions,  but  which  are  not  intended for use by external callers.
60           Their names all begin with "_pcre_", which hopefully will  not  provoke
61           any name clashes. In some environments, it is possible to control which
62           external symbols are exported when a shared library is  built,  and  in
63           these cases the undocumented symbols are not exported.
64    
65    
66    USER DOCUMENTATION
67    
68           The  user  documentation  for PCRE comprises a number of different sec-
69           tions. In the "man" format, each of these is a separate "man page".  In
70           the  HTML  format, each is a separate page, linked from the index page.
71           In the plain text format, all the sections are concatenated,  for  ease
72           of searching. The sections are as follows:
73    
74             pcre              this document
75             pcreapi           details of PCRE's native C API
76             pcrebuild         options for building PCRE
77             pcrecallout       details of the callout feature
78             pcrecompat        discussion of Perl compatibility
79             pcrecpp           details of the C++ wrapper
80             pcregrep          description of the pcregrep command
81             pcrematching      discussion of the two matching algorithms
82             pcrepartial       details of the partial matching facility
83             pcrepattern       syntax and semantics of supported
84                                 regular expressions
85             pcreperform       discussion of performance issues
86             pcreposix         the POSIX-compatible C API
87             pcreprecompile    details of saving and re-using precompiled patterns
88             pcresample        discussion of the sample program
89             pcrestack         discussion of stack usage
90             pcretest          description of the pcretest testing command
91    
92       pcre *pcre_compile(const char *pattern, int options,         In addition, in the "man" and HTML formats, there is a short  page  for
93            const char **errptr, int *erroffset,         each C library function, listing its arguments and results.
           const unsigned char *tableptr);  
94    
      pcre_extra *pcre_study(const pcre *code, int options,  
           const char **errptr);  
95    
96       int pcre_exec(const pcre *code, const pcre_extra *extra,  LIMITATIONS
           const char *subject, int length, int startoffset,  
           int options, int *ovector, int ovecsize);  
97    
98       int pcre_copy_substring(const char *subject, int *ovector,         There  are some size limitations in PCRE but it is hoped that they will
99            int stringcount, int stringnumber, char *buffer,         never in practice be relevant.
           int buffersize);  
100    
101       int pcre_get_substring(const char *subject, int *ovector,         The maximum length of a compiled pattern is 65539 (sic) bytes  if  PCRE
102            int stringcount, int stringnumber,         is compiled with the default internal linkage size of 2. If you want to
103            const char **stringptr);         process regular expressions that are truly enormous,  you  can  compile
104           PCRE  with  an  internal linkage size of 3 or 4 (see the README file in
105           the source distribution and the pcrebuild documentation  for  details).
106           In  these  cases the limit is substantially larger.  However, the speed
107           of execution is slower.
108    
109           All values in repeating quantifiers must be less than 65536. The  maxi-
110           mum  compiled  length  of  subpattern  with an explicit repeat count is
111           30000 bytes. The maximum number of capturing subpatterns is 65535.
112    
113           There is no limit to the number of parenthesized subpatterns, but there
114           can be no more than 65535 capturing subpatterns.
115    
116           The maximum length of name for a named subpattern is 32 characters, and
117           the maximum number of named subpatterns is 10000.
118    
119           The maximum length of a subject string is the largest  positive  number
120           that  an integer variable can hold. However, when using the traditional
121           matching function, PCRE uses recursion to handle subpatterns and indef-
122           inite  repetition.  This means that the available stack space may limit
123           the size of a subject string that can be processed by certain patterns.
124           For a discussion of stack issues, see the pcrestack documentation.
125    
126    
127    UTF-8 AND UNICODE PROPERTY SUPPORT
128    
129           From  release  3.3,  PCRE  has  had  some support for character strings
130           encoded in the UTF-8 format. For release 4.0 this was greatly  extended
131           to  cover  most common requirements, and in release 5.0 additional sup-
132           port for Unicode general category properties was added.
133    
134           In order process UTF-8 strings, you must build PCRE  to  include  UTF-8
135           support  in  the  code,  and, in addition, you must call pcre_compile()
136           with the PCRE_UTF8 option flag. When you do this, both the pattern  and
137           any  subject  strings  that are matched against it are treated as UTF-8
138           strings instead of just strings of bytes.
139    
140           If you compile PCRE with UTF-8 support, but do not use it at run  time,
141           the  library will be a bit bigger, but the additional run time overhead
142           is limited to testing the PCRE_UTF8 flag occasionally, so should not be
143           very big.
144    
145           If PCRE is built with Unicode character property support (which implies
146           UTF-8 support), the escape sequences \p{..}, \P{..}, and  \X  are  sup-
147           ported.  The available properties that can be tested are limited to the
148           general category properties such as Lu for an upper case letter  or  Nd
149           for  a  decimal number, the Unicode script names such as Arabic or Han,
150           and the derived properties Any and L&. A full  list  is  given  in  the
151           pcrepattern documentation. Only the short names for properties are sup-
152           ported. For example, \p{L} matches a letter. Its Perl synonym,  \p{Let-
153           ter},  is  not  supported.   Furthermore,  in Perl, many properties may
154           optionally be prefixed by "Is", for compatibility with Perl  5.6.  PCRE
155           does not support this.
156    
157           The following comments apply when PCRE is running in UTF-8 mode:
158    
159           1.  When you set the PCRE_UTF8 flag, the strings passed as patterns and
160           subjects are checked for validity on entry to the  relevant  functions.
161           If an invalid UTF-8 string is passed, an error return is given. In some
162           situations, you may already know  that  your  strings  are  valid,  and
163           therefore want to skip these checks in order to improve performance. If
164           you set the PCRE_NO_UTF8_CHECK flag at compile time  or  at  run  time,
165           PCRE  assumes  that  the  pattern or subject it is given (respectively)
166           contains only valid UTF-8 codes. In this case, it does not diagnose  an
167           invalid  UTF-8 string. If you pass an invalid UTF-8 string to PCRE when
168           PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program  may
169           crash.
170    
171           2.  An  unbraced  hexadecimal  escape sequence (such as \xb3) matches a
172           two-byte UTF-8 character if the value is greater than 127.
173    
174           3. Octal numbers up to \777 are recognized, and  match  two-byte  UTF-8
175           characters for values greater than \177.
176    
177           4.  Repeat quantifiers apply to complete UTF-8 characters, not to indi-
178           vidual bytes, for example: \x{100}{3}.
179    
180           5. The dot metacharacter matches one UTF-8 character instead of a  sin-
181           gle byte.
182    
183           6.  The  escape sequence \C can be used to match a single byte in UTF-8
184           mode, but its use can lead to some strange effects.  This  facility  is
185           not available in the alternative matching function, pcre_dfa_exec().
186    
187           7.  The  character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
188           test characters of any code value, but the characters that PCRE  recog-
189           nizes  as  digits,  spaces,  or  word characters remain the same set as
190           before, all with values less than 256. This remains true even when PCRE
191           includes  Unicode  property support, because to do otherwise would slow
192           down PCRE in many common cases. If you really want to test for a  wider
193           sense  of,  say,  "digit",  you must use Unicode property tests such as
194           \p{Nd}.
195    
196           8. Similarly, characters that match the POSIX named  character  classes
197           are all low-valued characters.
198    
199           9.  Case-insensitive  matching  applies only to characters whose values
200           are less than 128, unless PCRE is built with Unicode property  support.
201           Even  when  Unicode  property support is available, PCRE still uses its
202           own character tables when checking the case of  low-valued  characters,
203           so  as not to degrade performance.  The Unicode property information is
204           used only for characters with higher values. Even when Unicode property
205           support is available, PCRE supports case-insensitive matching only when
206           there is a one-to-one mapping between a letter's  cases.  There  are  a
207           small  number  of  many-to-one  mappings in Unicode; these are not sup-
208           ported by PCRE.
209    
      int pcre_get_substring_list(const char *subject,  
           int *ovector, int stringcount, const char ***listptr);  
210    
211       const unsigned char *pcre_maketables(void);  AUTHOR
212    
213       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,         Philip Hazel
214            int what, void *where);         University Computing Service
215           Cambridge CB2 3QH, England.
216    
217       int pcre_info(const pcre *code, int *optptr, *firstcharptr);         Putting an actual email address here seems to have been a spam  magnet,
218           so I've taken it away. If you want to email me, use my initial and sur-
219           name, separated by a dot, at the domain ucs.cam.ac.uk.
220    
      char *pcre_version(void);  
221    
222       void *(*pcre_malloc)(size_t);  REVISION
223    
224       void (*pcre_free)(void *);         Last updated: 06 March 2007
225           Copyright (c) 1997-2007 University of Cambridge.
226    ------------------------------------------------------------------------------
227    
228    
229    PCREBUILD(3)                                                      PCREBUILD(3)
230    
231    
232  DESCRIPTION  NAME
233       The PCRE library is a set of functions that implement  regu-         PCRE - Perl-compatible regular expressions
234       lar  expression  pattern  matching using the same syntax and  
235       semantics as Perl  5,  with  just  a  few  differences  (see  
236       below).  The  current  implementation  corresponds  to  Perl  PCRE BUILD-TIME OPTIONS
237       5.005, with some additional features from the Perl  develop-  
238       ment release.         This  document  describes  the  optional  features  of PCRE that can be
239           selected when the library is compiled. They are all selected, or  dese-
240       PCRE has its own native API,  which  is  described  in  this         lected, by providing options to the configure script that is run before
241       document.  There  is  also  a  set of wrapper functions that         the make command. The complete list of  options  for  configure  (which
242       correspond to the POSIX regular expression API.   These  are         includes  the  standard  ones such as the selection of the installation
243       described in the pcreposix documentation.         directory) can be obtained by running
244    
245       The native API function prototypes are defined in the header           ./configure --help
246       file  pcre.h,  and  on  Unix  systems  the library itself is  
247       called libpcre.a, so can be accessed by adding -lpcre to the         The following sections include  descriptions  of  options  whose  names
248       command  for  linking  an  application  which  calls it. The         begin with --enable or --disable. These settings specify changes to the
249       header file defines the macros PCRE_MAJOR and PCRE_MINOR  to         defaults for the configure command. Because of the way  that  configure
250       contain the major and minor release numbers for the library.         works,  --enable  and --disable always come in pairs, so the complemen-
251       Applications can use these to include support for  different         tary option always exists as well, but as it specifies the default,  it
252       releases.         is not described.
253    
254       The functions pcre_compile(), pcre_study(), and  pcre_exec()  
255       are  used  for  compiling  and matching regular expressions,  C++ SUPPORT
256       while   pcre_copy_substring(),   pcre_get_substring(),   and  
257       pcre_get_substring_list()   are  convenience  functions  for         By default, the configure script will search for a C++ compiler and C++
258       extracting  captured  substrings  from  a  matched   subject         header files. If it finds them, it automatically builds the C++ wrapper
259       string.  The function pcre_maketables() is used (optionally)         library for PCRE. You can disable this by adding
260       to build a set of character tables in the current locale for  
261       passing to pcre_compile().           --disable-cpp
262    
263       The function pcre_fullinfo() is used to find out information         to the configure command.
264       about a compiled pattern; pcre_info() is an obsolete version  
265       which returns only some of the available information, but is  
266       retained   for   backwards   compatibility.    The  function  UTF-8 SUPPORT
267       pcre_version() returns a pointer to a string containing  the  
268       version of PCRE and its date of release.         To build PCRE with support for UTF-8 character strings, add
269    
270       The global variables  pcre_malloc  and  pcre_free  initially           --enable-utf8
271       contain the entry points of the standard malloc() and free()  
272       functions respectively. PCRE  calls  the  memory  management         to  the  configure  command.  Of  itself, this does not make PCRE treat
273       functions  via  these  variables,  so  a calling program can         strings as UTF-8. As well as compiling PCRE with this option, you  also
274       replace them if it  wishes  to  intercept  the  calls.  This         have  have to set the PCRE_UTF8 option when you call the pcre_compile()
275       should be done before calling any PCRE functions.         function.
276    
277    
278    UNICODE CHARACTER PROPERTY SUPPORT
279    
280           UTF-8 support allows PCRE to process character values greater than  255
281           in  the  strings that it handles. On its own, however, it does not pro-
282           vide any facilities for accessing the properties of such characters. If
283           you  want  to  be able to use the pattern escapes \P, \p, and \X, which
284           refer to Unicode character properties, you must add
285    
286             --enable-unicode-properties
287    
288           to the configure command. This implies UTF-8 support, even if you  have
289           not explicitly requested it.
290    
291           Including  Unicode  property  support  adds around 30K of tables to the
292           PCRE library. Only the general category properties such as  Lu  and  Nd
293           are supported. Details are given in the pcrepattern documentation.
294    
295    
296    CODE VALUE OF NEWLINE
297    
298           By  default,  PCRE interprets character 10 (linefeed, LF) as indicating
299           the end of a line. This is the normal newline  character  on  Unix-like
300           systems. You can compile PCRE to use character 13 (carriage return, CR)
301           instead, by adding
302    
303             --enable-newline-is-cr
304    
305           to the  configure  command.  There  is  also  a  --enable-newline-is-lf
306           option, which explicitly specifies linefeed as the newline character.
307    
308           Alternatively, you can specify that line endings are to be indicated by
309           the two character sequence CRLF. If you want this, add
310    
311             --enable-newline-is-crlf
312    
313           to the configure command. There is a fourth option, specified by
314    
315             --enable-newline-is-any
316    
317           which causes PCRE to recognize any Unicode newline sequence.
318    
319           Whatever line ending convention is selected when PCRE is built  can  be
320           overridden  when  the library functions are called. At build time it is
321           conventional to use the standard for your operating system.
322    
323    
324    BUILDING SHARED AND STATIC LIBRARIES
325    
326           The PCRE building process uses libtool to build both shared and  static
327           Unix  libraries by default. You can suppress one of these by adding one
328           of
329    
330             --disable-shared
331             --disable-static
332    
333           to the configure command, as required.
334    
335    
336    POSIX MALLOC USAGE
337    
338           When PCRE is called through the POSIX interface (see the pcreposix doc-
339           umentation),  additional  working  storage  is required for holding the
340           pointers to capturing substrings, because PCRE requires three  integers
341           per  substring,  whereas  the POSIX interface provides only two. If the
342           number of expected substrings is small, the wrapper function uses space
343           on the stack, because this is faster than using malloc() for each call.
344           The default threshold above which the stack is no longer used is 10; it
345           can be changed by adding a setting such as
346    
347             --with-posix-malloc-threshold=20
348    
349           to the configure command.
350    
351    
352    HANDLING VERY LARGE PATTERNS
353    
354           Within  a  compiled  pattern,  offset values are used to point from one
355           part to another (for example, from an opening parenthesis to an  alter-
356           nation  metacharacter).  By default, two-byte values are used for these
357           offsets, leading to a maximum size for a  compiled  pattern  of  around
358           64K.  This  is sufficient to handle all but the most gigantic patterns.
359           Nevertheless, some people do want to process enormous patterns,  so  it
360           is  possible  to compile PCRE to use three-byte or four-byte offsets by
361           adding a setting such as
362    
363             --with-link-size=3
364    
365           to the configure command. The value given must be 2,  3,  or  4.  Using
366           longer  offsets slows down the operation of PCRE because it has to load
367           additional bytes when handling them.
368    
369    
370    AVOIDING EXCESSIVE STACK USAGE
371    
372           When matching with the pcre_exec() function, PCRE implements backtrack-
373           ing  by  making recursive calls to an internal function called match().
374           In environments where the size of the stack is limited,  this  can  se-
375           verely  limit  PCRE's operation. (The Unix environment does not usually
376           suffer from this problem, but it may sometimes be necessary to increase
377           the  maximum  stack size.  There is a discussion in the pcrestack docu-
378           mentation.) An alternative approach to recursion that uses memory  from
379           the  heap  to remember data, instead of using recursive function calls,
380           has been implemented to work round the problem of limited  stack  size.
381           If you want to build a version of PCRE that works this way, add
382    
383             --disable-stack-for-recursion
384    
385           to  the  configure  command. With this configuration, PCRE will use the
386           pcre_stack_malloc and pcre_stack_free variables to call memory  manage-
387           ment  functions.  Separate  functions are provided because the usage is
388           very predictable: the block sizes requested are always  the  same,  and
389           the  blocks  are always freed in reverse order. A calling program might
390           be able to implement optimized functions that perform better  than  the
391           standard  malloc()  and  free()  functions.  PCRE  runs noticeably more
392           slowly when built in this way. This option affects only the pcre_exec()
393           function; it is not relevant for the the pcre_dfa_exec() function.
394    
395    
396    LIMITING PCRE RESOURCE USAGE
397    
398           Internally,  PCRE has a function called match(), which it calls repeat-
399           edly  (sometimes  recursively)  when  matching  a  pattern   with   the
400           pcre_exec()  function.  By controlling the maximum number of times this
401           function may be called during a single matching operation, a limit  can
402           be  placed  on  the resources used by a single call to pcre_exec(). The
403           limit can be changed at run time, as described in the pcreapi  documen-
404           tation.  The default is 10 million, but this can be changed by adding a
405           setting such as
406    
407             --with-match-limit=500000
408    
409           to  the  configure  command.  This  setting  has  no  effect   on   the
410           pcre_dfa_exec() matching function.
411    
412           In  some  environments  it is desirable to limit the depth of recursive
413           calls of match() more strictly than the total number of calls, in order
414           to  restrict  the maximum amount of stack (or heap, if --disable-stack-
415           for-recursion is specified) that is used. A second limit controls this;
416           it  defaults  to  the  value  that is set for --with-match-limit, which
417           imposes no additional constraints. However, you can set a  lower  limit
418           by adding, for example,
419    
420             --with-match-limit-recursion=10000
421    
422           to  the  configure  command.  This  value can also be overridden at run
423           time.
424    
425    
426    CREATING CHARACTER TABLES AT BUILD TIME
427    
428           PCRE uses fixed tables for processing characters whose code values  are
429           less  than 256. By default, PCRE is built with a set of tables that are
430           distributed in the file pcre_chartables.c.dist. These  tables  are  for
431           ASCII codes only. If you add
432    
433             --enable-rebuild-chartables
434    
435           to  the  configure  command, the distributed tables are no longer used.
436           Instead, a program called dftables is compiled and  run.  This  outputs
437           the source for new set of tables, created in the default locale of your
438           C runtime system. (This method of replacing the tables does not work if
439           you  are cross compiling, because dftables is run on the local host. If
440           you need to create alternative tables when cross  compiling,  you  will
441           have to do so "by hand".)
442    
443    
444    USING EBCDIC CODE
445    
446           PCRE  assumes  by  default that it will run in an environment where the
447           character code is ASCII (or Unicode, which is  a  superset  of  ASCII).
448           PCRE  can,  however,  be  compiled  to  run in an EBCDIC environment by
449           adding
450    
451             --enable-ebcdic
452    
453           to the configure command. This setting implies --enable-rebuild-charta-
454           bles.
455    
456    
457    SEE ALSO
458    
459           pcreapi(3), pcre_config(3).
460    
461    
462    AUTHOR
463    
464           Philip Hazel
465           University Computing Service
466           Cambridge CB2 3QH, England.
467    
468    
469    REVISION
470    
471           Last updated: 20 March 2007
472           Copyright (c) 1997-2007 University of Cambridge.
473    ------------------------------------------------------------------------------
474    
475    
476    PCREMATCHING(3)                                                PCREMATCHING(3)
477    
478    
479    NAME
480           PCRE - Perl-compatible regular expressions
481    
482    
483    PCRE MATCHING ALGORITHMS
484    
485           This document describes the two different algorithms that are available
486           in PCRE for matching a compiled regular expression against a given sub-
487           ject  string.  The  "standard"  algorithm  is  the  one provided by the
488           pcre_exec() function.  This works in the same was  as  Perl's  matching
489           function, and provides a Perl-compatible matching operation.
490    
491           An  alternative  algorithm is provided by the pcre_dfa_exec() function;
492           this operates in a different way, and is not  Perl-compatible.  It  has
493           advantages  and disadvantages compared with the standard algorithm, and
494           these are described below.
495    
496           When there is only one possible way in which a given subject string can
497           match  a pattern, the two algorithms give the same answer. A difference
498           arises, however, when there are multiple possibilities. For example, if
499           the pattern
500    
501             ^<.*>
502    
503           is matched against the string
504    
505             <something> <something else> <something further>
506    
507           there are three possible answers. The standard algorithm finds only one
508           of them, whereas the alternative algorithm finds all three.
509    
510    
511    REGULAR EXPRESSIONS AS TREES
512    
513           The set of strings that are matched by a regular expression can be rep-
514           resented  as  a  tree structure. An unlimited repetition in the pattern
515           makes the tree of infinite size, but it is still a tree.  Matching  the
516           pattern  to a given subject string (from a given starting point) can be
517           thought of as a search of the tree.  There are two  ways  to  search  a
518           tree:  depth-first  and  breadth-first, and these correspond to the two
519           matching algorithms provided by PCRE.
520    
521    
522    THE STANDARD MATCHING ALGORITHM
523    
524           In the terminology of Jeffrey Friedl's book Mastering  Regular  Expres-
525           sions,  the  standard  algorithm  is  an "NFA algorithm". It conducts a
526           depth-first search of the pattern tree. That is, it  proceeds  along  a
527           single path through the tree, checking that the subject matches what is
528           required. When there is a mismatch, the algorithm  tries  any  alterna-
529           tives  at  the  current point, and if they all fail, it backs up to the
530           previous branch point in the  tree,  and  tries  the  next  alternative
531           branch  at  that  level.  This often involves backing up (moving to the
532           left) in the subject string as well.  The  order  in  which  repetition
533           branches  are  tried  is controlled by the greedy or ungreedy nature of
534           the quantifier.
535    
536           If a leaf node is reached, a matching string has  been  found,  and  at
537           that  point the algorithm stops. Thus, if there is more than one possi-
538           ble match, this algorithm returns the first one that it finds.  Whether
539           this  is the shortest, the longest, or some intermediate length depends
540           on the way the greedy and ungreedy repetition quantifiers are specified
541           in the pattern.
542    
543           Because  it  ends  up  with a single path through the tree, it is rela-
544           tively straightforward for this algorithm to keep  track  of  the  sub-
545           strings  that  are  matched  by portions of the pattern in parentheses.
546           This provides support for capturing parentheses and back references.
547    
548    
549    THE ALTERNATIVE MATCHING ALGORITHM
550    
551           This algorithm conducts a breadth-first search of  the  tree.  Starting
552           from  the  first  matching  point  in the subject, it scans the subject
553           string from left to right, once, character by character, and as it does
554           this,  it remembers all the paths through the tree that represent valid
555           matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
556           though  it is not implemented as a traditional finite state machine (it
557           keeps multiple states active simultaneously).
558    
559           The scan continues until either the end of the subject is  reached,  or
560           there  are  no more unterminated paths. At this point, terminated paths
561           represent the different matching possibilities (if there are none,  the
562           match  has  failed).   Thus,  if there is more than one possible match,
563           this algorithm finds all of them, and in particular, it finds the long-
564           est.  In PCRE, there is an option to stop the algorithm after the first
565           match (which is necessarily the shortest) has been found.
566    
567           Note that all the matches that are found start at the same point in the
568           subject. If the pattern
569    
570             cat(er(pillar)?)
571    
572           is  matched  against the string "the caterpillar catchment", the result
573           will be the three strings "cat", "cater", and "caterpillar" that  start
574           at the fourth character of the subject. The algorithm does not automat-
575           ically move on to find matches that start at later positions.
576    
577           There are a number of features of PCRE regular expressions that are not
578           supported by the alternative matching algorithm. They are as follows:
579    
580           1.  Because  the  algorithm  finds  all possible matches, the greedy or
581           ungreedy nature of repetition quantifiers is not relevant.  Greedy  and
582           ungreedy quantifiers are treated in exactly the same way. However, pos-
583           sessive quantifiers can make a difference when what follows could  also
584           match what is quantified, for example in a pattern like this:
585    
586             ^a++\w!
587    
588           This  pattern matches "aaab!" but not "aaa!", which would be matched by
589           a non-possessive quantifier. Similarly, if an atomic group is  present,
590           it  is matched as if it were a standalone pattern at the current point,
591           and the longest match is then "locked in" for the rest of  the  overall
592           pattern.
593    
594           2. When dealing with multiple paths through the tree simultaneously, it
595           is not straightforward to keep track of  captured  substrings  for  the
596           different  matching  possibilities,  and  PCRE's implementation of this
597           algorithm does not attempt to do this. This means that no captured sub-
598           strings are available.
599    
600           3.  Because no substrings are captured, back references within the pat-
601           tern are not supported, and cause errors if encountered.
602    
603           4. For the same reason, conditional expressions that use  a  backrefer-
604           ence  as  the  condition or test for a specific group recursion are not
605           supported.
606    
607           5. Callouts are supported, but the value of the  capture_top  field  is
608           always 1, and the value of the capture_last field is always -1.
609    
610           6.  The \C escape sequence, which (in the standard algorithm) matches a
611           single byte, even in UTF-8 mode, is not supported because the  alterna-
612           tive  algorithm  moves  through  the  subject string one character at a
613           time, for all active paths through the tree.
614    
615    
616    ADVANTAGES OF THE ALTERNATIVE ALGORITHM
617    
618           Using the alternative matching algorithm provides the following  advan-
619           tages:
620    
621           1. All possible matches (at a single point in the subject) are automat-
622           ically found, and in particular, the longest match is  found.  To  find
623           more than one match using the standard algorithm, you have to do kludgy
624           things with callouts.
625    
626  MULTI-THREADING         2. There is much better support for partial matching. The  restrictions
627       The PCRE functions can be used in  multi-threading  applica-         on  the content of the pattern that apply when using the standard algo-
628       tions, with the proviso that the memory management functions         rithm for partial matching do not apply to the  alternative  algorithm.
629       pointed to by pcre_malloc and pcre_free are  shared  by  all         For  non-anchored patterns, the starting position of a partial match is
630       threads.         available.
   
      The compiled form of a regular  expression  is  not  altered  
      during  matching, so the same compiled pattern can safely be  
      used by several threads at once.  
631    
632           3. Because the alternative algorithm  scans  the  subject  string  just
633           once,  and  never  needs to backtrack, it is possible to pass very long
634           subject strings to the matching function in  several  pieces,  checking
635           for partial matching each time.
636    
637    
638    DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
639    
640           The alternative algorithm suffers from a number of disadvantages:
641    
642           1.  It  is  substantially  slower  than the standard algorithm. This is
643           partly because it has to search for all possible matches, but  is  also
644           because it is less susceptible to optimization.
645    
646           2. Capturing parentheses and back references are not supported.
647    
648           3. Although atomic groups are supported, their use does not provide the
649           performance advantage that it does for the standard algorithm.
650    
651    
652    AUTHOR
653    
654           Philip Hazel
655           University Computing Service
656           Cambridge CB2 3QH, England.
657    
658    
659    REVISION
660    
661           Last updated: 06 March 2007
662           Copyright (c) 1997-2007 University of Cambridge.
663    ------------------------------------------------------------------------------
664    
665    
666    PCREAPI(3)                                                          PCREAPI(3)
667    
668    
669    NAME
670           PCRE - Perl-compatible regular expressions
671    
672    
673    PCRE NATIVE API
674    
675           #include <pcre.h>
676    
677           pcre *pcre_compile(const char *pattern, int options,
678                const char **errptr, int *erroffset,
679                const unsigned char *tableptr);
680    
681           pcre *pcre_compile2(const char *pattern, int options,
682                int *errorcodeptr,
683                const char **errptr, int *erroffset,
684                const unsigned char *tableptr);
685    
686           pcre_extra *pcre_study(const pcre *code, int options,
687                const char **errptr);
688    
689           int pcre_exec(const pcre *code, const pcre_extra *extra,
690                const char *subject, int length, int startoffset,
691                int options, int *ovector, int ovecsize);
692    
693           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
694                const char *subject, int length, int startoffset,
695                int options, int *ovector, int ovecsize,
696                int *workspace, int wscount);
697    
698           int pcre_copy_named_substring(const pcre *code,
699                const char *subject, int *ovector,
700                int stringcount, const char *stringname,
701                char *buffer, int buffersize);
702    
703           int pcre_copy_substring(const char *subject, int *ovector,
704                int stringcount, int stringnumber, char *buffer,
705                int buffersize);
706    
707           int pcre_get_named_substring(const pcre *code,
708                const char *subject, int *ovector,
709                int stringcount, const char *stringname,
710                const char **stringptr);
711    
712           int pcre_get_stringnumber(const pcre *code,
713                const char *name);
714    
715           int pcre_get_stringtable_entries(const pcre *code,
716                const char *name, char **first, char **last);
717    
718           int pcre_get_substring(const char *subject, int *ovector,
719                int stringcount, int stringnumber,
720                const char **stringptr);
721    
722           int pcre_get_substring_list(const char *subject,
723                int *ovector, int stringcount, const char ***listptr);
724    
725           void pcre_free_substring(const char *stringptr);
726    
727           void pcre_free_substring_list(const char **stringptr);
728    
729           const unsigned char *pcre_maketables(void);
730    
731           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
732                int what, void *where);
733    
734           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
735    
736           int pcre_refcount(pcre *code, int adjust);
737    
738           int pcre_config(int what, void *where);
739    
740           char *pcre_version(void);
741    
742           void *(*pcre_malloc)(size_t);
743    
744           void (*pcre_free)(void *);
745    
746           void *(*pcre_stack_malloc)(size_t);
747    
748           void (*pcre_stack_free)(void *);
749    
750           int (*pcre_callout)(pcre_callout_block *);
751    
752    
753    PCRE API OVERVIEW
754    
755           PCRE has its own native API, which is described in this document. There
756           are also some wrapper functions that correspond to  the  POSIX  regular
757           expression  API.  These  are  described in the pcreposix documentation.
758           Both of these APIs define a set of C function calls. A C++  wrapper  is
759           distributed with PCRE. It is documented in the pcrecpp page.
760    
761           The  native  API  C  function prototypes are defined in the header file
762           pcre.h, and on Unix systems the library itself is called  libpcre.   It
763           can normally be accessed by adding -lpcre to the command for linking an
764           application  that  uses  PCRE.  The  header  file  defines  the  macros
765           PCRE_MAJOR  and  PCRE_MINOR to contain the major and minor release num-
766           bers for the library.  Applications can use these  to  include  support
767           for different releases of PCRE.
768    
769           The   functions   pcre_compile(),  pcre_compile2(),  pcre_study(),  and
770           pcre_exec() are used for compiling and matching regular expressions  in
771           a  Perl-compatible  manner. A sample program that demonstrates the sim-
772           plest way of using them is provided in the file  called  pcredemo.c  in
773           the  source distribution. The pcresample documentation describes how to
774           run it.
775    
776           A second matching function, pcre_dfa_exec(), which is not Perl-compati-
777           ble,  is  also provided. This uses a different algorithm for the match-
778           ing. The alternative algorithm finds all possible matches (at  a  given
779           point  in  the subject), and scans the subject just once. However, this
780           algorithm does not return captured substrings. A description of the two
781           matching  algorithms and their advantages and disadvantages is given in
782           the pcrematching documentation.
783    
784           In addition to the main compiling and  matching  functions,  there  are
785           convenience functions for extracting captured substrings from a subject
786           string that is matched by pcre_exec(). They are:
787    
788             pcre_copy_substring()
789             pcre_copy_named_substring()
790             pcre_get_substring()
791             pcre_get_named_substring()
792             pcre_get_substring_list()
793             pcre_get_stringnumber()
794             pcre_get_stringtable_entries()
795    
796           pcre_free_substring() and pcre_free_substring_list() are also provided,
797           to free the memory used for extracted strings.
798    
799           The  function  pcre_maketables()  is  used  to build a set of character
800           tables  in  the  current  locale   for   passing   to   pcre_compile(),
801           pcre_exec(),  or  pcre_dfa_exec(). This is an optional facility that is
802           provided for specialist use.  Most  commonly,  no  special  tables  are
803           passed,  in  which case internal tables that are generated when PCRE is
804           built are used.
805    
806           The function pcre_fullinfo() is used to find out  information  about  a
807           compiled  pattern; pcre_info() is an obsolete version that returns only
808           some of the available information, but is retained for  backwards  com-
809           patibility.   The function pcre_version() returns a pointer to a string
810           containing the version of PCRE and its date of release.
811    
812           The function pcre_refcount() maintains a  reference  count  in  a  data
813           block  containing  a compiled pattern. This is provided for the benefit
814           of object-oriented applications.
815    
816           The global variables pcre_malloc and pcre_free  initially  contain  the
817           entry  points  of  the  standard malloc() and free() functions, respec-
818           tively. PCRE calls the memory management functions via these variables,
819           so  a  calling  program  can replace them if it wishes to intercept the
820           calls. This should be done before calling any PCRE functions.
821    
822           The global variables pcre_stack_malloc  and  pcre_stack_free  are  also
823           indirections  to  memory  management functions. These special functions
824           are used only when PCRE is compiled to use  the  heap  for  remembering
825           data, instead of recursive function calls, when running the pcre_exec()
826           function. See the pcrebuild documentation for  details  of  how  to  do
827           this.  It  is  a non-standard way of building PCRE, for use in environ-
828           ments that have limited stacks. Because of the greater  use  of  memory
829           management,  it  runs  more  slowly. Separate functions are provided so
830           that special-purpose external code can be  used  for  this  case.  When
831           used,  these  functions  are always called in a stack-like manner (last
832           obtained, first freed), and always for memory blocks of the same  size.
833           There  is  a discussion about PCRE's stack usage in the pcrestack docu-
834           mentation.
835    
836           The global variable pcre_callout initially contains NULL. It can be set
837           by  the  caller  to  a "callout" function, which PCRE will then call at
838           specified points during a matching operation. Details are given in  the
839           pcrecallout documentation.
840    
841    
842    NEWLINES
843    
844           PCRE  supports four different conventions for indicating line breaks in
845           strings: a single CR (carriage return) character, a  single  LF  (line-
846           feed)  character,  the two-character sequence CRLF, or any Unicode new-
847           line sequence.  The Unicode newline sequences are the three  just  men-
848           tioned, plus the single characters VT (vertical tab, U+000B), FF (form-
849           feed, U+000C), NEL (next line, U+0085), LS  (line  separator,  U+2028),
850           and PS (paragraph separator, U+2029).
851    
852           Each  of  the first three conventions is used by at least one operating
853           system as its standard newline sequence. When PCRE is built, a  default
854           can  be  specified.  The default default is LF, which is the Unix stan-
855           dard. When PCRE is run, the default can be overridden,  either  when  a
856           pattern is compiled, or when it is matched.
857    
858           In the PCRE documentation the word "newline" is used to mean "the char-
859           acter or pair of characters that indicate a line break". The choice  of
860           newline  convention  affects  the  handling of the dot, circumflex, and
861           dollar metacharacters, the handling of #-comments in /x mode, and, when
862           CRLF  is a recognized line ending sequence, the match position advance-
863           ment for a non-anchored pattern. The choice of newline convention  does
864           not affect the interpretation of the \n or \r escape sequences.
865    
866    
867    MULTITHREADING
868    
869           The  PCRE  functions  can be used in multi-threading applications, with
870           the  proviso  that  the  memory  management  functions  pointed  to  by
871           pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
872           callout function pointed to by pcre_callout, are shared by all threads.
873    
874           The  compiled form of a regular expression is not altered during match-
875           ing, so the same compiled pattern can safely be used by several threads
876           at once.
877    
878    
879    SAVING PRECOMPILED PATTERNS FOR LATER USE
880    
881           The compiled form of a regular expression can be saved and re-used at a
882           later time, possibly by a different program, and even on a  host  other
883           than  the  one  on  which  it  was  compiled.  Details are given in the
884           pcreprecompile documentation.
885    
886    
887    CHECKING BUILD-TIME OPTIONS
888    
889           int pcre_config(int what, void *where);
890    
891           The function pcre_config() makes it possible for a PCRE client to  dis-
892           cover which optional features have been compiled into the PCRE library.
893           The pcrebuild documentation has more details about these optional  fea-
894           tures.
895    
896           The  first  argument  for pcre_config() is an integer, specifying which
897           information is required; the second argument is a pointer to a variable
898           into  which  the  information  is  placed. The following information is
899           available:
900    
901             PCRE_CONFIG_UTF8
902    
903           The output is an integer that is set to one if UTF-8 support is  avail-
904           able; otherwise it is set to zero.
905    
906             PCRE_CONFIG_UNICODE_PROPERTIES
907    
908           The  output  is  an  integer  that is set to one if support for Unicode
909           character properties is available; otherwise it is set to zero.
910    
911             PCRE_CONFIG_NEWLINE
912    
913           The output is an integer whose value specifies  the  default  character
914           sequence  that is recognized as meaning "newline". The four values that
915           are supported are: 10 for LF, 13 for CR, 3338 for CRLF, and -1 for ANY.
916           The default should normally be the standard sequence for your operating
917           system.
918    
919             PCRE_CONFIG_LINK_SIZE
920    
921           The output is an integer that contains the number  of  bytes  used  for
922           internal linkage in compiled regular expressions. The value is 2, 3, or
923           4. Larger values allow larger regular expressions to  be  compiled,  at
924           the  expense  of  slower matching. The default value of 2 is sufficient
925           for all but the most massive patterns, since  it  allows  the  compiled
926           pattern to be up to 64K in size.
927    
928             PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
929    
930           The  output  is  an integer that contains the threshold above which the
931           POSIX interface uses malloc() for output vectors. Further  details  are
932           given in the pcreposix documentation.
933    
934             PCRE_CONFIG_MATCH_LIMIT
935    
936           The output is an integer that gives the default limit for the number of
937           internal matching function calls in a  pcre_exec()  execution.  Further
938           details are given with pcre_exec() below.
939    
940             PCRE_CONFIG_MATCH_LIMIT_RECURSION
941    
942           The  output is an integer that gives the default limit for the depth of
943           recursion when calling the internal matching function in a  pcre_exec()
944           execution. Further details are given with pcre_exec() below.
945    
946             PCRE_CONFIG_STACKRECURSE
947    
948           The  output is an integer that is set to one if internal recursion when
949           running pcre_exec() is implemented by recursive function calls that use
950           the  stack  to remember their state. This is the usual way that PCRE is
951           compiled. The output is zero if PCRE was compiled to use blocks of data
952           on  the  heap  instead  of  recursive  function  calls.  In  this case,
953           pcre_stack_malloc and  pcre_stack_free  are  called  to  manage  memory
954           blocks on the heap, thus avoiding the use of the stack.
955    
956    
957  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.  
958    
959           pcre *pcre_compile(const char *pattern, int options,
960                const char **errptr, int *erroffset,
961                const unsigned char *tableptr);
962    
963           pcre *pcre_compile2(const char *pattern, int options,
964                int *errorcodeptr,
965                const char **errptr, int *erroffset,
966                const unsigned char *tableptr);
967    
968           Either of the functions pcre_compile() or pcre_compile2() can be called
969           to compile a pattern into an internal form. The only difference between
970           the  two interfaces is that pcre_compile2() has an additional argument,
971           errorcodeptr, via which a numerical error code can be returned.
972    
973           The pattern is a C string terminated by a binary zero, and is passed in
974           the  pattern  argument.  A  pointer to a single block of memory that is
975           obtained via pcre_malloc is returned. This contains the  compiled  code
976           and related data. The pcre type is defined for the returned block; this
977           is a typedef for a structure whose contents are not externally defined.
978           It is up to the caller to free the memory (via pcre_free) when it is no
979           longer required.
980    
981           Although the compiled code of a PCRE regex is relocatable, that is,  it
982           does not depend on memory location, the complete pcre data block is not
983           fully relocatable, because it may contain a copy of the tableptr  argu-
984           ment, which is an address (see below).
985    
986           The options argument contains various bit settings that affect the com-
987           pilation. It should be zero if no options are required.  The  available
988           options  are  described  below. Some of them, in particular, those that
989           are compatible with Perl, can also be set and  unset  from  within  the
990           pattern  (see  the  detailed  description in the pcrepattern documenta-
991           tion). For these options, the contents of the options  argument  speci-
992           fies  their initial settings at the start of compilation and execution.
993           The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at  the  time
994           of matching as well as at compile time.
995    
996           If errptr is NULL, pcre_compile() returns NULL immediately.  Otherwise,
997           if compilation of a pattern fails,  pcre_compile()  returns  NULL,  and
998           sets the variable pointed to by errptr to point to a textual error mes-
999           sage. This is a static string that is part of the library. You must not
1000           try to free it. The offset from the start of the pattern to the charac-
1001           ter where the error was discovered is placed in the variable pointed to
1002           by  erroffset,  which must not be NULL. If it is, an immediate error is
1003           given.
1004    
1005           If pcre_compile2() is used instead of pcre_compile(),  and  the  error-
1006           codeptr  argument is not NULL, a non-zero error code number is returned
1007           via this argument in the event of an error. This is in addition to  the
1008           textual error message. Error codes and messages are listed below.
1009    
1010           If  the  final  argument, tableptr, is NULL, PCRE uses a default set of
1011           character tables that are  built  when  PCRE  is  compiled,  using  the
1012           default  C  locale.  Otherwise, tableptr must be an address that is the
1013           result of a call to pcre_maketables(). This value is  stored  with  the
1014           compiled  pattern,  and used again by pcre_exec(), unless another table
1015           pointer is passed to it. For more discussion, see the section on locale
1016           support below.
1017    
1018           This  code  fragment  shows a typical straightforward call to pcre_com-
1019           pile():
1020    
1021             pcre *re;
1022             const char *error;
1023             int erroffset;
1024             re = pcre_compile(
1025               "^A.*Z",          /* the pattern */
1026               0,                /* default options */
1027               &error,           /* for error message */
1028               &erroffset,       /* for error offset */
1029               NULL);            /* use default character tables */
1030    
1031           The following names for option bits are defined in  the  pcre.h  header
1032           file:
1033    
1034             PCRE_ANCHORED
1035    
1036           If this bit is set, the pattern is forced to be "anchored", that is, it
1037           is constrained to match only at the first matching point in the  string
1038           that  is being searched (the "subject string"). This effect can also be
1039           achieved by appropriate constructs in the pattern itself, which is  the
1040           only way to do it in Perl.
1041    
1042             PCRE_AUTO_CALLOUT
1043    
1044           If this bit is set, pcre_compile() automatically inserts callout items,
1045           all with number 255, before each pattern item. For  discussion  of  the
1046           callout facility, see the pcrecallout documentation.
1047    
1048             PCRE_CASELESS
1049    
1050           If  this  bit is set, letters in the pattern match both upper and lower
1051           case letters. It is equivalent to Perl's  /i  option,  and  it  can  be
1052           changed  within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1053           always understands the concept of case for characters whose values  are
1054           less  than 128, so caseless matching is always possible. For characters
1055           with higher values, the concept of case is supported if  PCRE  is  com-
1056           piled  with Unicode property support, but not otherwise. If you want to
1057           use caseless matching for characters 128 and  above,  you  must  ensure
1058           that  PCRE  is  compiled  with Unicode property support as well as with
1059           UTF-8 support.
1060    
1061             PCRE_DOLLAR_ENDONLY
1062    
1063           If this bit is set, a dollar metacharacter in the pattern matches  only
1064           at  the  end  of the subject string. Without this option, a dollar also
1065           matches immediately before a newline at the end of the string (but  not
1066           before  any  other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1067           if PCRE_MULTILINE is set.  There is no equivalent  to  this  option  in
1068           Perl, and no way to set it within a pattern.
1069    
1070             PCRE_DOTALL
1071    
1072           If this bit is set, a dot metacharater in the pattern matches all char-
1073           acters, including those that indicate newline. Without it, a  dot  does
1074           not  match  when  the  current position is at a newline. This option is
1075           equivalent to Perl's /s option, and it can be changed within a  pattern
1076           by  a (?s) option setting. A negative class such as [^a] always matches
1077           newline characters, independent of the setting of this option.
1078    
1079             PCRE_DUPNAMES
1080    
1081           If this bit is set, names used to identify capturing  subpatterns  need
1082           not be unique. This can be helpful for certain types of pattern when it
1083           is known that only one instance of the named  subpattern  can  ever  be
1084           matched.  There  are  more details of named subpatterns below; see also
1085           the pcrepattern documentation.
1086    
1087             PCRE_EXTENDED
1088    
1089           If this bit is set, whitespace  data  characters  in  the  pattern  are
1090           totally ignored except when escaped or inside a character class. White-
1091           space does not include the VT character (code 11). In addition, charac-
1092           ters between an unescaped # outside a character class and the next new-
1093           line, inclusive, are also ignored. This  is  equivalent  to  Perl's  /x
1094           option,  and  it  can be changed within a pattern by a (?x) option set-
1095           ting.
1096    
1097           This option makes it possible to include  comments  inside  complicated
1098           patterns.   Note,  however,  that this applies only to data characters.
1099           Whitespace  characters  may  never  appear  within  special   character
1100           sequences  in  a  pattern,  for  example  within the sequence (?( which
1101           introduces a conditional subpattern.
1102    
1103             PCRE_EXTRA
1104    
1105           This option was invented in order to turn on  additional  functionality
1106           of  PCRE  that  is  incompatible with Perl, but it is currently of very
1107           little use. When set, any backslash in a pattern that is followed by  a
1108           letter  that  has  no  special  meaning causes an error, thus reserving
1109           these combinations for future expansion. By  default,  as  in  Perl,  a
1110           backslash  followed by a letter with no special meaning is treated as a
1111           literal. (Perl can, however, be persuaded to give a warning for  this.)
1112           There  are  at  present no other features controlled by this option. It
1113           can also be set by a (?X) option setting within a pattern.
1114    
1115             PCRE_FIRSTLINE
1116    
1117           If this option is set, an  unanchored  pattern  is  required  to  match
1118           before  or  at  the  first  newline  in  the subject string, though the
1119           matched text may continue over the newline.
1120    
1121             PCRE_MULTILINE
1122    
1123           By default, PCRE treats the subject string as consisting  of  a  single
1124           line  of characters (even if it actually contains newlines). The "start
1125           of line" metacharacter (^) matches only at the  start  of  the  string,
1126           while  the  "end  of line" metacharacter ($) matches only at the end of
1127           the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1128           is set). This is the same as Perl.
1129    
1130           When  PCRE_MULTILINE  it  is set, the "start of line" and "end of line"
1131           constructs match immediately following or immediately  before  internal
1132           newlines  in  the  subject string, respectively, as well as at the very
1133           start and end. This is equivalent to Perl's /m option, and  it  can  be
1134           changed within a pattern by a (?m) option setting. If there are no new-
1135           lines in a subject string, or no occurrences of ^ or $  in  a  pattern,
1136           setting PCRE_MULTILINE has no effect.
1137    
1138             PCRE_NEWLINE_CR
1139             PCRE_NEWLINE_LF
1140             PCRE_NEWLINE_CRLF
1141             PCRE_NEWLINE_ANY
1142    
1143           These  options  override the default newline definition that was chosen
1144           when PCRE was built. Setting the first or the second specifies  that  a
1145           newline  is  indicated  by a single character (CR or LF, respectively).
1146           Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by  the
1147           two-character  CRLF  sequence.  Setting PCRE_NEWLINE_ANY specifies that
1148           any Unicode newline sequence should be recognized. The Unicode  newline
1149           sequences  are  the three just mentioned, plus the single characters VT
1150           (vertical tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085),
1151           LS  (line separator, U+2028), and PS (paragraph separator, U+2029). The
1152           last two are recognized only in UTF-8 mode.
1153    
1154           The newline setting in the  options  word  uses  three  bits  that  are
1155           treated  as  a  number, giving eight possibilities. Currently only five
1156           are used (default plus the four values above). This means that  if  you
1157           set  more  than  one  newline option, the combination may or may not be
1158           sensible. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is  equiva-
1159           lent  to PCRE_NEWLINE_CRLF, but other combinations yield unused numbers
1160           and cause an error.
1161    
1162           The only time that a line break is specially recognized when  compiling
1163           a  pattern  is  if  PCRE_EXTENDED  is set, and an unescaped # outside a
1164           character class is encountered. This indicates  a  comment  that  lasts
1165           until  after the next line break sequence. In other circumstances, line
1166           break  sequences  are  treated  as  literal  data,   except   that   in
1167           PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1168           and are therefore ignored.
1169    
1170           The newline option that is set at compile time becomes the default that
1171           is  used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1172    
1173             PCRE_NO_AUTO_CAPTURE
1174    
1175           If this option is set, it disables the use of numbered capturing paren-
1176           theses  in the pattern. Any opening parenthesis that is not followed by
1177           ? behaves as if it were followed by ?: but named parentheses can  still
1178           be  used  for  capturing  (and  they acquire numbers in the usual way).
1179           There is no equivalent of this option in Perl.
1180    
1181             PCRE_UNGREEDY
1182    
1183           This option inverts the "greediness" of the quantifiers  so  that  they
1184           are  not greedy by default, but become greedy if followed by "?". It is
1185           not compatible with Perl. It can also be set by a (?U)  option  setting
1186           within the pattern.
1187    
1188             PCRE_UTF8
1189    
1190           This  option  causes PCRE to regard both the pattern and the subject as
1191           strings of UTF-8 characters instead of single-byte  character  strings.
1192           However,  it is available only when PCRE is built to include UTF-8 sup-
1193           port. If not, the use of this option provokes an error. Details of  how
1194           this  option  changes the behaviour of PCRE are given in the section on
1195           UTF-8 support in the main pcre page.
1196    
1197             PCRE_NO_UTF8_CHECK
1198    
1199           When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1200           automatically  checked. If an invalid UTF-8 sequence of bytes is found,
1201           pcre_compile() returns an error. If you already know that your  pattern
1202           is  valid, and you want to skip this check for performance reasons, you
1203           can set the PCRE_NO_UTF8_CHECK option. When it is set,  the  effect  of
1204           passing an invalid UTF-8 string as a pattern is undefined. It may cause
1205           your program to crash.  Note that this option can  also  be  passed  to
1206           pcre_exec()  and pcre_dfa_exec(), to suppress the UTF-8 validity check-
1207           ing of subject strings.
1208    
1209    
1210    COMPILATION ERROR CODES
1211    
1212           The following table lists the error  codes  than  may  be  returned  by
1213           pcre_compile2(),  along with the error messages that may be returned by
1214           both compiling functions. As PCRE has developed, some error codes  have
1215           fallen out of use. To avoid confusion, they have not been re-used.
1216    
1217              0  no error
1218              1  \ at end of pattern
1219              2  \c at end of pattern
1220              3  unrecognized character follows \
1221              4  numbers out of order in {} quantifier
1222              5  number too big in {} quantifier
1223              6  missing terminating ] for character class
1224              7  invalid escape sequence in character class
1225              8  range out of order in character class
1226              9  nothing to repeat
1227             10  [this code is not in use]
1228             11  internal error: unexpected repeat
1229             12  unrecognized character after (?
1230             13  POSIX named classes are supported only within a class
1231             14  missing )
1232             15  reference to non-existent subpattern
1233             16  erroffset passed as NULL
1234             17  unknown option bit(s) set
1235             18  missing ) after comment
1236             19  [this code is not in use]
1237             20  regular expression too large
1238             21  failed to get memory
1239             22  unmatched parentheses
1240             23  internal error: code overflow
1241             24  unrecognized character after (?<
1242             25  lookbehind assertion is not fixed length
1243             26  malformed number or name after (?(
1244             27  conditional group contains more than two branches
1245             28  assertion expected after (?(
1246             29  (?R or (?digits must be followed by )
1247             30  unknown POSIX class name
1248             31  POSIX collating elements are not supported
1249             32  this version of PCRE is not compiled with PCRE_UTF8 support
1250             33  [this code is not in use]
1251             34  character value in \x{...} sequence is too large
1252             35  invalid condition (?(0)
1253             36  \C not allowed in lookbehind assertion
1254             37  PCRE does not support \L, \l, \N, \U, or \u
1255             38  number after (?C is > 255
1256             39  closing ) for (?C expected
1257             40  recursive call could loop indefinitely
1258             41  unrecognized character after (?P
1259             42  syntax error in subpattern name (missing terminator)
1260             43  two named subpatterns have the same name
1261             44  invalid UTF-8 string
1262             45  support for \P, \p, and \X has not been compiled
1263             46  malformed \P or \p sequence
1264             47  unknown property name after \P or \p
1265             48  subpattern name is too long (maximum 32 characters)
1266             49  too many named subpatterns (maximum 10,000)
1267             50  repeated subpattern is too long
1268             51  octal value is greater than \377 (not in UTF-8 mode)
1269             52  internal error: overran compiling workspace
1270             53   internal  error:  previously-checked  referenced  subpattern not
1271           found
1272             54  DEFINE group contains more than one branch
1273             55  repeating a DEFINE group is not allowed
1274             56  inconsistent NEWLINE options"
1275    
1276    
1277  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.  
1278    
1279           pcre_extra *pcre_study(const pcre *code, int options
1280                const char **errptr);
1281    
1282           If a compiled pattern is going to be used several times,  it  is  worth
1283           spending more time analyzing it in order to speed up the time taken for
1284           matching. The function pcre_study() takes a pointer to a compiled  pat-
1285           tern as its first argument. If studying the pattern produces additional
1286           information that will help speed up matching,  pcre_study()  returns  a
1287           pointer  to a pcre_extra block, in which the study_data field points to
1288           the results of the study.
1289    
1290           The  returned  value  from  pcre_study()  can  be  passed  directly  to
1291           pcre_exec().  However,  a  pcre_extra  block also contains other fields
1292           that can be set by the caller before the block  is  passed;  these  are
1293           described below in the section on matching a pattern.
1294    
1295           If  studying  the  pattern  does not produce any additional information
1296           pcre_study() returns NULL. In that circumstance, if the calling program
1297           wants  to  pass  any of the other fields to pcre_exec(), it must set up
1298           its own pcre_extra block.
1299    
1300           The second argument of pcre_study() contains option bits.  At  present,
1301           no options are defined, and this argument should always be zero.
1302    
1303           The  third argument for pcre_study() is a pointer for an error message.
1304           If studying succeeds (even if no data is  returned),  the  variable  it
1305           points  to  is  set  to NULL. Otherwise it is set to point to a textual
1306           error message. This is a static string that is part of the library. You
1307           must  not  try  to  free it. You should test the error pointer for NULL
1308           after calling pcre_study(), to be sure that it has run successfully.
1309    
1310           This is a typical call to pcre_study():
1311    
1312             pcre_extra *pe;
1313             pe = pcre_study(
1314               re,             /* result of pcre_compile() */
1315               0,              /* no options exist */
1316               &error);        /* set to NULL or points to a message */
1317    
1318           At present, studying a pattern is useful only for non-anchored patterns
1319           that  do not have a single fixed starting character. A bitmap of possi-
1320           ble starting bytes is created.
1321    
1322    
1323  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.  
1324    
1325           PCRE handles caseless matching, and determines whether  characters  are
1326           letters  digits,  or whatever, by reference to a set of tables, indexed
1327           by character value. When running in UTF-8 mode, this  applies  only  to
1328           characters  with  codes  less than 128. Higher-valued codes never match
1329           escapes such as \w or \d, but can be tested with \p if  PCRE  is  built
1330           with  Unicode  character property support. The use of locales with Uni-
1331           code is discouraged.
1332    
1333           An internal set of tables is created in the default C locale when  PCRE
1334           is  built.  This  is  used when the final argument of pcre_compile() is
1335           NULL, and is sufficient for many applications. An  alternative  set  of
1336           tables  can,  however, be supplied. These may be created in a different
1337           locale from the default. As more and more applications change to  using
1338           Unicode, the need for this locale support is expected to die away.
1339    
1340           External  tables  are  built by calling the pcre_maketables() function,
1341           which has no arguments, in the relevant locale. The result can then  be
1342           passed  to  pcre_compile()  or  pcre_exec()  as often as necessary. For
1343           example, to build and use tables that are appropriate  for  the  French
1344           locale  (where  accented  characters  with  values greater than 128 are
1345           treated as letters), the following code could be used:
1346    
1347             setlocale(LC_CTYPE, "fr_FR");
1348             tables = pcre_maketables();
1349             re = pcre_compile(..., tables);
1350    
1351           When pcre_maketables() runs, the tables are built  in  memory  that  is
1352           obtained  via  pcre_malloc. It is the caller's responsibility to ensure
1353           that the memory containing the tables remains available for as long  as
1354           it is needed.
1355    
1356           The pointer that is passed to pcre_compile() is saved with the compiled
1357           pattern, and the same tables are used via this pointer by  pcre_study()
1358           and normally also by pcre_exec(). Thus, by default, for any single pat-
1359           tern, compilation, studying and matching all happen in the same locale,
1360           but different patterns can be compiled in different locales.
1361    
1362           It  is  possible to pass a table pointer or NULL (indicating the use of
1363           the internal tables) to pcre_exec(). Although  not  intended  for  this
1364           purpose,  this facility could be used to match a pattern in a different
1365           locale from the one in which it was compiled. Passing table pointers at
1366           run time is discussed below in the section on matching a pattern.
1367    
1368    
1369  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  pat-  
      tern.  The  fourth argument should point to an int variable.  
      Zero is returned if there are no back references.  
   
        PCRE_INFO_FIRSTCHAR  
   
      Return information about the first character of any  matched  
      string,  for  a  non-anchored  pattern.  If there is a fixed  
      first   character,   e.g.   from   a   pattern    such    as  
      (cat|cow|coyote),  it  is returned in the integer pointed to  
      by where. Otherwise, if either  
   
      (a) the pattern was compiled with the PCRE_MULTILINE option,  
      and every branch starts with "^", or  
   
      (b) every  branch  of  the  pattern  starts  with  ".*"  and  
      PCRE_DOTALL is not set (if it were set, the pattern would be  
      anchored),  
   
      -1 is returned, indicating that the pattern matches only  at  
      the  start  of a subject string or after any "\n" within the  
      string. Otherwise -2 is returned.  For anchored patterns, -2  
      is returned.  
   
        PCRE_INFO_FIRSTTABLE  
   
      If the pattern was studied, and this resulted  in  the  con-  
      struction of a 256-bit table indicating a fixed set of char-  
      acters for the first character in  any  matching  string,  a  
      pointer   to  the  table  is  returned.  Otherwise  NULL  is  
      returned. The fourth argument should point  to  an  unsigned  
      char * variable.  
   
        PCRE_INFO_LASTLITERAL  
   
      For a non-anchored pattern, return the value of  the  right-  
      most  literal  character  which  must  exist  in any matched  
      string, other than at its start. The fourth argument  should  
      point  to an int variable. If there is no such character, or  
      if the pattern is anchored, -1 is returned. For example, for  
      the pattern /a\d+z\d+/ the returned value is 'z'.  
   
      The pcre_info() function is now obsolete because its  inter-  
      face  is  too  restrictive  to return all the available data  
      about  a  compiled  pattern.   New   programs   should   use  
      pcre_fullinfo()  instead.  The  yield  of pcre_info() is the  
      number of capturing subpatterns, or  one  of  the  following  
      negative numbers:  
   
        PCRE_ERROR_NULL       the argument code was NULL  
        PCRE_ERROR_BADMAGIC   the "magic number" was not found  
   
      If the optptr argument is not NULL, a copy  of  the  options  
      with which the pattern was compiled is placed in the integer  
      it points to (see PCRE_INFO_OPTIONS above).  
   
      If the pattern is not anchored and the firstcharptr argument  
      is  not  NULL, it is used to pass back information about the  
      first    character    of    any    matched    string    (see  
      PCRE_INFO_FIRSTCHAR above).  
1370    
1371           int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1372                int what, void *where);
1373    
1374           The  pcre_fullinfo() function returns information about a compiled pat-
1375           tern. It replaces the obsolete pcre_info() function, which is neverthe-
1376           less retained for backwards compability (and is documented below).
1377    
1378           The  first  argument  for  pcre_fullinfo() is a pointer to the compiled
1379           pattern. The second argument is the result of pcre_study(), or NULL  if
1380           the  pattern  was not studied. The third argument specifies which piece
1381           of information is required, and the fourth argument is a pointer  to  a
1382           variable  to  receive  the  data. The yield of the function is zero for
1383           success, or one of the following negative numbers:
1384    
1385             PCRE_ERROR_NULL       the argument code was NULL
1386                                   the argument where was NULL
1387             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1388             PCRE_ERROR_BADOPTION  the value of what was invalid
1389    
1390           The "magic number" is placed at the start of each compiled  pattern  as
1391           an  simple check against passing an arbitrary memory pointer. Here is a
1392           typical call of pcre_fullinfo(), to obtain the length of  the  compiled
1393           pattern:
1394    
1395             int rc;
1396             size_t length;
1397             rc = pcre_fullinfo(
1398               re,               /* result of pcre_compile() */
1399               pe,               /* result of pcre_study(), or NULL */
1400               PCRE_INFO_SIZE,   /* what is required */
1401               &length);         /* where to put the data */
1402    
1403           The  possible  values for the third argument are defined in pcre.h, and
1404           are as follows:
1405    
1406             PCRE_INFO_BACKREFMAX
1407    
1408           Return the number of the highest back reference  in  the  pattern.  The
1409           fourth  argument  should  point to an int variable. Zero is returned if
1410           there are no back references.
1411    
1412             PCRE_INFO_CAPTURECOUNT
1413    
1414           Return the number of capturing subpatterns in the pattern.  The  fourth
1415           argument should point to an int variable.
1416    
1417             PCRE_INFO_DEFAULT_TABLES
1418    
1419           Return  a pointer to the internal default character tables within PCRE.
1420           The fourth argument should point to an unsigned char *  variable.  This
1421           information call is provided for internal use by the pcre_study() func-
1422           tion. External callers can cause PCRE to use  its  internal  tables  by
1423           passing a NULL table pointer.
1424    
1425             PCRE_INFO_FIRSTBYTE
1426    
1427           Return  information  about  the first byte of any matched string, for a
1428           non-anchored pattern. The fourth argument should point to an int  vari-
1429           able.  (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1430           is still recognized for backwards compatibility.)
1431    
1432           If there is a fixed first byte, for example, from  a  pattern  such  as
1433           (cat|cow|coyote), its value is returned. Otherwise, if either
1434    
1435           (a)  the pattern was compiled with the PCRE_MULTILINE option, and every
1436           branch starts with "^", or
1437    
1438           (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1439           set (if it were set, the pattern would be anchored),
1440    
1441           -1  is  returned, indicating that the pattern matches only at the start
1442           of a subject string or after any newline within the  string.  Otherwise
1443           -2 is returned. For anchored patterns, -2 is returned.
1444    
1445             PCRE_INFO_FIRSTTABLE
1446    
1447           If  the pattern was studied, and this resulted in the construction of a
1448           256-bit table indicating a fixed set of bytes for the first byte in any
1449           matching  string, a pointer to the table is returned. Otherwise NULL is
1450           returned. The fourth argument should point to an unsigned char *  vari-
1451           able.
1452    
1453             PCRE_INFO_LASTLITERAL
1454    
1455           Return  the  value of the rightmost literal byte that must exist in any
1456           matched string, other than at its  start,  if  such  a  byte  has  been
1457           recorded. The fourth argument should point to an int variable. If there
1458           is no such byte, -1 is returned. For anchored patterns, a last  literal
1459           byte  is  recorded only if it follows something of variable length. For
1460           example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1461           /^a\dz\d/ the returned value is -1.
1462    
1463             PCRE_INFO_NAMECOUNT
1464             PCRE_INFO_NAMEENTRYSIZE
1465             PCRE_INFO_NAMETABLE
1466    
1467           PCRE  supports the use of named as well as numbered capturing parenthe-
1468           ses. The names are just an additional way of identifying the  parenthe-
1469           ses, which still acquire numbers. Several convenience functions such as
1470           pcre_get_named_substring() are provided for  extracting  captured  sub-
1471           strings  by  name. It is also possible to extract the data directly, by
1472           first converting the name to a number in order to  access  the  correct
1473           pointers in the output vector (described with pcre_exec() below). To do
1474           the conversion, you need  to  use  the  name-to-number  map,  which  is
1475           described by these three values.
1476    
1477           The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1478           gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1479           of  each  entry;  both  of  these  return  an int value. The entry size
1480           depends on the length of the longest name. PCRE_INFO_NAMETABLE  returns
1481           a  pointer  to  the  first  entry of the table (a pointer to char). The
1482           first two bytes of each entry are the number of the capturing parenthe-
1483           sis,  most  significant byte first. The rest of the entry is the corre-
1484           sponding name, zero terminated. The names are  in  alphabetical  order.
1485           When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1486           theses numbers. For example, consider  the  following  pattern  (assume
1487           PCRE_EXTENDED  is  set,  so  white  space  -  including  newlines  - is
1488           ignored):
1489    
1490             (?<date> (?<year>(\d\d)?\d\d) -
1491             (?<month>\d\d) - (?<day>\d\d) )
1492    
1493           There are four named subpatterns, so the table has  four  entries,  and
1494           each  entry  in the table is eight bytes long. The table is as follows,
1495           with non-printing bytes shows in hexadecimal, and undefined bytes shown
1496           as ??:
1497    
1498             00 01 d  a  t  e  00 ??
1499             00 05 d  a  y  00 ?? ??
1500             00 04 m  o  n  t  h  00
1501             00 02 y  e  a  r  00 ??
1502    
1503           When  writing  code  to  extract  data from named subpatterns using the
1504           name-to-number map, remember that the length of the entries  is  likely
1505           to be different for each compiled pattern.
1506    
1507             PCRE_INFO_OPTIONS
1508    
1509           Return  a  copy of the options with which the pattern was compiled. The
1510           fourth argument should point to an unsigned long  int  variable.  These
1511           option bits are those specified in the call to pcre_compile(), modified
1512           by any top-level option settings within the pattern itself.
1513    
1514           A pattern is automatically anchored by PCRE if  all  of  its  top-level
1515           alternatives begin with one of the following:
1516    
1517             ^     unless PCRE_MULTILINE is set
1518             \A    always
1519             \G    always
1520             .*    if PCRE_DOTALL is set and there are no back
1521                     references to the subpattern in which .* appears
1522    
1523           For such patterns, the PCRE_ANCHORED bit is set in the options returned
1524           by pcre_fullinfo().
1525    
1526             PCRE_INFO_SIZE
1527    
1528           Return the size of the compiled pattern, that is, the  value  that  was
1529           passed as the argument to pcre_malloc() when PCRE was getting memory in
1530           which to place the compiled data. The fourth argument should point to a
1531           size_t variable.
1532    
1533             PCRE_INFO_STUDYSIZE
1534    
1535           Return the size of the data block pointed to by the study_data field in
1536           a pcre_extra block. That is,  it  is  the  value  that  was  passed  to
1537           pcre_malloc() when PCRE was getting memory into which to place the data
1538           created by pcre_study(). The fourth argument should point to  a  size_t
1539           variable.
1540    
1541    
1542    OBSOLETE INFO FUNCTION
1543    
1544           int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1545    
1546           The  pcre_info()  function is now obsolete because its interface is too
1547           restrictive to return all the available data about a compiled  pattern.
1548           New   programs   should  use  pcre_fullinfo()  instead.  The  yield  of
1549           pcre_info() is the number of capturing subpatterns, or one of the  fol-
1550           lowing negative numbers:
1551    
1552             PCRE_ERROR_NULL       the argument code was NULL
1553             PCRE_ERROR_BADMAGIC   the "magic number" was not found
1554    
1555           If  the  optptr  argument is not NULL, a copy of the options with which
1556           the pattern was compiled is placed in the integer  it  points  to  (see
1557           PCRE_INFO_OPTIONS above).
1558    
1559           If  the  pattern  is  not anchored and the firstcharptr argument is not
1560           NULL, it is used to pass back information about the first character  of
1561           any matched string (see PCRE_INFO_FIRSTBYTE above).
1562    
1563    
1564    REFERENCE COUNTS
1565    
1566           int pcre_refcount(pcre *code, int adjust);
1567    
1568           The  pcre_refcount()  function is used to maintain a reference count in
1569           the data block that contains a compiled pattern. It is provided for the
1570           benefit  of  applications  that  operate  in an object-oriented manner,
1571           where different parts of the application may be using the same compiled
1572           pattern, but you want to free the block when they are all done.
1573    
1574           When a pattern is compiled, the reference count field is initialized to
1575           zero.  It is changed only by calling this function, whose action is  to
1576           add  the  adjust  value  (which may be positive or negative) to it. The
1577           yield of the function is the new value. However, the value of the count
1578           is  constrained to lie between 0 and 65535, inclusive. If the new value
1579           is outside these limits, it is forced to the appropriate limit value.
1580    
1581           Except when it is zero, the reference count is not correctly  preserved
1582           if  a  pattern  is  compiled on one host and then transferred to a host
1583           whose byte-order is different. (This seems a highly unlikely scenario.)
1584    
1585    
1586    MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1587    
1588           int pcre_exec(const pcre *code, const pcre_extra *extra,
1589                const char *subject, int length, int startoffset,
1590                int options, int *ovector, int ovecsize);
1591    
1592           The  function pcre_exec() is called to match a subject string against a
1593           compiled pattern, which is passed in the code argument. If the  pattern
1594           has been studied, the result of the study should be passed in the extra
1595           argument. This function is the main matching facility of  the  library,
1596           and it operates in a Perl-like manner. For specialist use there is also
1597           an alternative matching function, which is described below in the  sec-
1598           tion about the pcre_dfa_exec() function.
1599    
1600           In  most applications, the pattern will have been compiled (and option-
1601           ally studied) in the same process that calls pcre_exec().  However,  it
1602           is possible to save compiled patterns and study data, and then use them
1603           later in different processes, possibly even on different hosts.  For  a
1604           discussion about this, see the pcreprecompile documentation.
1605    
1606           Here is an example of a simple call to pcre_exec():
1607    
1608             int rc;
1609             int ovector[30];
1610             rc = pcre_exec(
1611               re,             /* result of pcre_compile() */
1612               NULL,           /* we didn't study the pattern */
1613               "some string",  /* the subject string */
1614               11,             /* the length of the subject string */
1615               0,              /* start at offset 0 in the subject */
1616               0,              /* default options */
1617               ovector,        /* vector of integers for substring information */
1618               30);            /* number of elements (NOT size in bytes) */
1619    
1620       Extra data for pcre_exec()
1621    
1622           If  the  extra argument is not NULL, it must point to a pcre_extra data
1623           block. The pcre_study() function returns such a block (when it  doesn't
1624           return  NULL), but you can also create one for yourself, and pass addi-
1625           tional information in it. The pcre_extra block contains  the  following
1626           fields (not necessarily in this order):
1627    
1628             unsigned long int flags;
1629             void *study_data;
1630             unsigned long int match_limit;
1631             unsigned long int match_limit_recursion;
1632             void *callout_data;
1633             const unsigned char *tables;
1634    
1635           The  flags  field  is a bitmap that specifies which of the other fields
1636           are set. The flag bits are:
1637    
1638             PCRE_EXTRA_STUDY_DATA
1639             PCRE_EXTRA_MATCH_LIMIT
1640             PCRE_EXTRA_MATCH_LIMIT_RECURSION
1641             PCRE_EXTRA_CALLOUT_DATA
1642             PCRE_EXTRA_TABLES
1643    
1644           Other flag bits should be set to zero. The study_data field is  set  in
1645           the  pcre_extra  block  that is returned by pcre_study(), together with
1646           the appropriate flag bit. You should not set this yourself, but you may
1647           add  to  the  block by setting the other fields and their corresponding
1648           flag bits.
1649    
1650           The match_limit field provides a means of preventing PCRE from using up
1651           a  vast amount of resources when running patterns that are not going to
1652           match, but which have a very large number  of  possibilities  in  their
1653           search  trees.  The  classic  example  is  the  use of nested unlimited
1654           repeats.
1655    
1656           Internally, PCRE uses a function called match() which it calls  repeat-
1657           edly  (sometimes  recursively). The limit set by match_limit is imposed
1658           on the number of times this function is called during  a  match,  which
1659           has  the  effect  of  limiting the amount of backtracking that can take
1660           place. For patterns that are not anchored, the count restarts from zero
1661           for each position in the subject string.
1662    
1663           The  default  value  for  the  limit can be set when PCRE is built; the
1664           default default is 10 million, which handles all but the  most  extreme
1665           cases.  You  can  override  the  default by suppling pcre_exec() with a
1666           pcre_extra    block    in    which    match_limit    is    set,     and
1667           PCRE_EXTRA_MATCH_LIMIT  is  set  in  the  flags  field. If the limit is
1668           exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1669    
1670           The match_limit_recursion field is similar to match_limit, but  instead
1671           of limiting the total number of times that match() is called, it limits
1672           the depth of recursion. The recursion depth is a  smaller  number  than
1673           the  total number of calls, because not all calls to match() are recur-
1674           sive.  This limit is of use only if it is set smaller than match_limit.
1675    
1676           Limiting  the  recursion  depth  limits the amount of stack that can be
1677           used, or, when PCRE has been compiled to use memory on the heap instead
1678           of the stack, the amount of heap memory that can be used.
1679    
1680           The  default  value  for  match_limit_recursion can be set when PCRE is
1681           built; the default default  is  the  same  value  as  the  default  for
1682           match_limit.  You can override the default by suppling pcre_exec() with
1683           a  pcre_extra  block  in  which  match_limit_recursion  is   set,   and
1684           PCRE_EXTRA_MATCH_LIMIT_RECURSION  is  set  in  the  flags field. If the
1685           limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1686    
1687           The pcre_callout field is used in conjunction with the  "callout"  fea-
1688           ture, which is described in the pcrecallout documentation.
1689    
1690           The  tables  field  is  used  to  pass  a  character  tables pointer to
1691           pcre_exec(); this overrides the value that is stored with the  compiled
1692           pattern.  A  non-NULL value is stored with the compiled pattern only if
1693           custom tables were supplied to pcre_compile() via  its  tableptr  argu-
1694           ment.  If NULL is passed to pcre_exec() using this mechanism, it forces
1695           PCRE's internal tables to be used. This facility is  helpful  when  re-
1696           using  patterns  that  have been saved after compiling with an external
1697           set of tables, because the external tables  might  be  at  a  different
1698           address  when  pcre_exec() is called. See the pcreprecompile documenta-
1699           tion for a discussion of saving compiled patterns for later use.
1700    
1701       Option bits for pcre_exec()
1702    
1703           The unused bits of the options argument for pcre_exec() must  be  zero.
1704           The  only  bits  that  may  be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
1705           PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NO_UTF8_CHECK   and
1706           PCRE_PARTIAL.
1707    
1708             PCRE_ANCHORED
1709    
1710           The  PCRE_ANCHORED  option  limits pcre_exec() to matching at the first
1711           matching position. If a pattern was  compiled  with  PCRE_ANCHORED,  or
1712           turned  out to be anchored by virtue of its contents, it cannot be made
1713           unachored at matching time.
1714    
1715             PCRE_NEWLINE_CR
1716             PCRE_NEWLINE_LF
1717             PCRE_NEWLINE_CRLF
1718             PCRE_NEWLINE_ANY
1719    
1720           These options override  the  newline  definition  that  was  chosen  or
1721           defaulted  when the pattern was compiled. For details, see the descrip-
1722           tion of pcre_compile()  above.  During  matching,  the  newline  choice
1723           affects  the  behaviour  of the dot, circumflex, and dollar metacharac-
1724           ters. It may also alter the way the match position is advanced after  a
1725           match  failure  for  an  unanchored  pattern. When PCRE_NEWLINE_CRLF or
1726           PCRE_NEWLINE_ANY is set, and a match attempt  fails  when  the  current
1727           position  is  at a CRLF sequence, the match position is advanced by two
1728           characters instead of one, in other words, to after the CRLF.
1729    
1730             PCRE_NOTBOL
1731    
1732           This option specifies that first character of the subject string is not
1733           the  beginning  of  a  line, so the circumflex metacharacter should not
1734           match before it. Setting this without PCRE_MULTILINE (at compile  time)
1735           causes  circumflex  never to match. This option affects only the behav-
1736           iour of the circumflex metacharacter. It does not affect \A.
1737    
1738             PCRE_NOTEOL
1739    
1740           This option specifies that the end of the subject string is not the end
1741           of  a line, so the dollar metacharacter should not match it nor (except
1742           in multiline mode) a newline immediately before it. Setting this  with-
1743           out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1744           option affects only the behaviour of the dollar metacharacter. It  does
1745           not affect \Z or \z.
1746    
1747             PCRE_NOTEMPTY
1748    
1749           An empty string is not considered to be a valid match if this option is
1750           set. If there are alternatives in the pattern, they are tried.  If  all
1751           the  alternatives  match  the empty string, the entire match fails. For
1752           example, if the pattern
1753    
1754             a?b?
1755    
1756           is applied to a string not beginning with "a" or "b",  it  matches  the
1757           empty  string at the start of the subject. With PCRE_NOTEMPTY set, this
1758           match is not valid, so PCRE searches further into the string for occur-
1759           rences of "a" or "b".
1760    
1761           Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1762           cial case of a pattern match of the empty  string  within  its  split()
1763           function,  and  when  using  the /g modifier. It is possible to emulate
1764           Perl's behaviour after matching a null string by first trying the match
1765           again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1766           if that fails by advancing the starting offset (see below)  and  trying
1767           an ordinary match again. There is some code that demonstrates how to do
1768           this in the pcredemo.c sample program.
1769    
1770             PCRE_NO_UTF8_CHECK
1771    
1772           When PCRE_UTF8 is set at compile time, the validity of the subject as a
1773           UTF-8  string is automatically checked when pcre_exec() is subsequently
1774           called.  The value of startoffset is also checked  to  ensure  that  it
1775           points  to the start of a UTF-8 character. If an invalid UTF-8 sequence
1776           of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If
1777           startoffset  contains  an  invalid  value, PCRE_ERROR_BADUTF8_OFFSET is
1778           returned.
1779    
1780           If you already know that your subject is valid, and you  want  to  skip
1781           these    checks    for   performance   reasons,   you   can   set   the
1782           PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might  want  to
1783           do  this  for the second and subsequent calls to pcre_exec() if you are
1784           making repeated calls to find all  the  matches  in  a  single  subject
1785           string.  However,  you  should  be  sure  that the value of startoffset
1786           points to the start of a UTF-8 character.  When  PCRE_NO_UTF8_CHECK  is
1787           set,  the  effect of passing an invalid UTF-8 string as a subject, or a
1788           value of startoffset that does not point to the start of a UTF-8  char-
1789           acter, is undefined. Your program may crash.
1790    
1791             PCRE_PARTIAL
1792    
1793           This  option  turns  on  the  partial  matching feature. If the subject
1794           string fails to match the pattern, but at some point during the  match-
1795           ing  process  the  end of the subject was reached (that is, the subject
1796           partially matches the pattern and the failure to  match  occurred  only
1797           because  there were not enough subject characters), pcre_exec() returns
1798           PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL  is
1799           used,  there  are restrictions on what may appear in the pattern. These
1800           are discussed in the pcrepartial documentation.
1801    
1802       The string to be matched by pcre_exec()
1803    
1804           The subject string is passed to pcre_exec() as a pointer in subject,  a
1805           length  in  length, and a starting byte offset in startoffset. In UTF-8
1806           mode, the byte offset must point to the start  of  a  UTF-8  character.
1807           Unlike  the  pattern string, the subject may contain binary zero bytes.
1808           When the starting offset is zero, the search for a match starts at  the
1809           beginning of the subject, and this is by far the most common case.
1810    
1811           A  non-zero  starting offset is useful when searching for another match
1812           in the same subject by calling pcre_exec() again after a previous  suc-
1813           cess.   Setting  startoffset differs from just passing over a shortened
1814           string and setting PCRE_NOTBOL in the case of  a  pattern  that  begins
1815           with any kind of lookbehind. For example, consider the pattern
1816    
1817             \Biss\B
1818    
1819           which  finds  occurrences  of "iss" in the middle of words. (\B matches
1820           only if the current position in the subject is not  a  word  boundary.)
1821           When  applied  to the string "Mississipi" the first call to pcre_exec()
1822           finds the first occurrence. If pcre_exec() is called  again  with  just
1823           the  remainder  of  the  subject,  namely  "issipi", it does not match,
1824           because \B is always false at the start of the subject, which is deemed
1825           to  be  a  word  boundary. However, if pcre_exec() is passed the entire
1826           string again, but with startoffset set to 4, it finds the second occur-
1827           rence  of "iss" because it is able to look behind the starting point to
1828           discover that it is preceded by a letter.
1829    
1830           If a non-zero starting offset is passed when the pattern  is  anchored,
1831           one attempt to match at the given offset is made. This can only succeed
1832           if the pattern does not require the match to be at  the  start  of  the
1833           subject.
1834    
1835       How pcre_exec() returns captured substrings
1836    
1837           In  general, a pattern matches a certain portion of the subject, and in
1838           addition, further substrings from the subject  may  be  picked  out  by
1839           parts  of  the  pattern.  Following the usage in Jeffrey Friedl's book,
1840           this is called "capturing" in what follows, and the  phrase  "capturing
1841           subpattern"  is  used for a fragment of a pattern that picks out a sub-
1842           string. PCRE supports several other kinds of  parenthesized  subpattern
1843           that do not cause substrings to be captured.
1844    
1845           Captured  substrings are returned to the caller via a vector of integer
1846           offsets whose address is passed in ovector. The number of  elements  in
1847           the  vector is passed in ovecsize, which must be a non-negative number.
1848           Note: this argument is NOT the size of ovector in bytes.
1849    
1850           The first two-thirds of the vector is used to pass back  captured  sub-
1851           strings,  each  substring using a pair of integers. The remaining third
1852           of the vector is used as workspace by pcre_exec() while  matching  cap-
1853           turing  subpatterns, and is not available for passing back information.
1854           The length passed in ovecsize should always be a multiple of three.  If
1855           it is not, it is rounded down.
1856    
1857           When  a  match  is successful, information about captured substrings is
1858           returned in pairs of integers, starting at the  beginning  of  ovector,
1859           and  continuing  up  to two-thirds of its length at the most. The first
1860           element of a pair is set to the offset of the first character in a sub-
1861           string,  and  the  second  is  set to the offset of the first character
1862           after the end of a substring. The  first  pair,  ovector[0]  and  ovec-
1863           tor[1],  identify  the  portion  of  the  subject string matched by the
1864           entire pattern. The next pair is used for the first  capturing  subpat-
1865           tern, and so on. The value returned by pcre_exec() is one more than the
1866           highest numbered pair that has been set. For example, if two substrings
1867           have  been captured, the returned value is 3. If there are no capturing
1868           subpatterns, the return value from a successful match is 1,  indicating
1869           that just the first pair of offsets has been set.
1870    
1871           If a capturing subpattern is matched repeatedly, it is the last portion
1872           of the string that it matched that is returned.
1873    
1874           If the vector is too small to hold all the captured substring  offsets,
1875           it is used as far as possible (up to two-thirds of its length), and the
1876           function returns a value of zero. In particular, if the substring  off-
1877           sets are not of interest, pcre_exec() may be called with ovector passed
1878           as NULL and ovecsize as zero. However, if  the  pattern  contains  back
1879           references  and  the  ovector is not big enough to remember the related
1880           substrings, PCRE has to get additional memory for use during  matching.
1881           Thus it is usually advisable to supply an ovector.
1882    
1883           The  pcre_info()  function  can  be used to find out how many capturing
1884           subpatterns there are in a compiled  pattern.  The  smallest  size  for
1885           ovector  that  will allow for n captured substrings, in addition to the
1886           offsets of the substring matched by the whole pattern, is (n+1)*3.
1887    
1888           It is possible for capturing subpattern number n+1 to match  some  part
1889           of the subject when subpattern n has not been used at all. For example,
1890           if the string "abc" is matched  against  the  pattern  (a|(z))(bc)  the
1891           return from the function is 4, and subpatterns 1 and 3 are matched, but
1892           2 is not. When this happens, both values in  the  offset  pairs  corre-
1893           sponding to unused subpatterns are set to -1.
1894    
1895           Offset  values  that correspond to unused subpatterns at the end of the
1896           expression are also set to -1. For example,  if  the  string  "abc"  is
1897           matched  against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
1898           matched. The return from the function is 2, because  the  highest  used
1899           capturing subpattern number is 1. However, you can refer to the offsets
1900           for the second and third capturing subpatterns if  you  wish  (assuming
1901           the vector is large enough, of course).
1902    
1903           Some  convenience  functions  are  provided for extracting the captured
1904           substrings as separate strings. These are described below.
1905    
1906       Error return values from pcre_exec()
1907    
1908           If pcre_exec() fails, it returns a negative number. The  following  are
1909           defined in the header file:
1910    
1911             PCRE_ERROR_NOMATCH        (-1)
1912    
1913           The subject string did not match the pattern.
1914    
1915             PCRE_ERROR_NULL           (-2)
1916    
1917           Either  code  or  subject  was  passed as NULL, or ovector was NULL and
1918           ovecsize was not zero.
1919    
1920             PCRE_ERROR_BADOPTION      (-3)
1921    
1922           An unrecognized bit was set in the options argument.
1923    
1924             PCRE_ERROR_BADMAGIC       (-4)
1925    
1926           PCRE stores a 4-byte "magic number" at the start of the compiled  code,
1927           to catch the case when it is passed a junk pointer and to detect when a
1928           pattern that was compiled in an environment of one endianness is run in
1929           an  environment  with the other endianness. This is the error that PCRE
1930           gives when the magic number is not present.
1931    
1932             PCRE_ERROR_UNKNOWN_OPCODE (-5)
1933    
1934           While running the pattern match, an unknown item was encountered in the
1935           compiled  pattern.  This  error  could be caused by a bug in PCRE or by
1936           overwriting of the compiled pattern.
1937    
1938             PCRE_ERROR_NOMEMORY       (-6)
1939    
1940           If a pattern contains back references, but the ovector that  is  passed
1941           to pcre_exec() is not big enough to remember the referenced substrings,
1942           PCRE gets a block of memory at the start of matching to  use  for  this
1943           purpose.  If the call via pcre_malloc() fails, this error is given. The
1944           memory is automatically freed at the end of matching.
1945    
1946             PCRE_ERROR_NOSUBSTRING    (-7)
1947    
1948           This error is used by the pcre_copy_substring(),  pcre_get_substring(),
1949           and  pcre_get_substring_list()  functions  (see  below).  It  is  never
1950           returned by pcre_exec().
1951    
1952             PCRE_ERROR_MATCHLIMIT     (-8)
1953    
1954           The backtracking limit, as specified by  the  match_limit  field  in  a
1955           pcre_extra  structure  (or  defaulted) was reached. See the description
1956           above.
1957    
1958             PCRE_ERROR_CALLOUT        (-9)
1959    
1960           This error is never generated by pcre_exec() itself. It is provided for
1961           use  by  callout functions that want to yield a distinctive error code.
1962           See the pcrecallout documentation for details.
1963    
1964             PCRE_ERROR_BADUTF8        (-10)
1965    
1966           A string that contains an invalid UTF-8 byte sequence was passed  as  a
1967           subject.
1968    
1969             PCRE_ERROR_BADUTF8_OFFSET (-11)
1970    
1971           The UTF-8 byte sequence that was passed as a subject was valid, but the
1972           value of startoffset did not point to the beginning of a UTF-8  charac-
1973           ter.
1974    
1975             PCRE_ERROR_PARTIAL        (-12)
1976    
1977           The  subject  string did not match, but it did match partially. See the
1978           pcrepartial documentation for details of partial matching.
1979    
1980             PCRE_ERROR_BADPARTIAL     (-13)
1981    
1982           The PCRE_PARTIAL option was used with  a  compiled  pattern  containing
1983           items  that are not supported for partial matching. See the pcrepartial
1984           documentation for details of partial matching.
1985    
1986             PCRE_ERROR_INTERNAL       (-14)
1987    
1988           An unexpected internal error has occurred. This error could  be  caused
1989           by a bug in PCRE or by overwriting of the compiled pattern.
1990    
1991             PCRE_ERROR_BADCOUNT       (-15)
1992    
1993           This  error is given if the value of the ovecsize argument is negative.
1994    
1995             PCRE_ERROR_RECURSIONLIMIT (-21)
1996    
1997           The internal recursion limit, as specified by the match_limit_recursion
1998           field  in  a  pcre_extra  structure (or defaulted) was reached. See the
1999           description above.
2000    
2001             PCRE_ERROR_NULLWSLIMIT    (-22)
2002    
2003           When a group that can match an empty  substring  is  repeated  with  an
2004           unbounded  upper  limit, the subject position at the start of the group
2005           must be remembered, so that a test for an empty string can be made when
2006           the  end  of the group is reached. Some workspace is required for this;
2007           if it runs out, this error is given.
2008    
2009             PCRE_ERROR_BADNEWLINE     (-23)
2010    
2011           An invalid combination of PCRE_NEWLINE_xxx options was given.
2012    
2013           Error numbers -16 to -20 are not used by pcre_exec().
2014    
2015    
2016    EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2017    
2018           int pcre_copy_substring(const char *subject, int *ovector,
2019                int stringcount, int stringnumber, char *buffer,
2020                int buffersize);
2021    
2022           int pcre_get_substring(const char *subject, int *ovector,
2023                int stringcount, int stringnumber,
2024                const char **stringptr);
2025    
2026           int pcre_get_substring_list(const char *subject,
2027                int *ovector, int stringcount, const char ***listptr);
2028    
2029           Captured substrings can be  accessed  directly  by  using  the  offsets
2030           returned  by  pcre_exec()  in  ovector.  For convenience, the functions
2031           pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
2032           string_list()  are  provided for extracting captured substrings as new,
2033           separate, zero-terminated strings. These functions identify  substrings
2034           by  number.  The  next section describes functions for extracting named
2035           substrings.
2036    
2037           A substring that contains a binary zero is correctly extracted and  has
2038           a  further zero added on the end, but the result is not, of course, a C
2039           string.  However, you can process such a string  by  referring  to  the
2040           length  that  is  returned  by  pcre_copy_substring() and pcre_get_sub-
2041           string().  Unfortunately, the interface to pcre_get_substring_list() is
2042           not  adequate for handling strings containing binary zeros, because the
2043           end of the final string is not independently indicated.
2044    
2045           The first three arguments are the same for all  three  of  these  func-
2046           tions:  subject  is  the subject string that has just been successfully
2047           matched, ovector is a pointer to the vector of integer offsets that was
2048           passed to pcre_exec(), and stringcount is the number of substrings that
2049           were captured by the match, including the substring  that  matched  the
2050           entire regular expression. This is the value returned by pcre_exec() if
2051           it is greater than zero. If pcre_exec() returned zero, indicating  that
2052           it  ran out of space in ovector, the value passed as stringcount should
2053           be the number of elements in the vector divided by three.
2054    
2055           The functions pcre_copy_substring() and pcre_get_substring() extract  a
2056           single  substring,  whose  number  is given as stringnumber. A value of
2057           zero extracts the substring that matched the  entire  pattern,  whereas
2058           higher  values  extract  the  captured  substrings.  For pcre_copy_sub-
2059           string(), the string is placed in buffer,  whose  length  is  given  by
2060           buffersize,  while  for  pcre_get_substring()  a new block of memory is
2061           obtained via pcre_malloc, and its address is  returned  via  stringptr.
2062           The  yield  of  the function is the length of the string, not including
2063           the terminating zero, or one of these error codes:
2064    
2065             PCRE_ERROR_NOMEMORY       (-6)
2066    
2067           The buffer was too small for pcre_copy_substring(), or the  attempt  to
2068           get memory failed for pcre_get_substring().
2069    
2070             PCRE_ERROR_NOSUBSTRING    (-7)
2071    
2072           There is no substring whose number is stringnumber.
2073    
2074           The  pcre_get_substring_list()  function  extracts  all  available sub-
2075           strings and builds a list of pointers to them. All this is  done  in  a
2076           single block of memory that is obtained via pcre_malloc. The address of
2077           the memory block is returned via listptr, which is also  the  start  of
2078           the  list  of  string pointers. The end of the list is marked by a NULL
2079           pointer. The yield of the function is zero if all  went  well,  or  the
2080           error code
2081    
2082             PCRE_ERROR_NOMEMORY       (-6)
2083    
2084           if the attempt to get the memory block failed.
2085    
2086           When  any of these functions encounter a substring that is unset, which
2087           can happen when capturing subpattern number n+1 matches  some  part  of
2088           the  subject, but subpattern n has not been used at all, they return an
2089           empty string. This can be distinguished from a genuine zero-length sub-
2090           string  by inspecting the appropriate offset in ovector, which is nega-
2091           tive for unset substrings.
2092    
2093           The two convenience functions pcre_free_substring() and  pcre_free_sub-
2094           string_list()  can  be  used  to free the memory returned by a previous
2095           call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
2096           tively.  They  do  nothing  more  than  call the function pointed to by
2097           pcre_free, which of course could be called directly from a  C  program.
2098           However,  PCRE is used in some situations where it is linked via a spe-
2099           cial  interface  to  another  programming  language  that  cannot   use
2100           pcre_free  directly;  it is for these cases that the functions are pro-
2101           vided.
2102    
2103    
2104    EXTRACTING CAPTURED SUBSTRINGS BY NAME
2105    
2106           int pcre_get_stringnumber(const pcre *code,
2107                const char *name);
2108    
2109           int pcre_copy_named_substring(const pcre *code,
2110                const char *subject, int *ovector,
2111                int stringcount, const char *stringname,
2112                char *buffer, int buffersize);
2113    
2114           int pcre_get_named_substring(const pcre *code,
2115                const char *subject, int *ovector,
2116                int stringcount, const char *stringname,
2117                const char **stringptr);
2118    
2119           To extract a substring by name, you first have to find associated  num-
2120           ber.  For example, for this pattern
2121    
2122             (a+)b(?<xxx>\d+)...
2123    
2124           the number of the subpattern called "xxx" is 2. If the name is known to
2125           be unique (PCRE_DUPNAMES was not set), you can find the number from the
2126           name by calling pcre_get_stringnumber(). The first argument is the com-
2127           piled pattern, and the second is the name. The yield of the function is
2128           the  subpattern  number,  or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2129           subpattern of that name.
2130    
2131           Given the number, you can extract the substring directly, or use one of
2132           the functions described in the previous section. For convenience, there
2133           are also two functions that do the whole job.
2134    
2135           Most   of   the   arguments    of    pcre_copy_named_substring()    and
2136           pcre_get_named_substring()  are  the  same  as  those for the similarly
2137           named functions that extract by number. As these are described  in  the
2138           previous  section,  they  are not re-described here. There are just two
2139           differences:
2140    
2141           First, instead of a substring number, a substring name is  given.  Sec-
2142           ond, there is an extra argument, given at the start, which is a pointer
2143           to the compiled pattern. This is needed in order to gain access to  the
2144           name-to-number translation table.
2145    
2146           These  functions call pcre_get_stringnumber(), and if it succeeds, they
2147           then call pcre_copy_substring() or pcre_get_substring(),  as  appropri-
2148           ate.  NOTE:  If PCRE_DUPNAMES is set and there are duplicate names, the
2149           behaviour may not be what you want (see the next section).
2150    
2151    
2152    DUPLICATE SUBPATTERN NAMES
2153    
2154           int pcre_get_stringtable_entries(const pcre *code,
2155                const char *name, char **first, char **last);
2156    
2157           When a pattern is compiled with the  PCRE_DUPNAMES  option,  names  for
2158           subpatterns  are  not  required  to  be unique. Normally, patterns with
2159           duplicate names are such that in any one match, only one of  the  named
2160           subpatterns  participates. An example is shown in the pcrepattern docu-
2161           mentation. When duplicates are present, pcre_copy_named_substring() and
2162           pcre_get_named_substring()  return the first substring corresponding to
2163           the given name that is set.  If  none  are  set,  an  empty  string  is
2164           returned.  The pcre_get_stringnumber() function returns one of the num-
2165           bers that are associated with the name, but it is not defined which  it
2166           is.
2167    
2168           If  you want to get full details of all captured substrings for a given
2169           name, you must use  the  pcre_get_stringtable_entries()  function.  The
2170           first argument is the compiled pattern, and the second is the name. The
2171           third and fourth are pointers to variables which  are  updated  by  the
2172           function. After it has run, they point to the first and last entries in
2173           the name-to-number table  for  the  given  name.  The  function  itself
2174           returns  the  length  of  each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2175           there are none. The format of the table is described above in the  sec-
2176           tion  entitled  Information  about  a  pattern.  Given all the relevant
2177           entries for the name, you can extract each of their numbers, and  hence
2178           the captured data, if any.
2179    
2180    
2181    FINDING ALL POSSIBLE MATCHES
2182    
2183           The  traditional  matching  function  uses a similar algorithm to Perl,
2184           which stops when it finds the first match, starting at a given point in
2185           the  subject.  If you want to find all possible matches, or the longest
2186           possible match, consider using the alternative matching  function  (see
2187           below)  instead.  If you cannot use the alternative function, but still
2188           need to find all possible matches, you can kludge it up by  making  use
2189           of the callout facility, which is described in the pcrecallout documen-
2190           tation.
2191    
2192           What you have to do is to insert a callout right at the end of the pat-
2193           tern.   When your callout function is called, extract and save the cur-
2194           rent matched substring. Then return  1,  which  forces  pcre_exec()  to
2195           backtrack  and  try other alternatives. Ultimately, when it runs out of
2196           matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2197    
2198    
2199    MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2200    
2201           int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2202                const char *subject, int length, int startoffset,
2203                int options, int *ovector, int ovecsize,
2204                int *workspace, int wscount);
2205    
2206           The function pcre_dfa_exec()  is  called  to  match  a  subject  string
2207           against  a  compiled pattern, using a matching algorithm that scans the
2208           subject string just once, and does not backtrack.  This  has  different
2209           characteristics  to  the  normal  algorithm, and is not compatible with
2210           Perl. Some of the features of PCRE patterns are not  supported.  Never-
2211           theless,  there are times when this kind of matching can be useful. For
2212           a discussion of the two matching algorithms, see the pcrematching docu-
2213           mentation.
2214    
2215           The  arguments  for  the  pcre_dfa_exec()  function are the same as for
2216           pcre_exec(), plus two extras. The ovector argument is used in a differ-
2217           ent  way,  and  this is described below. The other common arguments are
2218           used in the same way as for pcre_exec(), so their  description  is  not
2219           repeated here.
2220    
2221           The  two  additional  arguments provide workspace for the function. The
2222           workspace vector should contain at least 20 elements. It  is  used  for
2223           keeping  track  of  multiple  paths  through  the  pattern  tree.  More
2224           workspace will be needed for patterns and subjects where  there  are  a
2225           lot of potential matches.
2226    
2227           Here is an example of a simple call to pcre_dfa_exec():
2228    
2229             int rc;
2230             int ovector[10];
2231             int wspace[20];
2232             rc = pcre_dfa_exec(
2233               re,             /* result of pcre_compile() */
2234               NULL,           /* we didn't study the pattern */
2235               "some string",  /* the subject string */
2236               11,             /* the length of the subject string */
2237               0,              /* start at offset 0 in the subject */
2238               0,              /* default options */
2239               ovector,        /* vector of integers for substring information */
2240               10,             /* number of elements (NOT size in bytes) */
2241               wspace,         /* working space vector */
2242               20);            /* number of elements (NOT size in bytes) */
2243    
2244       Option bits for pcre_dfa_exec()
2245    
2246           The  unused  bits  of  the options argument for pcre_dfa_exec() must be
2247           zero. The only bits  that  may  be  set  are  PCRE_ANCHORED,  PCRE_NEW-
2248           LINE_xxx,  PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
2249           PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2250           three of these are the same as for pcre_exec(), so their description is
2251           not repeated here.
2252    
2253             PCRE_PARTIAL
2254    
2255           This has the same general effect as it does for  pcre_exec(),  but  the
2256           details   are   slightly   different.  When  PCRE_PARTIAL  is  set  for
2257           pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is  converted  into
2258           PCRE_ERROR_PARTIAL  if  the  end  of the subject is reached, there have
2259           been no complete matches, but there is still at least one matching pos-
2260           sibility.  The portion of the string that provided the partial match is
2261           set as the first matching string.
2262    
2263             PCRE_DFA_SHORTEST
2264    
2265           Setting the PCRE_DFA_SHORTEST option causes the matching  algorithm  to
2266           stop as soon as it has found one match. Because of the way the alterna-
2267           tive algorithm works, this is necessarily the shortest  possible  match
2268           at the first possible matching point in the subject string.
2269    
2270             PCRE_DFA_RESTART
2271    
2272           When  pcre_dfa_exec()  is  called  with  the  PCRE_PARTIAL  option, and
2273           returns a partial match, it is possible to call it  again,  with  addi-
2274           tional  subject  characters,  and have it continue with the same match.
2275           The PCRE_DFA_RESTART option requests this action; when it is  set,  the
2276           workspace  and wscount options must reference the same vector as before
2277           because data about the match so far is left in  them  after  a  partial
2278           match.  There  is  more  discussion of this facility in the pcrepartial
2279           documentation.
2280    
2281       Successful returns from pcre_dfa_exec()
2282    
2283           When pcre_dfa_exec() succeeds, it may have matched more than  one  sub-
2284           string in the subject. Note, however, that all the matches from one run
2285           of the function start at the same point in  the  subject.  The  shorter
2286           matches  are all initial substrings of the longer matches. For example,
2287           if the pattern
2288    
2289             <.*>
2290    
2291           is matched against the string
2292    
2293             This is <something> <something else> <something further> no more
2294    
2295           the three matched strings are
2296    
2297             <something>
2298             <something> <something else>
2299             <something> <something else> <something further>
2300    
2301           On success, the yield of the function is a number  greater  than  zero,
2302           which  is  the  number of matched substrings. The substrings themselves
2303           are returned in ovector. Each string uses two elements;  the  first  is
2304           the  offset  to  the start, and the second is the offset to the end. In
2305           fact, all the strings have the same start  offset.  (Space  could  have
2306           been  saved by giving this only once, but it was decided to retain some
2307           compatibility with the way pcre_exec() returns data,  even  though  the
2308           meaning of the strings is different.)
2309    
2310           The strings are returned in reverse order of length; that is, the long-
2311           est matching string is given first. If there were too many  matches  to
2312           fit  into ovector, the yield of the function is zero, and the vector is
2313           filled with the longest matches.
2314    
2315       Error returns from pcre_dfa_exec()
2316    
2317           The pcre_dfa_exec() function returns a negative number when  it  fails.
2318           Many  of  the  errors  are  the  same as for pcre_exec(), and these are
2319           described above.  There are in addition the following errors  that  are
2320           specific to pcre_dfa_exec():
2321    
2322             PCRE_ERROR_DFA_UITEM      (-16)
2323    
2324           This  return is given if pcre_dfa_exec() encounters an item in the pat-
2325           tern that it does not support, for instance, the use of \C  or  a  back
2326           reference.
2327    
2328             PCRE_ERROR_DFA_UCOND      (-17)
2329    
2330           This  return  is  given  if pcre_dfa_exec() encounters a condition item
2331           that uses a back reference for the condition, or a test  for  recursion
2332           in a specific group. These are not supported.
2333    
2334             PCRE_ERROR_DFA_UMLIMIT    (-18)
2335    
2336           This  return  is given if pcre_dfa_exec() is called with an extra block
2337           that contains a setting of the match_limit field. This is not supported
2338           (it is meaningless).
2339    
2340             PCRE_ERROR_DFA_WSSIZE     (-19)
2341    
2342           This  return  is  given  if  pcre_dfa_exec()  runs  out of space in the
2343           workspace vector.
2344    
2345             PCRE_ERROR_DFA_RECURSE    (-20)
2346    
2347           When a recursive subpattern is processed, the matching  function  calls
2348           itself  recursively,  using  private vectors for ovector and workspace.
2349           This error is given if the output vector  is  not  large  enough.  This
2350           should be extremely rare, as a vector of size 1000 is used.
2351    
 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  
      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 store  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.  
2352    
2353    SEE ALSO
2354    
2355           pcrebuild(3),  pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2356           tial(3), pcreposix(3), pcreprecompile(3), pcresample(3),  pcrestack(3).
2357    
2358    
2359  LIMITATIONS  AUTHOR
      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.  
   
      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  
2360    
2361       The following sections describe  the  use  of  each  of  the         Philip Hazel
2362       meta-characters.         University Computing Service
2363           Cambridge CB2 3QH, England.
2364    
2365    
2366    REVISION
2367    
2368  BACKSLASH         Last updated: 06 March 2007
2369       The backslash character has several uses. Firstly, if it  is         Copyright (c) 1997-2007 University of Cambridge.
2370       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.  
2371    
2372    
2373    PCRECALLOUT(3)                                                  PCRECALLOUT(3)
2374    
 CIRCUMFLEX AND DOLLAR  
      Outside a character class, in the default matching mode, the  
      circumflex  character  is an assertion which is true only if  
      the current matching point is at the start  of  the  subject  
      string.  If  the startoffset argument of pcre_exec() is non-  
      zero, circumflex can never match. Inside a character  class,  
      circumflex has an entirely different meaning (see below).  
   
      Circumflex need not be the first character of the pattern if  
      a  number of alternatives are involved, but it should be the  
      first thing in each alternative in which it appears  if  the  
      pattern is ever to match that branch. If all possible alter-  
      natives start with a circumflex, that is, if the pattern  is  
      constrained to match only at the start of the subject, it is  
      said to be an "anchored" pattern. (There are also other con-  
      structs that can cause a pattern to be anchored.)  
   
      A dollar character is an assertion which is true only if the  
      current  matching point is at the end of the subject string,  
      or immediately before a newline character that is  the  last  
      character in the string (by default). Dollar need not be the  
      last character of the pattern if a  number  of  alternatives  
      are  involved,  but it should be the last item in any branch  
      in which it appears.  Dollar has no  special  meaning  in  a  
      character class.  
   
      The meaning of dollar can be changed so that it matches only  
      at   the   very   end   of   the   string,  by  setting  the  
      PCRE_DOLLAR_ENDONLY option at compile or matching time. This  
      does not affect the \Z assertion.  
   
      The meanings of the circumflex  and  dollar  characters  are  
      changed  if  the  PCRE_MULTILINE option is set. When this is  
      the case,  they  match  immediately  after  and  immediately  
      before an internal "\n" character, respectively, in addition  
      to matching at the start and end of the subject string.  For  
      example,  the  pattern  /^abc$/  matches  the subject string  
      "def\nabc" in multiline  mode,  but  not  otherwise.  Conse-  
      quently,  patterns  that  are  anchored  in single line mode  
      because all branches start with "^" are not anchored in mul-  
      tiline mode, and a match for circumflex is possible when the  
      startoffset  argument  of  pcre_exec()  is   non-zero.   The  
      PCRE_DOLLAR_ENDONLY  option  is ignored if PCRE_MULTILINE is  
      set.  
   
      Note that the sequences \A, \Z, and \z can be used to  match  
      the  start  and end of the subject in both modes, and if all  
      branches of a pattern start with \A is it  always  anchored,  
      whether PCRE_MULTILINE is set or not.  
2375    
2376    NAME
2377           PCRE - Perl-compatible regular expressions
2378    
2379    
2380  FULL STOP (PERIOD, DOT)  PCRE CALLOUTS
      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.  
2381    
2382           int (*pcre_callout)(pcre_callout_block *);
2383    
2384           PCRE provides a feature called "callout", which is a means of temporar-
2385           ily passing control to the caller of PCRE  in  the  middle  of  pattern
2386           matching.  The  caller of PCRE provides an external function by putting
2387           its entry point in the global variable pcre_callout. By  default,  this
2388           variable contains NULL, which disables all calling out.
2389    
2390           Within  a  regular  expression,  (?C) indicates the points at which the
2391           external function is to be called.  Different  callout  points  can  be
2392           identified  by  putting  a number less than 256 after the letter C. The
2393           default value is zero.  For  example,  this  pattern  has  two  callout
2394           points:
2395    
2396             (?C1)eabc(?C2)def
2397    
2398           If  the  PCRE_AUTO_CALLOUT  option  bit  is  set when pcre_compile() is
2399           called, PCRE automatically  inserts  callouts,  all  with  number  255,
2400           before  each  item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2401           used with the pattern
2402    
2403             A(\d{2}|--)
2404    
2405           it is processed as if it were
2406    
2407           (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2408    
2409           Notice that there is a callout before and after  each  parenthesis  and
2410           alternation  bar.  Automatic  callouts  can  be  used  for tracking the
2411           progress of pattern matching. The pcretest command has an  option  that
2412           sets  automatic callouts; when it is used, the output indicates how the
2413           pattern is matched. This is useful information when you are  trying  to
2414           optimize the performance of a particular pattern.
2415    
2416    
2417    MISSING CALLOUTS
2418    
2419           You  should  be  aware  that,  because of optimizations in the way PCRE
2420           matches patterns, callouts sometimes do not happen. For example, if the
2421           pattern is
2422    
2423             ab(?C4)cd
2424    
2425           PCRE knows that any matching string must contain the letter "d". If the
2426           subject string is "abyz", the lack of "d" means that  matching  doesn't
2427           ever  start,  and  the  callout is never reached. However, with "abyd",
2428           though the result is still no match, the callout is obeyed.
2429    
2430    
2431    THE CALLOUT INTERFACE
2432    
2433           During matching, when PCRE reaches a callout point, the external  func-
2434           tion  defined by pcre_callout is called (if it is set). This applies to
2435           both the pcre_exec() and the pcre_dfa_exec()  matching  functions.  The
2436           only  argument  to  the callout function is a pointer to a pcre_callout
2437           block. This structure contains the following fields:
2438    
2439             int          version;
2440             int          callout_number;
2441             int         *offset_vector;
2442             const char  *subject;
2443             int          subject_length;
2444             int          start_match;
2445             int          current_position;
2446             int          capture_top;
2447             int          capture_last;
2448             void        *callout_data;
2449             int          pattern_position;
2450             int          next_item_length;
2451    
2452           The version field is an integer containing the version  number  of  the
2453           block  format. The initial version was 0; the current version is 1. The
2454           version number will change again in future  if  additional  fields  are
2455           added, but the intention is never to remove any of the existing fields.
2456    
2457           The callout_number field contains the number of the  callout,  as  com-
2458           piled  into  the pattern (that is, the number after ?C for manual call-
2459           outs, and 255 for automatically generated callouts).
2460    
2461           The offset_vector field is a pointer to the vector of offsets that  was
2462           passed   by   the   caller  to  pcre_exec()  or  pcre_dfa_exec().  When
2463           pcre_exec() is used, the contents can be inspected in order to  extract
2464           substrings  that  have  been  matched  so  far,  in the same way as for
2465           extracting substrings after a match has completed. For  pcre_dfa_exec()
2466           this field is not useful.
2467    
2468           The subject and subject_length fields contain copies of the values that
2469           were passed to pcre_exec().
2470    
2471           The start_match field contains the offset within the subject  at  which
2472           the  current match attempt started. If the pattern is not anchored, the
2473           callout function may be called several times from the same point in the
2474           pattern for different starting points in the subject.
2475    
2476           The  current_position  field  contains the offset within the subject of
2477           the current match pointer.
2478    
2479           When the pcre_exec() function is used, the capture_top  field  contains
2480           one  more than the number of the highest numbered captured substring so
2481           far. If no substrings have been captured, the value of  capture_top  is
2482           one.  This  is always the case when pcre_dfa_exec() is used, because it
2483           does not support captured substrings.
2484    
2485           The capture_last field contains the number of the  most  recently  cap-
2486           tured  substring. If no substrings have been captured, its value is -1.
2487           This is always the case when pcre_dfa_exec() is used.
2488    
2489           The callout_data field contains a value that is passed  to  pcre_exec()
2490           or  pcre_dfa_exec() specifically so that it can be passed back in call-
2491           outs. It is passed in the pcre_callout field  of  the  pcre_extra  data
2492           structure.  If  no such data was passed, the value of callout_data in a
2493           pcre_callout block is NULL. There is a description  of  the  pcre_extra
2494           structure in the pcreapi documentation.
2495    
2496           The  pattern_position field is present from version 1 of the pcre_call-
2497           out structure. It contains the offset to the next item to be matched in
2498           the pattern string.
2499    
2500           The  next_item_length field is present from version 1 of the pcre_call-
2501           out structure. It contains the length of the next item to be matched in
2502           the  pattern  string. When the callout immediately precedes an alterna-
2503           tion bar, a closing parenthesis, or the end of the pattern, the  length
2504           is  zero.  When the callout precedes an opening parenthesis, the length
2505           is that of the entire subpattern.
2506    
2507           The pattern_position and next_item_length fields are intended  to  help
2508           in  distinguishing between different automatic callouts, which all have
2509           the same callout number. However, they are set for all callouts.
2510    
2511    
2512    RETURN VALUES
2513    
2514           The external callout function returns an integer to PCRE. If the  value
2515           is  zero,  matching  proceeds  as  normal. If the value is greater than
2516           zero, matching fails at the current point, but  the  testing  of  other
2517           matching possibilities goes ahead, just as if a lookahead assertion had
2518           failed. If the value is less than zero, the  match  is  abandoned,  and
2519           pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2520    
2521           Negative   values   should   normally   be   chosen  from  the  set  of
2522           PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2523           dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
2524           reserved for use by callout functions; it will never be  used  by  PCRE
2525           itself.
2526    
 POSIX CHARACTER CLASSES  
      Perl 5.6 (not yet released at the time of writing) is  going  
      to  support  the POSIX notation for character classes, which  
      uses names enclosed by  [:  and  :]   within  the  enclosing  
      square brackets. PCRE supports this notation. For example,  
   
        [01[:alpha:]%]  
   
      matches "0", "1", any alphabetic character, or "%". The sup-  
      ported class names are  
   
        alnum    letters and digits  
        alpha    letters  
        ascii    character codes 0 - 127  
        cntrl    control characters  
        digit    decimal digits (same as \d)  
        graph    printing characters, excluding space  
        lower    lower case letters  
        print    printing characters, including space  
        punct    printing characters, excluding letters and digits  
        space    white space (same as \s)  
        upper    upper case letters  
        word     "word" characters (same as \w)  
        xdigit   hexadecimal digits  
   
      The names "ascii" and "word" are  Perl  extensions.  Another  
      Perl  extension is negation, which is indicated by a ^ char-  
      acter after the colon. For example,  
   
        [12[:^digit:]]  
   
      matches "1", "2", or any non-digit.  PCRE  (and  Perl)  also  
      recogize  the POSIX syntax [.ch.] and [=ch=] where "ch" is a  
      "collating element", but these are  not  supported,  and  an  
      error is given if they are encountered.  
2527    
2528    AUTHOR
2529    
2530           Philip Hazel
2531           University Computing Service
2532           Cambridge CB2 3QH, England.
2533    
 VERTICAL BAR  
      Vertical bar characters are  used  to  separate  alternative  
      patterns. For example, the pattern  
2534    
2535         gilbert|sullivan  REVISION
2536    
2537       matches either "gilbert" or "sullivan". Any number of alter-         Last updated: 06 March 2007
2538       natives  may  appear,  and an empty alternative is permitted         Copyright (c) 1997-2007 University of Cambridge.
2539       (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.  
2540    
2541    
2542    PCRECOMPAT(3)                                                    PCRECOMPAT(3)
2543    
 INTERNAL OPTION SETTING  
      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.  
2544    
2545    NAME
2546           PCRE - Perl-compatible regular expressions
2547    
2548    
2549  SUBPATTERNS  DIFFERENCES BETWEEN PCRE AND PERL
      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".  
2550    
2551           This  document describes the differences in the ways that PCRE and Perl
2552           handle regular expressions. The differences described here  are  mainly
2553           with  respect  to  Perl 5.8, though PCRE version 7.0 contains some fea-
2554           tures that are expected to be in the forthcoming Perl 5.10.
2555    
2556           1. PCRE has only a subset of Perl's UTF-8 and Unicode support.  Details
2557           of  what  it does have are given in the section on UTF-8 support in the
2558           main pcre page.
2559    
2560           2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2561           permits  them,  but they do not mean what you might think. For example,
2562           (?!a){3} does not assert that the next three characters are not "a". It
2563           just asserts that the next character is not "a" three times.
2564    
2565           3.  Capturing  subpatterns  that occur inside negative lookahead asser-
2566           tions are counted, but their entries in the offsets  vector  are  never
2567           set.  Perl sets its numerical variables from any such patterns that are
2568           matched before the assertion fails to match something (thereby succeed-
2569           ing),  but  only  if the negative lookahead assertion contains just one
2570           branch.
2571    
2572           4. Though binary zero characters are supported in the  subject  string,
2573           they are not allowed in a pattern string because it is passed as a nor-
2574           mal C string, terminated by zero. The escape sequence \0 can be used in
2575           the pattern to represent a binary zero.
2576    
2577           5.  The  following Perl escape sequences are not supported: \l, \u, \L,
2578           \U, and \N. In fact these are implemented by Perl's general string-han-
2579           dling  and are not part of its pattern matching engine. If any of these
2580           are encountered by PCRE, an error is generated.
2581    
2582           6. The Perl escape sequences \p, \P, and \X are supported only if  PCRE
2583           is  built  with Unicode character property support. The properties that
2584           can be tested with \p and \P are limited to the general category  prop-
2585           erties  such  as  Lu and Nd, script names such as Greek or Han, and the
2586           derived properties Any and L&.
2587    
2588           7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2589           ters  in  between  are  treated as literals. This is slightly different
2590           from Perl in that $ and @ are  also  handled  as  literals  inside  the
2591           quotes.  In Perl, they cause variable interpolation (but of course PCRE
2592           does not have variables). Note the following examples:
2593    
2594               Pattern            PCRE matches      Perl matches
2595    
2596               \Qabc$xyz\E        abc$xyz           abc followed by the
2597                                                      contents of $xyz
2598               \Qabc\$xyz\E       abc\$xyz          abc\$xyz
2599               \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
2600    
2601           The \Q...\E sequence is recognized both inside  and  outside  character
2602           classes.
2603    
2604           8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2605           constructions. However, there is support for recursive  patterns.  This
2606           is  not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
2607           "callout" feature allows an external function to be called during  pat-
2608           tern matching. See the pcrecallout documentation for details.
2609    
2610           9.  Subpatterns  that  are  called  recursively or as "subroutines" are
2611           always treated as atomic groups in  PCRE.  This  is  like  Python,  but
2612           unlike Perl.
2613    
2614           10.  There are some differences that are concerned with the settings of
2615           captured strings when part of  a  pattern  is  repeated.  For  example,
2616           matching  "aba"  against  the  pattern  /^(a(b)?)+$/  in Perl leaves $2
2617           unset, but in PCRE it is set to "b".
2618    
2619           11. PCRE provides some extensions to the Perl regular expression facil-
2620           ities.   Perl  5.10  will  include new features that are not in earlier
2621           versions, some of which (such as named parentheses) have been  in  PCRE
2622           for some time. This list is with respect to Perl 5.10:
2623    
2624           (a)  Although  lookbehind  assertions  must match fixed length strings,
2625           each alternative branch of a lookbehind assertion can match a different
2626           length of string. Perl requires them all to have the same length.
2627    
2628           (b)  If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
2629           meta-character matches only at the very end of the string.
2630    
2631           (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2632           cial  meaning  is  faulted.  Otherwise,  like  Perl,  the  backslash is
2633           ignored. (Perl can be made to issue a warning.)
2634    
2635           (d) If PCRE_UNGREEDY is set, the greediness of the  repetition  quanti-
2636           fiers is inverted, that is, by default they are not greedy, but if fol-
2637           lowed by a question mark they are.
2638    
2639           (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2640           tried only at the first matching position in the subject string.
2641    
2642           (f)  The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2643           TURE options for pcre_exec() have no Perl equivalents.
2644    
2645           (g) The callout facility is PCRE-specific.
2646    
2647           (h) The partial matching facility is PCRE-specific.
2648    
2649  REPETITION         (i) Patterns compiled by PCRE can be saved and re-used at a later time,
2650       Repetition is specified by quantifiers, which can follow any         even on different hosts that have the other endianness.
      of the following items:  
2651    
2652         a single character, possibly escaped         (j)  The  alternative  matching function (pcre_dfa_exec()) matches in a
2653         the . metacharacter         different way and is not Perl-compatible.
        a character class  
        a back reference (see next section)  
        a parenthesized subpattern (unless it is  an  assertion  -  
      see below)  
   
      The general repetition quantifier specifies  a  minimum  and  
      maximum  number  of  permitted  matches,  by  giving the two  
      numbers in curly brackets (braces), separated  by  a  comma.  
      The  numbers  must be less than 65536, and the first must be  
      less than or equal to the second. For example:  
   
        z{2,4}  
   
      matches "zz", "zzz", or "zzzz". A closing brace on  its  own  
      is not a special character. If the second number is omitted,  
      but the comma is present, there is no upper  limit;  if  the  
      second number and the comma are both omitted, the quantifier  
      specifies an exact number of required matches. Thus  
   
        [aeiou]{3,}  
   
      matches at least 3 successive vowels,  but  may  match  many  
      more, while  
   
        \d{8}  
   
      matches exactly 8 digits.  An  opening  curly  bracket  that  
      appears  in a position where a quantifier is not allowed, or  
      one that does not match the syntax of a quantifier, is taken  
      as  a literal character. For example, {,6} is not a quantif-  
      ier, but a literal string of four characters.  
   
      The quantifier {0} is permitted, causing the  expression  to  
      behave  as  if the previous item and the quantifier were not  
      present.  
   
      For convenience (and  historical  compatibility)  the  three  
      most common quantifiers have single-character abbreviations:  
   
        *    is equivalent to {0,}  
        +    is equivalent to {1,}  
        ?    is equivalent to {0,1}  
   
      It is possible to construct infinite loops  by  following  a  
      subpattern  that  can  match no characters with a quantifier  
      that has no upper limit, for example:  
   
        (a?)*  
   
      Earlier versions of Perl and PCRE used to give an  error  at  
      compile  time  for such patterns. However, because there are  
      cases where this  can  be  useful,  such  patterns  are  now  
      accepted,  but  if  any repetition of the subpattern does in  
      fact match no characters, the loop is forcibly broken.  
   
      By default, the quantifiers  are  "greedy",  that  is,  they  
      match  as much as possible (up to the maximum number of per-  
      mitted times), without causing the rest of  the  pattern  to  
      fail. The classic example of where this gives problems is in  
      trying to match comments in C programs. These appear between  
      the  sequences /* and */ and within the sequence, individual  
      * and / characters may appear. An attempt to  match  C  com-  
      ments by applying the pattern  
   
        /\*.*\*/  
   
      to the string  
   
        /* first command */  not comment  /* second comment */  
   
      fails, because it matches  the  entire  string  due  to  the  
      greediness of the .*  item.  
   
      However, if a quantifier is followed by a question mark,  it  
      ceases  to be greedy, and instead matches the minimum number  
      of times possible, so the pattern  
   
        /\*.*?\*/  
   
      does the right thing with the C comments. The meaning of the  
      various  quantifiers is not otherwise changed, just the pre-  
      ferred number of matches.  Do not confuse this use of  ques-  
      tion  mark  with  its  use as a quantifier in its own right.  
      Because it has two uses, it can sometimes appear doubled, as  
      in  
   
        \d??\d  
   
      which matches one digit by preference, but can match two  if  
      that is the only way the rest of the pattern matches.  
   
      If the PCRE_UNGREEDY option is set (an option which  is  not  
      available  in  Perl),  the  quantifiers  are  not  greedy by  
      default, but individual ones can be made greedy by following  
      them  with  a  question mark. In other words, it inverts the  
      default behaviour.  
   
      When a parenthesized subpattern is quantified with a minimum  
      repeat  count  that is greater than 1 or with a limited max-  
      imum, more store is required for the  compiled  pattern,  in  
      proportion to the size of the minimum or maximum.  
   
      If a pattern starts with .* or  .{0,}  and  the  PCRE_DOTALL  
      option (equivalent to Perl's /s) is set, thus allowing the .  
      to match  newlines,  the  pattern  is  implicitly  anchored,  
      because whatever follows will be tried against every charac-  
      ter position in the subject string, so there is no point  in  
      retrying  the overall match at any position after the first.  
      PCRE treats such a pattern as though it were preceded by \A.  
      In  cases where it is known that the subject string contains  
      no newlines, it is worth setting PCRE_DOTALL when  the  pat-  
      tern begins with .* in order to obtain this optimization, or  
      alternatively using ^ to indicate anchoring explicitly.  
   
      When a capturing subpattern is repeated, the value  captured  
      is the substring that matched the final iteration. For exam-  
      ple, after  
   
        (tweedle[dume]{3}\s*)+  
   
      has matched "tweedledum tweedledee" the value  of  the  cap-  
      tured  substring  is  "tweedledee".  However,  if  there are  
      nested capturing  subpatterns,  the  corresponding  captured