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