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

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

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

revision 71 by nigel, Sat Feb 24 21:40:24 2007 UTC revision 79 by nigel, Sat Feb 24 21:40:52 2007 UTC
# Line 1  Line 1 
1    -----------------------------------------------------------------------------
2  This file contains a concatenation of the PCRE man pages, converted to plain  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  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  that do not have a man page processor. The small individual files that give
# Line 5  synopses of each function in the library Line 6  synopses of each function in the library
6  separate text files for the pcregrep and pcretest commands.  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  DESCRIPTION  INTRODUCTION
18    
19       The PCRE library is a set of functions that implement  regu-         The  PCRE  library is a set of functions that implement regular expres-
20       lar  expression  pattern  matching using the same syntax and         sion pattern matching using the same syntax and semantics as Perl, with
21       semantics as Perl, with just a few differences. The  current         just  a  few  differences.  The current implementation of PCRE (release
22       implementation  of  PCRE  (release 4.x) corresponds approxi-         6.x) corresponds approximately with Perl  5.8,  including  support  for
23       mately with Perl 5.8, including support  for  UTF-8  encoded         UTF-8 encoded strings and Unicode general category properties. However,
24       strings.    However,  this  support  has  to  be  explicitly         this support has to be explicitly enabled; it is not the default.
25       enabled; it is not the default.  
26           In addition to the Perl-compatible matching function,  PCRE  also  con-
27       PCRE is written in C and released as a C library. However, a         tains  an  alternative matching function that matches the same compiled
28       number  of  people  have  written wrappers and interfaces of         patterns in a different way. In certain circumstances, the  alternative
29       various kinds. A C++ class is included  in  these  contribu-         function  has  some  advantages.  For  a discussion of the two matching
30       tions,  which  can  be found in the Contrib directory at the         algorithms, see the pcrematching page.
31       primary FTP site, which is:  
32           PCRE is written in C and released as a C library. A  number  of  people
33       ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre         have  written  wrappers and interfaces of various kinds. In particular,
34           Google Inc.  have provided a comprehensive C++  wrapper.  This  is  now
35       Details of exactly which Perl  regular  expression  features         included as part of the PCRE distribution. The pcrecpp page has details
36       are  and  are  not  supported  by PCRE are given in separate         of this interface. Other people's contributions can  be  found  in  the
37       documents. See the pcrepattern and pcrecompat pages.         Contrib directory at the primary FTP site, which is:
38    
39       Some features of PCRE can be included, excluded, or  changed         ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
40       when  the library is built. The pcre_config() function makes  
41       it possible for a client  to  discover  which  features  are         Details  of  exactly which Perl regular expression features are and are
42       available.  Documentation  about  building  PCRE for various         not supported by PCRE are given in separate documents. See the pcrepat-
43       operating systems can be found in the  README  file  in  the         tern and pcrecompat pages.
44       source distribution.  
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  USER DOCUMENTATION
60    
61       The user documentation for PCRE has been  split  up  into  a         The user documentation for PCRE comprises a number  of  different  sec-
62       number  of  different sections. In the "man" format, each of         tions.  In the "man" format, each of these is a separate "man page". In
63       these is a separate "man page". In the HTML format, each  is         the HTML format, each is a separate page, linked from the  index  page.
64       a  separate  page,  linked from the index page. In the plain         In  the  plain text format, all the sections are concatenated, for ease
65       text format, all the sections are concatenated, for ease  of         of searching. The sections are as follows:
66       searching. The sections are as follows:  
67             pcre              this document
68         pcre              this document           pcreapi           details of PCRE's native C API
69         pcreapi           details of PCRE's native API           pcrebuild         options for building PCRE
70         pcrebuild         options for building PCRE           pcrecallout       details of the callout feature
71         pcrecallout       details of the callout feature           pcrecompat        discussion of Perl compatibility
72         pcrecompat        discussion of Perl compatibility           pcrecpp           details of the C++ wrapper
73         pcregrep          description of the pcregrep command           pcregrep          description of the pcregrep command
74         pcrepattern       syntax and semantics of supported           pcrematching      discussion of the two matching algorithms
75                             regular expressions           pcrepartial       details of the partial matching facility
76         pcreperform       discussion of performance issues           pcrepattern       syntax and semantics of supported
77         pcreposix         the POSIX-compatible API                               regular expressions
78         pcresample        discussion of the sample program           pcreperform       discussion of performance issues
79         pcretest          the pcretest testing command           pcreposix         the POSIX-compatible C API
80             pcreprecompile    details of saving and re-using precompiled patterns
81       In addition, in the "man" and HTML formats, there is a short           pcresample        discussion of the sample program
82       page  for  each  library function, listing its arguments and           pcretest          description of the pcretest testing command
83       results.  
84           In  addition,  in the "man" and HTML formats, there is a short page for
85           each C library function, listing its arguments and results.
86    
87    
88  LIMITATIONS  LIMITATIONS
89    
90       There are some size limitations in PCRE but it is hoped that         There are some size limitations in PCRE but it is hoped that they  will
91       they will never in practice be relevant.         never in practice be relevant.
92    
93       The maximum length of a  compiled  pattern  is  65539  (sic)         The  maximum  length of a compiled pattern is 65539 (sic) bytes if PCRE
94       bytes  if PCRE is compiled with the default internal linkage         is compiled with the default internal linkage size of 2. If you want to
95       size of 2. If you want to process regular  expressions  that         process  regular  expressions  that are truly enormous, you can compile
96       are  truly  enormous,  you can compile PCRE with an internal         PCRE with an internal linkage size of 3 or 4 (see the  README  file  in
97       linkage size of 3 or 4 (see the README file  in  the  source         the  source  distribution and the pcrebuild documentation for details).
98       distribution  and  the pcrebuild documentation for details).         In these cases the limit is substantially larger.  However,  the  speed
99       If these cases the limit is substantially larger.   However,         of execution will be slower.
100       the speed of execution will be slower.  
101           All values in repeating quantifiers must be less than 65536.  The maxi-
102       All values in repeating quantifiers must be less than 65536.         mum number of capturing subpatterns is 65535.
103       The maximum number of capturing subpatterns is 65535.  
104           There is no limit to the number of non-capturing subpatterns,  but  the
105       There is no limit to the  number  of  non-capturing  subpat-         maximum  depth  of  nesting  of  all kinds of parenthesized subpattern,
106       terns,  but  the  maximum  depth  of nesting of all kinds of         including capturing subpatterns, assertions, and other types of subpat-
107       parenthesized subpattern, including  capturing  subpatterns,         tern, is 200.
108       assertions, and other types of subpattern, is 200.  
109           The  maximum  length of a subject string is the largest positive number
110       The maximum length of a subject string is the largest  posi-         that an integer variable can hold. However, when using the  traditional
111       tive number that an integer variable can hold. However, PCRE         matching function, PCRE uses recursion to handle subpatterns and indef-
112       uses recursion to handle subpatterns and indefinite  repeti-         inite repetition.  This means that the available stack space may  limit
113       tion.  This  means  that the available stack space may limit         the size of a subject string that can be processed by certain patterns.
114       the size of a subject string that can be processed  by  cer-  
115       tain patterns.  
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    
195    
196  UTF-8 SUPPORT  AUTHOR
197    
198       Starting at release 3.3, PCRE has had some support for char-         Philip Hazel
199       acter  strings  encoded in the UTF-8 format. For release 4.0         University Computing Service,
200       this has been greatly extended to cover most common require-         Cambridge CB2 3QG, England.
      ments.  
   
      In order process UTF-8  strings,  you  must  build  PCRE  to  
      include  UTF-8  support  in  the code, and, in addition, you  
      must call pcre_compile() with  the  PCRE_UTF8  option  flag.  
      When  you  do this, both the pattern and any subject strings  
      that are matched against it are  treated  as  UTF-8  strings  
      instead of just strings of bytes.  
   
      If you compile PCRE with UTF-8 support, but do not use it at  
      run  time,  the  library will be a bit bigger, but the addi-  
      tional run time overhead is limited to testing the PCRE_UTF8  
      flag in several places, so should not be very large.  
   
      The following comments apply when PCRE is running  in  UTF-8  
      mode:  
   
      1. When you set the PCRE_UTF8 flag, the  strings  passed  as  
      patterns  and  subjects are checked for validity on entry to  
      the relevant  functions.  If  an  invalid  UTF-8  string  is  
      passed,  an  error  return is given. In some situations, you  
      may already know that your strings are valid, and  therefore  
      want  to  skip these checks in order to improve performance.  
      If you set the PCRE_NO_UTF8_CHECK flag at compile time or at  
      run  time,  PCRE  assumes  that the pattern or subject it is  
      given (respectively) contains only  valid  UTF-8  codes.  In  
      this  case, it does not diagnose an invalid UTF-8 string. If  
      you  pass   an   invalid   UTF-8   string   to   PCRE   when  
      PCRE_NO_UTF8_CHECK  is  set, the results are undefined. Your  
      program may crash.  
   
      2. In a pattern, the escape sequence \x{...}, where the con-  
      tents  of  the  braces is a string of hexadecimal digits, is  
      interpreted as a UTF-8 character whose code  number  is  the  
      given  hexadecimal  number, for example: \x{1234}. If a non-  
      hexadecimal digit appears between the braces,  the  item  is  
      not  recognized.  This escape sequence can be used either as  
      a literal, or within a character class.  
   
      3. The original hexadecimal escape sequence, \xhh, matches a  
      two-byte UTF-8 character if the value is greater than 127.  
   
      4. Repeat quantifiers apply to  complete  UTF-8  characters,  
      not to individual bytes, for example: \x{100}{3}.  
   
      5. The dot metacharacter matches one UTF-8 character instead  
      of a single byte.  
   
      6. The escape sequence \C can be used to match a single byte  
      in UTF-8 mode, but its use can lead to some strange effects.  
   
      7. The character escapes \b, \B, \d, \D, \s, \S, \w, and  \W  
      correctly test characters of any code value, but the charac-  
      ters that PCRE recognizes as digits, spaces, or word charac-  
      ters  remain  the  same  set as before, all with values less  
      than 256.  
   
      8. Case-insensitive  matching  applies  only  to  characters  
      whose  values  are  less than 256. PCRE does not support the  
      notion of "case" for higher-valued characters.  
201    
202       9. PCRE does not support the use of Unicode tables and  pro-         Putting  an actual email address here seems to have been a spam magnet,
203       perties or the Perl escapes \p, \P, and \X.         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    Last updated: 07 March 2005
207    Copyright (c) 1997-2005 University of Cambridge.
208    ------------------------------------------------------------------------------
209    
 AUTHOR  
210    
211       Philip Hazel <ph10@cam.ac.uk>  PCREBUILD(3)                                                      PCREBUILD(3)
      University Computing Service,  
      Cambridge CB2 3QG, England.  
      Phone: +44 1223 334714  
212    
 Last updated: 20 August 2003  
 Copyright (c) 1997-2003 University of Cambridge.  
 -----------------------------------------------------------------------------  
213    
214  NAME  NAME
215       PCRE - Perl-compatible regular expressions         PCRE - Perl-compatible regular expressions
216    
217    
218  PCRE BUILD-TIME OPTIONS  PCRE BUILD-TIME OPTIONS
219    
220       This document describes the optional features of  PCRE  that         This  document  describes  the  optional  features  of PCRE that can be
221       can  be  selected when the library is compiled. They are all         selected when the library is compiled. They are all selected, or  dese-
222       selected, or deselected, by providing options to the config-         lected, by providing options to the configure script that is run before
223       ure  script  which  is run before the make command. The com-         the make command. The complete list of  options  for  configure  (which
224       plete list of options  for  configure  (which  includes  the         includes  the  standard  ones such as the selection of the installation
225       standard  ones  such  as  the  selection of the installation         directory) can be obtained by running
226       directory) can be obtained by running  
227             ./configure --help
228         ./configure --help  
229           The following sections describe certain options whose names begin  with
230       The following sections describe certain options whose  names         --enable  or  --disable. These settings specify changes to the defaults
231       begin  with  --enable  or  --disable. These settings specify         for the configure command. Because of the  way  that  configure  works,
232       changes to the defaults for the configure  command.  Because         --enable  and  --disable  always  come  in  pairs, so the complementary
233       of  the  way  that  configure  works, --enable and --disable         option always exists as well, but as it specifies the  default,  it  is
234       always come in pairs, so  the  complementary  option  always         not described.
      exists  as  well, but as it specifies the default, it is not  
      described.  
235    
236    
237  UTF-8 SUPPORT  UTF-8 SUPPORT
238    
239       To build PCRE with support for UTF-8 character strings, add         To build PCRE with support for UTF-8 character strings, add
240    
241             --enable-utf8
242    
243           to  the  configure  command.  Of  itself, this does not make PCRE treat
244           strings as UTF-8. As well as compiling PCRE with this option, you  also
245           have  have to set the PCRE_UTF8 option when you call the pcre_compile()
246           function.
247    
248    
249    UNICODE CHARACTER PROPERTY SUPPORT
250    
251         --enable-utf8         UTF-8 support allows PCRE to process character values greater than  255
252           in  the  strings that it handles. On its own, however, it does not pro-
253           vide any facilities for accessing the properties of such characters. If
254           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    
257       to the configure command. Of itself, this does not make PCRE           --enable-unicode-properties
258       treat  strings as UTF-8. As well as compiling PCRE with this  
259       option, you also have have to set the PCRE_UTF8 option  when         to the configure command. This implies UTF-8 support, even if you  have
260       you call the pcre_compile() function.         not explicitly requested it.
261    
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  CODE VALUE OF NEWLINE
269    
270       By default, PCRE treats character 10 (linefeed) as the  new-         By default, PCRE treats character 10 (linefeed) as the newline  charac-
271       line  character.  This  is  the  normal newline character on         ter. This is the normal newline character on Unix-like systems. You can
272       Unix-like systems. You can compile PCRE to use character  13         compile PCRE to use character 13 (carriage return) instead by adding
273       (carriage return) instead by adding  
274             --enable-newline-is-cr
275         --enable-newline-is-cr  
276           to the configure command. For completeness there is  also  a  --enable-
277       to the configure command. For completeness there is  also  a         newline-is-lf  option,  which explicitly specifies linefeed as the new-
278       --enable-newline-is-lf  option,  which  explicitly specifies         line character.
      linefeed as the newline character.  
279    
280    
281  BUILDING SHARED AND STATIC LIBRARIES  BUILDING SHARED AND STATIC LIBRARIES
282    
283       The PCRE building process uses libtool to build both  shared         The PCRE building process uses libtool to build both shared and  static
284       and  static  Unix libraries by default. You can suppress one         Unix  libraries by default. You can suppress one of these by adding one
285       of these by adding one of         of
286    
287         --disable-shared           --disable-shared
288         --disable-static           --disable-static
289    
290       to the configure command, as required.         to the configure command, as required.
291    
292    
293  POSIX MALLOC USAGE  POSIX MALLOC USAGE
294    
295       When PCRE is called through the  POSIX  interface  (see  the         When PCRE is called through the POSIX interface (see the pcreposix doc-
296       pcreposix  documentation),  additional  working  storage  is         umentation),  additional  working  storage  is required for holding the
297       required for holding the pointers  to  capturing  substrings         pointers to capturing substrings, because PCRE requires three  integers
298       because  PCRE requires three integers per substring, whereas         per  substring,  whereas  the POSIX interface provides only two. If the
299       the POSIX interface provides only  two.  If  the  number  of         number of expected substrings is small, the wrapper function uses space
300       expected  substrings  is  small,  the  wrapper function uses         on the stack, because this is faster than using malloc() for each call.
301       space on the stack, because this is faster than  using  mal-         The default threshold above which the stack is no longer used is 10; it
302       loc()  for  each call. The default threshold above which the         can be changed by adding a setting such as
      stack is no longer used is 10; it can be changed by adding a  
      setting such as  
303    
304         --with-posix-malloc-threshold=20           --with-posix-malloc-threshold=20
305    
306       to the configure command.         to the configure command.
307    
308    
309  LIMITING PCRE RESOURCE USAGE  LIMITING PCRE RESOURCE USAGE
310    
311       Internally, PCRE has a  function  called  match()  which  it         Internally,  PCRE has a function called match(), which it calls repeat-
312       calls  repeatedly  (possibly  recursively) when performing a         edly  (possibly  recursively)  when  matching  a   pattern   with   the
313       matching operation. By limiting the  number  of  times  this         pcre_exec()  function.  By controlling the maximum number of times this
314       function  may  be  called,  a  limit  can  be  placed on the         function may be called during a single matching operation, a limit  can
315       resources used by a single call to  pcre_exec().  The  limit         be  placed  on  the resources used by a single call to pcre_exec(). The
316       can  be  changed  at  run  time, as described in the pcreapi         limit can be changed at run time, as described in the pcreapi  documen-
317       documentation. The default is 10 million, but  this  can  be         tation.  The default is 10 million, but this can be changed by adding a
318       changed by adding a setting such as         setting such as
319    
320         --with-match-limit=500000           --with-match-limit=500000
321    
322       to the configure command.         to  the  configure  command.  This  setting  has  no  effect   on   the
323           pcre_dfa_exec() matching function.
324    
325    
326  HANDLING VERY LARGE PATTERNS  HANDLING VERY LARGE PATTERNS
327    
328       Within a compiled pattern, offset values are used  to  point         Within  a  compiled  pattern,  offset values are used to point from one
329       from  one  part  to  another  (for  example, from an opening         part to another (for example, from an opening parenthesis to an  alter-
330       parenthesis to an  alternation  metacharacter).  By  default         nation  metacharacter).  By default, two-byte values are used for these
331       two-byte  values  are  used  for these offsets, leading to a         offsets, leading to a maximum size for a  compiled  pattern  of  around
332       maximum size for a compiled pattern of around 64K.  This  is         64K.  This  is sufficient to handle all but the most gigantic patterns.
333       sufficient  to  handle  all  but the most gigantic patterns.         Nevertheless, some people do want to process enormous patterns,  so  it
334       Nevertheless, some people do want to process  enormous  pat-         is  possible  to compile PCRE to use three-byte or four-byte offsets by
335       terns,  so  it is possible to compile PCRE to use three-byte         adding a setting such as
336       or four-byte offsets by adding a setting such as  
337             --with-link-size=3
338         --with-link-size=3  
339           to the configure command. The value given must be 2,  3,  or  4.  Using
340       to the configure command. The value given must be 2,  3,  or         longer  offsets slows down the operation of PCRE because it has to load
341       4.  Using  longer  offsets  slows down the operation of PCRE         additional bytes when handling them.
342       because it has to load additional bytes when handling them.  
343           If you build PCRE with an increased link size, test 2 (and  test  5  if
344       If you build PCRE with an increased link size, test  2  (and         you  are using UTF-8) will fail. Part of the output of these tests is a
345       test 5 if you are using UTF-8) will fail. Part of the output         representation of the compiled pattern, and this changes with the  link
346       of these tests is a representation of the compiled  pattern,         size.
347       and this changes with the link size.  
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    
 Last updated: 21 January 2003  
 Copyright (c) 1997-2003 University of Cambridge.  
 -----------------------------------------------------------------------------  
391    
392  NAME  NAME
393       PCRE - Perl-compatible regular expressions         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  SYNOPSIS OF PCRE API         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       #include <pcre.h>         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       pcre *pcre_compile(const char *pattern, int options,           ^<.*>
           const char **errptr, int *erroffset,  
           const unsigned char *tableptr);  
415    
416       pcre_extra *pcre_study(const pcre *code, int options,         is matched against the string
           const char **errptr);  
417    
418       int pcre_exec(const pcre *code, const pcre_extra *extra,           <something> <something else> <something further>
           const char *subject, int length, int startoffset,  
           int options, int *ovector, int ovecsize);  
419    
420       int pcre_copy_named_substring(const pcre *code,         there are three possible answers. The standard algorithm finds only one
421            const char *subject, int *ovector,         of them, whereas the DFA algorithm finds all three.
           int stringcount, const char *stringname,  
           char *buffer, int buffersize);  
422    
      int pcre_copy_substring(const char *subject, int *ovector,  
           int stringcount, int stringnumber, char *buffer,  
           int buffersize);  
423    
424       int pcre_get_named_substring(const pcre *code,  REGULAR EXPRESSIONS AS TREES
           const char *subject, int *ovector,  
           int stringcount, const char *stringname,  
           const char **stringptr);  
425    
426       int pcre_get_stringnumber(const pcre *code,         The set of strings that are matched by a regular expression can be rep-
427            const char *name);         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    
      int pcre_get_substring(const char *subject, int *ovector,  
           int stringcount, int stringnumber,  
           const char **stringptr);  
434    
435       int pcre_get_substring_list(const char *subject,  THE STANDARD MATCHING ALGORITHM
436            int *ovector, int stringcount, const char ***listptr);  
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    
      void pcre_free_substring(const char *stringptr);  
563    
564       void pcre_free_substring_list(const char **stringptr);  PCRE NATIVE API
565    
566       const unsigned char *pcre_maketables(void);         #include <pcre.h>
567    
568       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,         pcre *pcre_compile(const char *pattern, int options,
569            int what, void *where);              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       int pcre_info(const pcre *code, int *optptr, *firstcharptr);         pcre_extra *pcre_study(const pcre *code, int options,
578                const char **errptr);
579    
580       int pcre_config(int what, void *where);         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       char *pcre_version(void);         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       void *(*pcre_malloc)(size_t);         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       void (*pcre_free)(void *);         int pcre_copy_substring(const char *subject, int *ovector,
595                int stringcount, int stringnumber, char *buffer,
596                int buffersize);
597    
598       int (*pcre_callout)(pcre_callout_block *);         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  PCRE API         int pcre_get_substring(const char *subject, int *ovector,
607                int stringcount, int stringnumber,
608                const char **stringptr);
609    
610       PCRE has its own native API,  which  is  described  in  this         int pcre_get_substring_list(const char *subject,
611       document.  There  is  also  a  set of wrapper functions that              int *ovector, int stringcount, const char ***listptr);
      correspond to the POSIX regular expression API.   These  are  
      described in the pcreposix documentation.  
612    
613       The native API function prototypes are defined in the header         void pcre_free_substring(const char *stringptr);
      file  pcre.h,  and  on  Unix  systems  the library itself is  
      called libpcre.a, so can be accessed by adding -lpcre to the  
      command  for  linking  an  application  which  calls it. The  
      header file defines the macros PCRE_MAJOR and PCRE_MINOR  to  
      contain the major and minor release numbers for the library.  
      Applications can use these to include support for  different  
      releases.  
614    
615       The functions pcre_compile(), pcre_study(), and  pcre_exec()         void pcre_free_substring_list(const char **stringptr);
      are  used  for compiling and matching regular expressions. A  
      sample program that demonstrates the simplest way  of  using  
      them  is  given in the file pcredemo.c. The pcresample docu-  
      mentation describes how to run it.  
616    
617       There are convenience functions for extracting captured sub-         const unsigned char *pcre_maketables(void);
      strings from a matched subject string. They are:  
618    
619         pcre_copy_substring()         int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
620         pcre_copy_named_substring()              int what, void *where);
        pcre_get_substring()  
        pcre_get_named_substring()  
        pcre_get_substring_list()  
621    
622       pcre_free_substring()  and  pcre_free_substring_list()   are         int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
      also  provided,  to  free  the  memory  used  for  extracted  
      strings.  
623    
624       The function pcre_maketables() is used (optionally) to build         int pcre_refcount(pcre *code, int adjust);
      a  set of character tables in the current locale for passing  
      to pcre_compile().  
625    
626       The function pcre_fullinfo() is used to find out information         int pcre_config(int what, void *where);
      about a compiled pattern; pcre_info() is an obsolete version  
      which returns only some of the available information, but is  
      retained   for   backwards   compatibility.    The  function  
      pcre_version() returns a pointer to a string containing  the  
      version of PCRE and its date of release.  
627    
628       The global variables  pcre_malloc  and  pcre_free  initially         char *pcre_version(void);
      contain the entry points of the standard malloc() and free()  
      functions respectively. PCRE  calls  the  memory  management  
      functions  via  these  variables,  so  a calling program can  
      replace them if it  wishes  to  intercept  the  calls.  This  
      should be done before calling any PCRE functions.  
629    
630       The global variable pcre_callout initially contains NULL. It         void *(*pcre_malloc)(size_t);
631       can be set by the caller to a "callout" function, which PCRE  
632       will then call at specified points during a matching  opera-         void (*pcre_free)(void *);
633       tion. Details are given in the pcrecallout documentation.  
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  MULTITHREADING
727    
728       The PCRE functions can be used in  multi-threading  applica-         The  PCRE  functions  can be used in multi-threading applications, with
729       tions, with the proviso that the memory management functions         the  proviso  that  the  memory  management  functions  pointed  to  by
730       pointed to by pcre_malloc and  pcre_free,  and  the  callout         pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
731       function  pointed  to  by  pcre_callout,  are  shared by all         callout function pointed to by pcre_callout, are shared by all threads.
732       threads.  
733           The  compiled form of a regular expression is not altered during match-
734       The compiled form of a regular  expression  is  not  altered         ing, so the same compiled pattern can safely be used by several threads
735       during  matching, so the same compiled pattern can safely be         at once.
736       used by several threads at once.  
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  CHECKING BUILD-TIME OPTIONS
747    
748       int pcre_config(int what, void *where);         int pcre_config(int what, void *where);
749    
750       The function pcre_config() makes  it  possible  for  a  PCRE         The function pcre_config() makes it possible for a PCRE client to  dis-
751       client  to  discover  which optional features have been com-         cover which optional features have been compiled into the PCRE library.
752       piled into the PCRE library. The pcrebuild documentation has         The pcrebuild documentation has more details about these optional  fea-
753       more details about these optional features.         tures.
754    
755       The first argument for pcre_config() is an integer, specify-         The  first  argument  for pcre_config() is an integer, specifying which
756       ing  which information is required; the second argument is a         information is required; the second argument is a pointer to a variable
757       pointer to a variable into which the information is  placed.         into  which  the  information  is  placed. The following information is
758       The following information is available:         available:
759    
760         PCRE_CONFIG_UTF8           PCRE_CONFIG_UTF8
761    
762       The output is an integer that is set to one if UTF-8 support         The output is an integer that is set to one if UTF-8 support is  avail-
763       is available; otherwise it is set to zero.         able; otherwise it is set to zero.
764    
765         PCRE_CONFIG_NEWLINE           PCRE_CONFIG_UNICODE_PROPERTIES
766    
767       The output is an integer that is set to  the  value  of  the         The  output  is  an  integer  that is set to one if support for Unicode
768       code  that  is  used for the newline character. It is either         character properties is available; otherwise it is set to zero.
      linefeed (10) or carriage return (13), and  should  normally  
      be the standard character for your operating system.  
769    
770         PCRE_CONFIG_LINK_SIZE           PCRE_CONFIG_NEWLINE
771    
772       The output is an integer that contains the number  of  bytes         The output is an integer that is set to the value of the code  that  is
773       used  for  internal linkage in compiled regular expressions.         used  for the newline character. It is either linefeed (10) or carriage
774       The value is 2, 3, or 4. Larger values allow larger  regular         return (13), and should normally be the  standard  character  for  your
775       expressions  to be compiled, at the expense of slower match-         operating system.
      ing. The default value of 2 is sufficient for  all  but  the  
      most  massive patterns, since it allows the compiled pattern  
      to be up to 64K in size.  
776    
777         PCRE_CONFIG_POSIX_MALLOC_THRESHOLD           PCRE_CONFIG_LINK_SIZE
778    
779       The output is an integer that contains the  threshold  above         The  output  is  an  integer that contains the number of bytes used for
780       which  the POSIX interface uses malloc() for output vectors.         internal linkage in compiled regular expressions. The value is 2, 3, or
781       Further details are given in the pcreposix documentation.         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_MATCH_LIMIT           PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
787    
788       The output is an integer that gives the  default  limit  for         The output is an integer that contains the threshold  above  which  the
789       the   number  of  internal  matching  function  calls  in  a         POSIX  interface  uses malloc() for output vectors. Further details are
790       pcre_exec()  execution.  Further  details  are  given   with         given in the pcreposix documentation.
      pcre_exec() below.  
791    
792             PCRE_CONFIG_MATCH_LIMIT
793    
794  COMPILING A PATTERN         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       pcre *pcre_compile(const char *pattern, int options,         The output is an integer that is set to one if internal recursion  when
801            const char **errptr, int *erroffset,         running pcre_exec() is implemented by recursive function calls that use
802            const unsigned char *tableptr);         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       The function pcre_compile() is called to compile  a  pattern         on the  heap  instead  of  recursive  function  calls.  In  this  case,
805       into  an internal form. The pattern is a C string terminated         pcre_stack_malloc  and  pcre_stack_free  are  called  to  manage memory
806       by a binary zero, and is passed in the argument  pattern.  A         blocks on the heap, thus avoiding the use of the stack.
      pointer  to  a  single  block of memory that is obtained via  
      pcre_malloc is returned. This contains the compiled code and  
      related  data.  The  pcre  type  is defined for the returned  
      block; this is a typedef for a structure whose contents  are  
      not  externally  defined. It is up to the caller to free the  
      memory when it is no longer required.  
   
      Although the compiled code of a PCRE regex  is  relocatable,  
      that is, it does not depend on memory location, the complete  
      pcre data block is not fully relocatable,  because  it  con-  
      tains  a  copy of the tableptr argument, which is an address  
      (see below).  
      The 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  in the pcrepattern documentation). For these options,  
      the contents of the options argument specifies their initial  
      settings  at  the  start  of  compilation and execution. The  
      PCRE_ANCHORED option can be set at the time of  matching  as  
      well as at compile time.  
   
      If errptr is NULL, pcre_compile() returns NULL  immediately.  
      Otherwise, if compilation of a pattern fails, pcre_compile()  
      returns NULL, and sets the variable pointed to by errptr  to  
      point  to a textual error message. The offset from the start  
      of  the  pattern  to  the  character  where  the  error  was  
      discovered   is   placed  in  the  variable  pointed  to  by  
      erroffset, which must not be NULL. If it  is,  an  immediate  
      error is given.  
   
      If the final  argument,  tableptr,  is  NULL,  PCRE  uses  a  
      default  set  of character tables which are built when it is  
      compiled, using the default C  locale.  Otherwise,  tableptr  
      must  be  the result of a call to pcre_maketables(). See the  
      section on locale support below.  
   
      This code fragment shows a typical straightforward  call  to  
      pcre_compile():  
   
        pcre *re;  
        const char *error;  
        int erroffset;  
        re = pcre_compile(  
          "^A.*Z",          /* the pattern */  
          0,                /* default options */  
          &error,           /* for error message */  
          &erroffset,       /* for error offset */  
          NULL);            /* use default character tables */  
   
      The following option bits are defined:  
   
        PCRE_ANCHORED  
   
      If this bit is set, the pattern is forced to be  "anchored",  
      that is, it is constrained to match only at the first match-  
      ing point in the string which is being searched  (the  "sub-  
      ject string"). This effect can also be achieved by appropri-  
      ate 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, and it can be changed within a  pattern  by  a  (?i)  
      option setting.  
   
        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, and no way to set it within a pattern.  
   
        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,  and  it  can  be changed within a pattern by a (?s)  
      option setting. A negative class such as [^a] always matches  
      a  newline  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. Whitespace does not include the VT  charac-  
      ter  (code 11). In addition, characters between an unescaped  
      # outside a character class and the next newline  character,  
      inclusive, are also ignored. This is equivalent to Perl's /x  
      option, and it can be changed within a  pattern  by  a  (?x)  
      option setting.  
   
      This option 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, and it can be changed within  
      a  pattern  by  a  (?m) option setting. If there are no "\n"  
      characters in a subject string, or no occurrences of ^ or  $  
      in a pattern, setting PCRE_MULTILINE has no effect.  
   
        PCRE_NO_AUTO_CAPTURE  
   
      If this option is set, it disables the use of numbered  cap-  
      turing  parentheses  in the pattern. Any opening parenthesis  
      that is not followed by ? behaves as if it were followed  by  
      ?:  but  named  parentheses  can still be used for capturing  
      (and they acquire numbers in the usual  way).  There  is  no  
      equivalent of this option in Perl.  
   
        PCRE_UNGREEDY  
   
      This option inverts the "greediness" of the  quantifiers  so  
      that  they  are  not greedy by default, but become greedy if  
      followed by "?". It is not compatible with Perl. It can also  
      be set by a (?U) option setting within the pattern.  
   
        PCRE_UTF8  
   
      This option causes PCRE to regard both the pattern  and  the  
      subject  as  strings  of UTF-8 characters instead of single-  
      byte character strings. However, it  is  available  only  if  
      PCRE  has  been  built to include UTF-8 support. If not, the  
      use of this option provokes an error. Details  of  how  this  
      option  changes  the behaviour of PCRE are given in the sec-  
      tion on UTF-8 support in the main pcre page.  
   
        PCRE_NO_UTF8_CHECK  
   
      When PCRE_UTF8 is set, the validity  of  the  pattern  as  a  
      UTF-8  string  is automatically checked. If an invalid UTF-8  
      sequence of bytes is found, pcre_compile() returns an error.  
      If you already know that your pattern is valid, and you want  
      to skip this check for performance reasons, you can set  the  
      PCRE_NO_UTF8_CHECK  option.  When  it  is set, the effect of  
      passing an invalid UTF-8 string as a pattern  is  undefined.  
      It  may  cause  your program to crash.  Note that there is a  
      similar option  for  suppressing  the  checking  of  subject  
      strings passed to pcre_exec().  
807    
808    
809    COMPILING A 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
1074    
1075       pcre_extra *pcre_study(const pcre *code, int options,         pcre_extra *pcre_study(const pcre *code, int options
1076            const char **errptr);              const char **errptr);
1077    
1078       When a pattern is going to be  used  several  times,  it  is         If a compiled pattern is going to be used several times,  it  is  worth
1079       worth  spending  more time analyzing it in order to speed up         spending more time analyzing it in order to speed up the time taken for
1080       the time taken for matching. The function pcre_study() takes         matching. The function pcre_study() takes a pointer to a compiled  pat-
1081       a  pointer  to  a compiled pattern as its first argument. If         tern as its first argument. If studying the pattern produces additional
1082       studing the pattern  produces  additional  information  that         information that will help speed up matching,  pcre_study()  returns  a
1083       will  help speed up matching, pcre_study() returns a pointer         pointer  to a pcre_extra block, in which the study_data field points to
1084       to a pcre_extra block, in which the study_data field  points         the results of the study.
1085       to the results of the study.  
1086           The  returned  value  from  pcre_study()  can  be  passed  directly  to
1087       The  returned  value  from  a  pcre_study()  can  be  passed         pcre_exec().  However,  a  pcre_extra  block also contains other fields
1088       directly  to pcre_exec(). However, the pcre_extra block also         that can be set by the caller before the block  is  passed;  these  are
1089       contains other fields that can be set by the  caller  before         described below in the section on matching a pattern.
1090       the  block is passed; these are described below. If studying  
1091       the pattern does not  produce  any  additional  information,         If  studying  the  pattern  does not produce any additional information
1092       pcre_study() returns NULL. In that circumstance, if the cal-         pcre_study() returns NULL. In that circumstance, if the calling program
1093       ling program wants to pass  some  of  the  other  fields  to         wants  to  pass  any of the other fields to pcre_exec(), it must set up
1094       pcre_exec(), it must set up its own pcre_extra block.         its own pcre_extra block.
1095    
1096       The second argument contains option  bits.  At  present,  no         The second argument of pcre_study() contains option bits.  At  present,
1097       options  are  defined  for  pcre_study(),  and this argument         no options are defined, and this argument should always be zero.
1098       should always be zero.  
1099           The  third argument for pcre_study() is a pointer for an error message.
1100       The third argument for pcre_study()  is  a  pointer  for  an         If studying succeeds (even if no data is  returned),  the  variable  it
1101       error  message.  If  studying  succeeds  (even if no data is         points  to  is set to NULL. Otherwise it points to a textual error mes-
1102       returned), the variable it points to is set to NULL.  Other-         sage. You should therefore test the error pointer for NULL after  call-
1103       wise it points to a textual error message. You should there-         ing pcre_study(), to be sure that it has run successfully.
1104       fore  test  the  error  pointer  for  NULL   after   calling  
1105       pcre_study(), to be sure that it has run successfully.         This is a typical call to pcre_study():
1106    
1107       This is a typical call to pcre_study():           pcre_extra *pe;
1108             pe = pcre_study(
1109         pcre_extra *pe;             re,             /* result of pcre_compile() */
1110         pe = pcre_study(             0,              /* no options exist */
1111           re,             /* result of pcre_compile() */             &error);        /* set to NULL or points to a message */
1112           0,              /* no options exist */  
1113           &error);        /* set to NULL or points to a message */         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       At present, studying a  pattern  is  useful  only  for  non-         ble starting bytes is created.
      anchored  patterns  that do not have a single fixed starting  
      character. A  bitmap  of  possible  starting  characters  is  
      created.  
1116    
1117    
1118  LOCALE SUPPORT  LOCALE SUPPORT
1119    
1120       PCRE handles caseless matching, and determines whether char-         PCRE  handles  caseless matching, and determines whether characters are
1121       acters  are  letters, digits, or whatever, by reference to a         letters digits, or whatever, by reference to a set of  tables,  indexed
1122       set of tables. When running in UTF-8 mode, this applies only         by  character  value.  When running in UTF-8 mode, this applies only to
1123       to characters with codes less than 256. The library contains         characters with codes less than 128. Higher-valued  codes  never  match
1124       a default set of tables that is created  in  the  default  C         escapes  such  as  \w or \d, but can be tested with \p if PCRE is built
1125       locale  when  PCRE  is compiled. This is used when the final         with Unicode character property support.
1126       argument of pcre_compile() is NULL, and  is  sufficient  for  
1127       many applications.         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       An alternative set of tables can, however, be supplied. Such         NULL, and is sufficient for many applications. An  alternative  set  of
1130       tables  are built by calling the pcre_maketables() function,         tables  can,  however, be supplied. These may be created in a different
1131       which has no arguments, in the relevant locale.  The  result         locale from the default. As more and more applications change to  using
1132       can  then be passed to pcre_compile() as often as necessary.         Unicode, the need for this locale support is expected to die away.
1133       For example, to build and use tables  that  are  appropriate  
1134       for  the French locale (where accented characters with codes         External  tables  are  built by calling the pcre_maketables() function,
1135       greater than 128 are treated as letters), the following code         which has no arguments, in the relevant locale. The result can then  be
1136       could be used:         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         setlocale(LC_CTYPE, "fr");         locale  (where  accented  characters  with  values greater than 128 are
1139         tables = pcre_maketables();         treated as letters), the following code could be used:
1140         re = pcre_compile(..., tables);  
1141             setlocale(LC_CTYPE, "fr_FR");
1142       The  tables  are  built  in  memory  that  is  obtained  via           tables = pcre_maketables();
1143       pcre_malloc.  The  pointer that is passed to pcre_compile is           re = pcre_compile(..., tables);
1144       saved with the compiled pattern, and  the  same  tables  are  
1145       used via this pointer by pcre_study() and pcre_exec(). Thus,         When pcre_maketables() runs, the tables are built  in  memory  that  is
1146       for any single pattern, compilation, studying  and  matching         obtained  via  pcre_malloc. It is the caller's responsibility to ensure
1147       all happen in the same locale, but different patterns can be         that the memory containing the tables remains available for as long  as
1148       compiled in different locales. It is the caller's  responsi-         it is needed.
1149       bility  to  ensure  that  the  memory  containing the tables  
1150       remains available for as long as it is needed.         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
1164    
1165       int pcre_fullinfo(const pcre *code, const pcre_extra *extra,         int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1166            int what, void *where);              int what, void *where);
1167    
1168       The pcre_fullinfo() function  returns  information  about  a         The  pcre_fullinfo() function returns information about a compiled pat-
1169       compiled pattern. It replaces the obsolete pcre_info() func-         tern. It replaces the obsolete pcre_info() function, which is neverthe-
1170       tion, which is nevertheless retained for backwards compabil-         less retained for backwards compability (and is documented below).
1171       ity (and is documented below).  
1172       The first argument for pcre_fullinfo() is a pointer  to  the         The  first  argument  for  pcre_fullinfo() is a pointer to the compiled
1173       compiled  pattern.  The  second  argument  is  the result of         pattern. The second argument is the result of pcre_study(), or NULL  if
1174       pcre_study(), or NULL if the pattern was  not  studied.  The         the  pattern  was not studied. The third argument specifies which piece
1175       third  argument  specifies  which  piece  of  information is         of information is required, and the fourth argument is a pointer  to  a
1176       required, and the fourth argument is a pointer to a variable         variable  to  receive  the  data. The yield of the function is zero for
1177       to  receive  the data. The yield of the function is zero for         success, or one of the following negative numbers:
1178       success, or one of the following negative numbers:  
1179             PCRE_ERROR_NULL       the argument code was NULL
1180         PCRE_ERROR_NULL       the argument code was NULL                                 the argument where was NULL
1181                               the argument where was NULL           PCRE_ERROR_BADMAGIC   the "magic number" was not found
1182         PCRE_ERROR_BADMAGIC   the "magic number" was not found           PCRE_ERROR_BADOPTION  the value of what was invalid
1183         PCRE_ERROR_BADOPTION  the value of what was invalid  
1184           The "magic number" is placed at the start of each compiled  pattern  as
1185       Here is a typical call of  pcre_fullinfo(),  to  obtain  the         an  simple check against passing an arbitrary memory pointer. Here is a
1186       length of the compiled pattern:         typical call of pcre_fullinfo(), to obtain the length of  the  compiled
1187           pattern:
1188         int rc;  
1189         unsigned long int length;           int rc;
1190         rc = pcre_fullinfo(           unsigned long int length;
1191           re,               /* result of pcre_compile() */           rc = pcre_fullinfo(
1192           pe,               /* result of pcre_study(), or NULL */             re,               /* result of pcre_compile() */
1193           PCRE_INFO_SIZE,   /* what is required */             pe,               /* result of pcre_study(), or NULL */
1194           &length);         /* where to put the data */             PCRE_INFO_SIZE,   /* what is required */
1195               &length);         /* where to put the data */
1196       The possible values for the third argument  are  defined  in  
1197       pcre.h, and are as follows:         The  possible  values for the third argument are defined in pcre.h, and
1198           are as follows:
1199         PCRE_INFO_BACKREFMAX  
1200             PCRE_INFO_BACKREFMAX
1201       Return the number of the highest back reference in the  pat-  
1202       tern.  The  fourth argument should point to an int variable.         Return the number of the highest back reference  in  the  pattern.  The
1203       Zero is returned if there are no back references.         fourth  argument  should  point to an int variable. Zero is returned if
1204           there are no back references.
1205         PCRE_INFO_CAPTURECOUNT  
1206             PCRE_INFO_CAPTURECOUNT
1207       Return the number of capturing subpatterns in  the  pattern.  
1208       The fourth argument should point to an int variable.         Return the number of capturing subpatterns in the pattern.  The  fourth
1209           argument should point to an int variable.
1210         PCRE_INFO_FIRSTBYTE  
1211             PCRE_INFO_DEFAULT_TABLES
1212       Return information about  the  first  byte  of  any  matched  
1213       string,  for a non-anchored pattern. (This option used to be         Return  a pointer to the internal default character tables within PCRE.
1214       called PCRE_INFO_FIRSTCHAR; the old name is still recognized         The fourth argument should point to an unsigned char *  variable.  This
1215       for backwards compatibility.)         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       If there is a fixed first byte, e.g. from a pattern such  as         passing a NULL table pointer.
1218       (cat|cow|coyote),  it  is returned in the integer pointed to  
1219       by where. Otherwise, if either           PCRE_INFO_FIRSTBYTE
1220    
1221       (a) the pattern was compiled with the PCRE_MULTILINE option,         Return  information  about  the first byte of any matched string, for a
1222       and every branch starts with "^", or         non-anchored   pattern.   (This    option    used    to    be    called
1223           PCRE_INFO_FIRSTCHAR;  the  old  name  is still recognized for backwards
1224       (b) every  branch  of  the  pattern  starts  with  ".*"  and         compatibility.)
1225       PCRE_DOTALL is not set (if it were set, the pattern would be  
1226       anchored),         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       -1 is returned, indicating that the pattern matches only  at         Otherwise, if either
1229       the  start  of  a subject string or after any newline within  
1230       the string. Otherwise -2 is returned. For anchored patterns,         (a) the pattern was compiled with the PCRE_MULTILINE option, and  every
1231       -2 is returned.         branch starts with "^", or
1232    
1233         PCRE_INFO_FIRSTTABLE         (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       If the pattern was studied, and this resulted  in  the  con-  
1236       struction of a 256-bit table indicating a fixed set of bytes         -1 is returned, indicating that the pattern matches only at  the  start
1237       for the first byte in any matching string, a pointer to  the         of  a  subject string or after any newline within the string. Otherwise
1238       table  is  returned.  Otherwise NULL is returned. The fourth         -2 is returned. For anchored patterns, -2 is returned.
1239       argument should point to an unsigned char * variable.  
1240             PCRE_INFO_FIRSTTABLE
1241         PCRE_INFO_LASTLITERAL  
1242           If the pattern was studied, and this resulted in the construction of  a
1243       Return the value of the rightmost  literal  byte  that  must         256-bit table indicating a fixed set of bytes for the first byte in any
1244       exist  in  any  matched  string, other than at its start, if         matching string, a pointer to the table is returned. Otherwise NULL  is
1245       such a byte has been recorded. The  fourth  argument  should         returned.  The fourth argument should point to an unsigned char * vari-
1246       point  to  an  int variable. If there is no such byte, -1 is         able.
1247       returned. For anchored patterns,  a  last  literal  byte  is  
1248       recorded  only  if  it follows something of variable length.           PCRE_INFO_LASTLITERAL
1249       For example, for the pattern /^a\d+z\d+/ the returned  value  
1250       is "z", but for /^a\dz\d/ the returned value is -1.         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         PCRE_INFO_NAMECOUNT         recorded. The fourth argument should point to an int variable. If there
1253         PCRE_INFO_NAMEENTRYSIZE         is  no such byte, -1 is returned. For anchored patterns, a last literal
1254         PCRE_INFO_NAMETABLE         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       PCRE supports the use of named as well as numbered capturing         /^a\dz\d/ the returned value is -1.
1257       parentheses. The names are just an additional way of identi-  
1258       fying the parentheses,  which  still  acquire  a  number.  A           PCRE_INFO_NAMECOUNT
1259       caller  that  wants  to extract data from a named subpattern           PCRE_INFO_NAMEENTRYSIZE
1260       must convert the name to a number in  order  to  access  the           PCRE_INFO_NAMETABLE
1261       correct  pointers  in  the  output  vector  (described  with  
1262       pcre_exec() below). In order to do this, it must  first  use         PCRE supports the use of named as well as numbered capturing  parenthe-
1263       these  three  values  to  obtain  the name-to-number mapping         ses.  The names are just an additional way of identifying the parenthe-
1264       table for the pattern.         ses,  which  still  acquire  numbers.  A  convenience  function  called
1265           pcre_get_named_substring()  is  provided  for  extracting an individual
1266       The  map  consists  of  a  number  of  fixed-size   entries.         captured substring by name. It is also possible  to  extract  the  data
1267       PCRE_INFO_NAMECOUNT   gives   the  number  of  entries,  and         directly,  by  first converting the name to a number in order to access
1268       PCRE_INFO_NAMEENTRYSIZE gives the size of each  entry;  both         the correct pointers in the output vector (described  with  pcre_exec()
1269       of  these return an int value. The entry size depends on the         below).  To  do the conversion, you need to use the name-to-number map,
1270       length of the longest name.  PCRE_INFO_NAMETABLE  returns  a         which is described by these three values.
1271       pointer to the first entry of the table (a pointer to char).  
1272       The first two bytes of each entry are the number of the cap-         The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1273       turing parenthesis, most significant byte first. The rest of         gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1274       the entry is the corresponding name,  zero  terminated.  The         of each entry; both of these  return  an  int  value.  The  entry  size
1275       names  are  in alphabetical order. For example, consider the         depends  on the length of the longest name. PCRE_INFO_NAMETABLE returns
1276       following pattern (assume PCRE_EXTENDED  is  set,  so  white         a pointer to the first entry of the table  (a  pointer  to  char).  The
1277       space - including newlines - is ignored):         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         (?P<date> (?P<year>(\d\d)?\d\d) -         sponding  name,  zero  terminated. The names are in alphabetical order.
1280         (?P<month>\d\d) - (?P<day>\d\d) )         For example, consider the following pattern  (assume  PCRE_EXTENDED  is
1281           set, so white space - including newlines - is ignored):
1282       There are four named subpatterns,  so  the  table  has  four  
1283       entries,  and  each  entry in the table is eight bytes long.           (?P<date> (?P<year>(\d\d)?\d\d) -
1284       The table is as follows, with non-printing  bytes  shows  in           (?P<month>\d\d) - (?P<day>\d\d) )
1285       hex, and undefined bytes shown as ??:  
1286           There  are  four  named subpatterns, so the table has four entries, and
1287         00 01 d  a  t  e  00 ??         each entry in the table is eight bytes long. The table is  as  follows,
1288         00 05 d  a  y  00 ?? ??         with non-printing bytes shows in hexadecimal, and undefined bytes shown
1289         00 04 m  o  n  t  h  00         as ??:
1290         00 02 y  e  a  r  00 ??  
1291             00 01 d  a  t  e  00 ??
1292       When writing code to extract data  from  named  subpatterns,           00 05 d  a  y  00 ?? ??
1293       remember  that the length of each entry may be different for           00 04 m  o  n  t  h  00
1294       each compiled pattern.           00 02 y  e  a  r  00 ??
1295    
1296         PCRE_INFO_OPTIONS         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       Return a copy of the options with which the pattern was com-         be different for each compiled pattern.
1299       piled.  The fourth argument should point to an unsigned long  
1300       int variable. These option bits are those specified  in  the           PCRE_INFO_OPTIONS
1301       call  to  pcre_compile(),  modified  by any top-level option  
1302       settings within the pattern itself.         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       A pattern is automatically anchored by PCRE if  all  of  its         option bits are those specified in the call to pcre_compile(), modified
1305       top-level alternatives begin with one of the following:         by any top-level option settings within the pattern itself.
1306    
1307         ^     unless PCRE_MULTILINE is set         A  pattern  is  automatically  anchored by PCRE if all of its top-level
1308         \A    always         alternatives begin with one of the following:
1309         \G    always  
1310         .*    if PCRE_DOTALL is set and there are no back           ^     unless PCRE_MULTILINE is set
1311                 references to the subpattern in which .* appears           \A    always
1312             \G    always
1313       For such patterns, the  PCRE_ANCHORED  bit  is  set  in  the           .*    if PCRE_DOTALL is set and there are no back
1314       options returned by pcre_fullinfo().                   references to the subpattern in which .* appears
1315    
1316         PCRE_INFO_SIZE         For such patterns, the PCRE_ANCHORED bit is set in the options returned
1317           by pcre_fullinfo().
1318       Return the size of the compiled pattern, that is, the  value  
1319       that  was  passed as the argument to pcre_malloc() when PCRE           PCRE_INFO_SIZE
1320       was getting memory in which to place the compiled data.  The  
1321       fourth argument should point to a size_t variable.         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         PCRE_INFO_STUDYSIZE         which to place the compiled data. The fourth argument should point to a
1324           size_t variable.
1325       Returns the size  of  the  data  block  pointed  to  by  the  
1326       study_data  field  in a pcre_extra block. That is, it is the           PCRE_INFO_STUDYSIZE
1327       value that was passed to pcre_malloc() when PCRE was getting  
1328       memory into which to place the data created by pcre_study().         Return the size of the data block pointed to by the study_data field in
1329       The fourth argument should point to a size_t variable.         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  OBSOLETE INFO FUNCTION
1336    
1337       int pcre_info(const pcre *code, int *optptr, *firstcharptr);         int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
   
      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_FIRSTBYTE above).  
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  MATCHING A PATTERN           PCRE_ERROR_INTERNAL       (-14)
1736    
1737       int pcre_exec(const pcre *code, const pcre_extra *extra,         An  unexpected  internal error has occurred. This error could be caused
1738            const char *subject, int length, int startoffset,         by a bug in PCRE or by overwriting of the compiled pattern.
           int options, int *ovector, int ovecsize);  
   
      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.  
   
      Here is an example of a simple call to pcre_exec():  
   
        int rc;  
        int ovector[30];  
        rc = pcre_exec(  
          re,             /* result of pcre_compile() */  
          NULL,           /* we didn't study the pattern */  
          "some string",  /* the subject string */  
          11,             /* the length of the subject string */  
          0,              /* start at offset 0 in the subject */  
          0,              /* default options */  
          ovector,        /* vector for substring information */  
          30);            /* number of elements in the vector */  
   
      If the extra argument is  not  NULL,  it  must  point  to  a  
      pcre_extra  data  block.  The  pcre_study() function returns  
      such a block (when it doesn't return NULL), but you can also  
      create  one for yourself, and pass additional information in  
      it. The fields in the block are as follows:  
   
        unsigned long int flags;  
        void *study_data;  
        unsigned long int match_limit;  
        void *callout_data;  
   
      The flags field is a bitmap  that  specifies  which  of  the  
      other fields are set. The flag bits are:  
   
        PCRE_EXTRA_STUDY_DATA  
        PCRE_EXTRA_MATCH_LIMIT  
        PCRE_EXTRA_CALLOUT_DATA  
   
      Other flag bits should be set to zero. The study_data  field  
      is   set  in  the  pcre_extra  block  that  is  returned  by  
      pcre_study(), together with the appropriate  flag  bit.  You  
      should  not  set this yourself, but you can add to the block  
      by setting the other fields.  
   
      The match_limit field provides a means  of  preventing  PCRE  
      from  using  up a vast amount of resources when running pat-  
      terns that are not going to match, but  which  have  a  very  
      large  number  of  possibilities  in their search trees. The  
      classic example is the  use  of  nested  unlimited  repeats.  
      Internally,  PCRE  uses  a  function called match() which it  
      calls  repeatedly  (sometimes  recursively).  The  limit  is  
      imposed  on the number of times this function is called dur-  
      ing a match, which has the effect of limiting the amount  of  
      recursion and backtracking that can take place. For patterns  
      that are not anchored, the count starts from zero  for  each  
      position in the subject string.  
   
      The default limit for the library can be set  when  PCRE  is  
      built;  the default default is 10 million, which handles all  
      but the most extreme cases. You can reduce  the  default  by  
      suppling  pcre_exec()  with  a  pcre_extra  block  in  which  
      match_limit   is   set   to    a    smaller    value,    and  
      PCRE_EXTRA_MATCH_LIMIT  is  set  in  the flags field. If the  
      limit      is      exceeded,       pcre_exec()       returns  
      PCRE_ERROR_MATCHLIMIT.  
   
      The pcre_callout field is used in conjunction with the "cal-  
      lout"  feature,  which is described in the pcrecallout docu-  
      mentation.  
   
      The PCRE_ANCHORED option can be passed in the options  argu-  
      ment,   whose   unused   bits  must  be  zero.  This  limits  
      pcre_exec() to matching at the first matching position. How-  
      ever,  if  a  pattern  was  compiled  with PCRE_ANCHORED, or  
      turned out to be anchored by virtue of its contents, it can-  
      not be made unachored at matching time.  
   
      When PCRE_UTF8 was set at compile time, the validity of  the  
      subject  as  a  UTF-8 string is automatically checked. If an  
      invalid  UTF-8  sequence  of  bytes  is  found,  pcre_exec()  
      returns  the  error  PCRE_ERROR_BADUTF8. If you already know  
      that your subject is valid, and you want to skip this  check  
      for  performance reasons, you can set the PCRE_NO_UTF8_CHECK  
      option when calling pcre_exec(). When this  option  is  set,  
      the  effect  of passing an invalid UTF-8 string as a subject  
      is undefined. It may cause your program to crash.  
   
      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 to pcre_exec() as a pointer  in  
      subject,  a length in length, and a starting offset in star-  
      toffset. Unlike the pattern string, the subject may  contain  
      binary  zero  bytes.  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.  
   
      If the pattern was compiled with the PCRE_UTF8  option,  the  
      subject  must  be  a sequence of bytes that is a valid UTF-8  
      string.  If  an  invalid  UTF-8  string  is  passed,  PCRE's  
      behaviour is not defined.  
   
      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.  
   
        PCRE_ERROR_NOSUBSTRING    (-7)  
   
      This   error   is   used   by   the   pcre_copy_substring(),  
      pcre_get_substring(),  and  pcre_get_substring_list()  func-  
      tions (see below). It is never returned by pcre_exec().  
   
        PCRE_ERROR_MATCHLIMIT     (-8)  
   
      The recursion and backtracking limit, as  specified  by  the  
      match_limit  field  in a pcre_extra structure (or defaulted)  
      was reached. See the description above.  
   
        PCRE_ERROR_CALLOUT        (-9)  
   
      This error is never generated by pcre_exec() itself.  It  is  
      provided  for  use by callout functions that want to yield a  
      distinctive error code. See  the  pcrecallout  documentation  
      for details.  
1739    
1740         PCRE_ERROR_BADUTF8       (-10)           PCRE_ERROR_BADCOUNT       (-15)
1741    
1742       A string that contains an invalid UTF-8  byte  sequence  was         This error is given if the value of the ovecsize argument is  negative.
      passed as a subject.  
1743    
1744    
1745  EXTRACTING CAPTURED SUBSTRINGS BY NUMBER  EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
1746    
1747       int pcre_copy_substring(const char *subject, int *ovector,         int pcre_copy_substring(const char *subject, int *ovector,
1748            int stringcount, int stringnumber, char *buffer,              int stringcount, int stringnumber, char *buffer,
1749            int buffersize);              int buffersize);
1750    
1751       int pcre_get_substring(const char *subject, int *ovector,         int pcre_get_substring(const char *subject, int *ovector,
1752            int stringcount, int stringnumber,              int stringcount, int stringnumber,
1753            const char **stringptr);              const char **stringptr);
1754    
1755       int pcre_get_substring_list(const char *subject,         int pcre_get_substring_list(const char *subject,
1756            int *ovector, int stringcount, const char ***listptr);              int *ovector, int stringcount, const char ***listptr);
1757    
1758       Captured substrings can be accessed directly  by  using  the         Captured  substrings  can  be  accessed  directly  by using the offsets
1759       offsets returned by pcre_exec() in ovector. For convenience,         returned by pcre_exec() in  ovector.  For  convenience,  the  functions
1760       the functions  pcre_copy_substring(),  pcre_get_substring(),         pcre_copy_substring(),    pcre_get_substring(),    and    pcre_get_sub-
1761       and  pcre_get_substring_list()  are  provided for extracting         string_list() are provided for extracting captured substrings  as  new,
1762       captured  substrings  as  new,   separate,   zero-terminated         separate,  zero-terminated strings. These functions identify substrings
1763       strings.  These functions identify substrings by number. The         by number. The next section describes functions  for  extracting  named
1764       next section describes functions for extracting  named  sub-         substrings.  A  substring  that  contains  a  binary  zero is correctly
1765       strings.   A  substring  that  contains  a  binary  zero  is         extracted and has a further zero added on the end, but  the  result  is
1766       correctly extracted and has a further zero added on the end,         not, of course, a C string.
1767       but the result is not, of course, a C string.  
1768           The  first  three  arguments  are the same for all three of these func-
1769       The first three arguments are the  same  for  all  three  of         tions: subject is the subject string that has  just  been  successfully
1770       these  functions:   subject  is the subject string which has         matched, ovector is a pointer to the vector of integer offsets that was
1771       just been successfully matched, ovector is a pointer to  the         passed to pcre_exec(), and stringcount is the number of substrings that
1772       vector  of  integer  offsets that was passed to pcre_exec(),         were  captured  by  the match, including the substring that matched the
1773       and stringcount is the number of substrings that  were  cap-         entire regular expression. This is the value returned by pcre_exec() if
1774       tured by the match, including the substring that matched the         it  is greater than zero. If pcre_exec() returned zero, indicating that
1775       entire regular expression. This is  the  value  returned  by         it ran out of space in ovector, the value passed as stringcount  should
1776       pcre_exec  if  it  is  greater  than  zero.  If  pcre_exec()         be the number of elements in the vector divided by three.
1777       returned zero, indicating that it ran out of space in  ovec-  
1778       tor,  the  value passed as stringcount should be the size of         The  functions pcre_copy_substring() and pcre_get_substring() extract a
1779       the vector divided by three.         single substring, whose number is given as  stringnumber.  A  value  of
1780       The functions pcre_copy_substring() and pcre_get_substring()         zero  extracts  the  substring that matched the entire pattern, whereas
1781       extract a single substring, whose number is given as string-         higher values  extract  the  captured  substrings.  For  pcre_copy_sub-
1782       number. A value of zero extracts the substring that  matched         string(),  the  string  is  placed  in buffer, whose length is given by
1783       the entire pattern, while higher values extract the captured         buffersize, while for pcre_get_substring() a new  block  of  memory  is
1784       substrings. For pcre_copy_substring(), the string is  placed         obtained  via  pcre_malloc,  and its address is returned via stringptr.
1785       in  buffer,  whose  length is given by buffersize, while for         The yield of the function is the length of the  string,  not  including
1786       pcre_get_substring() a new block of memory is  obtained  via         the terminating zero, or one of
1787       pcre_malloc,  and its address is returned via stringptr. The  
1788       yield of the function is  the  length  of  the  string,  not           PCRE_ERROR_NOMEMORY       (-6)
1789       including the terminating zero, or one of  
1790           The  buffer  was too small for pcre_copy_substring(), or the attempt to
1791         PCRE_ERROR_NOMEMORY       (-6)         get memory failed for pcre_get_substring().
1792    
1793       The buffer was too small for pcre_copy_substring(),  or  the           PCRE_ERROR_NOSUBSTRING    (-7)
1794       attempt to get memory failed for pcre_get_substring().  
1795           There is no substring whose number is stringnumber.
1796         PCRE_ERROR_NOSUBSTRING    (-7)  
1797           The pcre_get_substring_list()  function  extracts  all  available  sub-
1798       There is no substring whose number is stringnumber.         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 pcre_get_substring_list() function extracts  all  avail-         the  memory  block  is returned via listptr, which is also the start of
1801       able  substrings  and builds a list of pointers to them. All         the list of string pointers. The end of the list is marked  by  a  NULL
1802       this is done in a single block of memory which  is  obtained         pointer. The yield of the function is zero if all went well, or
1803       via pcre_malloc. The address of the memory block is returned  
1804       via listptr, which is also the start of the list  of  string           PCRE_ERROR_NOMEMORY       (-6)
1805       pointers.  The  end of the list is marked by a NULL pointer.  
1806       The yield of the function is zero if all went well, or         if the attempt to get the memory block failed.
1807    
1808         PCRE_ERROR_NOMEMORY       (-6)         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       if the attempt to get the memory block failed.         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       When any of these functions encounter a  substring  that  is         string  by inspecting the appropriate offset in ovector, which is nega-
1813       unset, which can happen when capturing subpattern number n+1         tive for unset substrings.
1814       matches some part of the subject, but subpattern n  has  not  
1815       been  used  at all, they return an empty string. This can be         The two convenience functions pcre_free_substring() and  pcre_free_sub-
1816       distinguished  from  a  genuine  zero-length  substring   by         string_list()  can  be  used  to free the memory returned by a previous
1817       inspecting the appropriate offset in ovector, which is nega-         call  of  pcre_get_substring()  or  pcre_get_substring_list(),  respec-
1818       tive for unset substrings.         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       The  two  convenience  functions  pcre_free_substring()  and         However,  PCRE is used in some situations where it is linked via a spe-
1821       pcre_free_substring_list()  can  be  used to free the memory         cial  interface  to  another  programming  language  which  cannot  use
1822       returned by  a  previous  call  of  pcre_get_substring()  or         pcre_free  directly;  it is for these cases that the functions are pro-
1823       pcre_get_substring_list(),  respectively.  They  do  nothing         vided.
      more than call the function pointed to by  pcre_free,  which  
      of  course  could  be called directly from a C program. How-  
      ever, PCRE is used in some situations where it is linked via  
      a  special  interface  to another programming language which  
      cannot use pcre_free directly; it is for  these  cases  that  
      the functions are provided.  
1824    
1825    
1826  EXTRACTING CAPTURED SUBSTRINGS BY NAME  EXTRACTING CAPTURED SUBSTRINGS BY NAME
1827    
1828       int pcre_copy_named_substring(const pcre *code,         int pcre_get_stringnumber(const pcre *code,
1829            const char *subject, int *ovector,              const char *name);
1830            int stringcount, const char *stringname,  
1831            char *buffer, int buffersize);         int pcre_copy_named_substring(const pcre *code,
1832                const char *subject, int *ovector,
1833       int pcre_get_stringnumber(const pcre *code,              int stringcount, const char *stringname,
1834            const char *name);              char *buffer, int buffersize);
1835    
1836       int pcre_get_named_substring(const pcre *code,         int pcre_get_named_substring(const pcre *code,
1837            const char *subject, int *ovector,              const char *subject, int *ovector,
1838            int stringcount, const char *stringname,              int stringcount, const char *stringname,
1839            const char **stringptr);              const char **stringptr);
1840    
1841       To extract a substring by name, you first have to find asso-         To extract a substring by name, you first have to find associated  num-
1842       ciated    number.    This    can    be   done   by   calling         ber.  For example, for this pattern
1843       pcre_get_stringnumber(). The first argument is the  compiled  
1844       pattern,  and  the second is the name. For example, for this           (a+)b(?P<xxx>\d+)...
1845       pattern  
1846           the number of the subpattern called "xxx" is 2. You can find the number
1847         ab(?<xxx>\d+)...         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       the number of the subpattern called "xxx" is  1.  Given  the         function is the subpattern number, or  PCRE_ERROR_NOSUBSTRING  (-7)  if
1850       number,  you can then extract the substring directly, or use         there is no subpattern of that name.
1851       one of the functions described in the previous section.  For  
1852       convenience,  there are also two functions that do the whole         Given the number, you can extract the substring directly, or use one of
1853       job.         the functions described in the previous section. For convenience, there
1854           are also two functions that do the whole job.
1855       Most of the  arguments  of  pcre_copy_named_substring()  and  
1856       pcre_get_named_substring()  are  the  same  as those for the         Most    of    the    arguments   of   pcre_copy_named_substring()   and
1857       functions that  extract  by  number,  and  so  are  not  re-         pcre_get_named_substring() are the same  as  those  for  the  similarly
1858       described here. There are just two differences.         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       First, instead of a substring number, a  substring  name  is         differences:
1861       given.  Second,  there  is  an  extra argument, given at the  
1862       start, which is a pointer to the compiled pattern.  This  is         First,  instead  of a substring number, a substring name is given. Sec-
1863       needed  in order to gain access to the name-to-number trans-         ond, there is an extra argument, given at the start, which is a pointer
1864       lation table.         to  the compiled pattern. This is needed in order to gain access to the
1865           name-to-number translation table.
1866       These functions  call  pcre_get_stringnumber(),  and  if  it  
1867       succeeds,    they   then   call   pcre_copy_substring()   or         These functions call pcre_get_stringnumber(), and if it succeeds,  they
1868       pcre_get_substring(), as appropriate.         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    
2046    
2047    PCRECALLOUT(3)                                                  PCRECALLOUT(3)
2048    
 Last updated: 20 August 2003  
 Copyright (c) 1997-2003 University of Cambridge.  
 -----------------------------------------------------------------------------  
2049    
2050  NAME  NAME
2051       PCRE - Perl-compatible regular expressions         PCRE - Perl-compatible regular expressions
2052    
2053    
2054  PCRE CALLOUTS  PCRE CALLOUTS
2055    
2056       int (*pcre_callout)(pcre_callout_block *);         int (*pcre_callout)(pcre_callout_block *);
   
      PCRE provides a feature called "callout", which is  a  means  
      of  temporarily passing control to the caller of PCRE in the  
      middle of pattern matching. The caller of PCRE  provides  an  
      external  function  by putting its entry point in the global  
      variable pcre_callout. By default,  this  variable  contains  
      NULL, which disables all calling out.  
   
      Within a regular expression, (?C) indicates  the  points  at  
      which  the external function is to be called. Different cal-  
      lout points can be identified by putting a number less  than  
      256  after  the  letter  C.  The default value is zero.  For  
      example, this pattern has two callout points:  
   
        (?C1)9abc(?C2)def  
   
      During matching, when PCRE  reaches  a  callout  point  (and  
      pcre_callout  is  set), the external function is called. Its  
      only argument is a pointer to  a  pcre_callout  block.  This  
      contains the following variables:  
   
        int          version;  
        int          callout_number;  
        int         *offset_vector;  
        const char  *subject;  
        int          subject_length;  
        int          start_match;  
        int          current_position;  
        int          capture_top;  
        int          capture_last;  
        void        *callout_data;  
   
      The version field  is  an  integer  containing  the  version  
      number of the block format. The current version is zero. The  
      version number may change in future if additional fields are  
      added,  but  the  intention  is  never  to remove any of the  
      existing fields.  
   
      The callout_number field contains the number of the callout,  
      as compiled into the pattern (that is, the number after ?C).  
   
      The offset_vector field  is  a  pointer  to  the  vector  of  
      offsets  that  was  passed by the caller to pcre_exec(). The  
      contents can be inspected in  order  to  extract  substrings  
      that  have  been  matched  so  far,  in  the same way as for  
      extracting substrings after a match has completed.  
      The subject and subject_length  fields  contain  copies  the  
      values that were passed to pcre_exec().  
   
      The start_match field contains the offset within the subject  
      at  which  the current match attempt started. If the pattern  
      is not anchored, the callout function may be called  several  
      times for different starting points.  
   
      The current_position field contains the  offset  within  the  
      subject of the current match pointer.  
   
      The capture_top field contains one more than the  number  of  
      the  highest  numbered captured substring so far. If no sub-  
      strings have been captured, the value of capture_top is one.  
   
      The capture_last field  contains  the  number  of  the  most  
      recently captured substring.  
   
      The callout_data field contains a value that  is  passed  to  
      pcre_exec()  by  the  caller  specifically so that it can be  
      passed back in callouts. It is passed  in  the  pcre_callout  
      field  of the pcre_extra data structure. If no such data was  
      passed, the value of callout_data in a pcre_callout block is  
      NULL.  There is a description of the pcre_extra structure in  
      the pcreapi documentation.  
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  RETURN VALUES
2187    
2188       The callout function returns an integer.  If  the  value  is         The external callout function returns an integer to PCRE. If the  value
2189       zero,  matching  proceeds as normal. If the value is greater         is  zero,  matching  proceeds  as  normal. If the value is greater than
2190       than zero, matching fails at the current  point,  but  back-         zero, matching fails at the current point, but  the  testing  of  other
2191       tracking  to test other possibilities goes ahead, just as if         matching possibilities goes ahead, just as if a lookahead assertion had
2192       a lookahead assertion had failed. If the value is less  than         failed. If the value is less than zero, the  match  is  abandoned,  and
2193       zero,  the  match  is abandoned, and pcre_exec() returns the         pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2194       value.  
2195           Negative   values   should   normally   be   chosen  from  the  set  of
2196       Negative values should normally be chosen from  the  set  of         PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2197       PCRE_ERROR_xxx  values.  In  particular,  PCRE_ERROR_NOMATCH         dard  "no  match"  failure.   The  error  number  PCRE_ERROR_CALLOUT is
2198       forces a standard "no  match"  failure.   The  error  number         reserved for use by callout functions; it will never be  used  by  PCRE
2199       PCRE_ERROR_CALLOUT is reserved for use by callout functions;         itself.
2200       it will never be used by PCRE itself.  
2201    Last updated: 28 February 2005
2202    Copyright (c) 1997-2005 University of Cambridge.
2203    ------------------------------------------------------------------------------
2204    
2205    
2206    PCRECOMPAT(3)                                                    PCRECOMPAT(3)
2207    
 Last updated: 21 January 2003  
 Copyright (c) 1997-2003 University of Cambridge.  
 -----------------------------------------------------------------------------  
2208    
2209  NAME  NAME
2210       PCRE - Perl-compatible regular expressions         PCRE - Perl-compatible regular expressions
2211    
2212    
2213  DIFFERENCES FROM PERL  DIFFERENCES BETWEEN PCRE AND PERL
2214    
2215       This document describes the differences  in  the  ways  that         This  document describes the differences in the ways that PCRE and Perl
2216       PCRE  and  Perl  handle regular expressions. The differences         handle regular expressions. The differences  described  here  are  with
2217       described here are with respect to Perl 5.8.         respect to Perl 5.8.
   
      1. 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.  
   
      2. 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.  
   
      3. 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.  
   
      4. The following Perl escape sequences  are  not  supported:  
      \l,  \u,  \L,  \U,  \P, \p, and \X. In fact these are imple-  
      mented by Perl's general string-handling and are not part of  
      its pattern matching engine. If any of these are encountered  
      by PCRE, an error is generated.  
   
      5. PCRE does support the \Q...\E  escape  for  quoting  sub-  
      strings. Characters in between are treated as literals. This  
      is slightly different from Perl in that $  and  @  are  also  
      handled  as  literals inside the quotes. In Perl, they cause  
      variable interpolation (but of course  PCRE  does  not  have  
      variables). Note the following examples:  
   
          Pattern            PCRE matches      Perl matches  
   
          \Qabc$xyz\E        abc$xyz           abc followed by the  
                                                 contents of $xyz  
          \Qabc\$xyz\E       abc\$xyz          abc\$xyz  
          \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz  
   
      In PCRE, the \Q...\E mechanism is not  recognized  inside  a  
      character class.  
   
      8. 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  items  
      (?R),  (?number)  and  (?P>name).  Also,  the PCRE "callout"  
      feature allows an external function to be called during pat-  
      tern matching.  
   
      9. There are some differences that are  concerned  with  the  
      settings  of  captured  strings  when  part  of a pattern is  
      repeated. For example, matching "aba"  against  the  pattern  
      /^(a(b)?)+$/  in Perl leaves $2 unset, but in PCRE it is set  
      to "b".  
   
      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  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 first matching position in the subject string.  
   
      (f)  The  PCRE_NOTBOL,   PCRE_NOTEOL,   PCRE_NOTEMPTY,   and  
      PCRE_NO_AUTO_CAPTURE  options  for  pcre_exec() have no Perl  
      equivalents.  
   
      (g) The (?R), (?number), and (?P>name) constructs allows for  
      recursive  pattern  matching  (Perl  can  do  this using the  
      (?p{code}) construct, which PCRE cannot support.)  
   
      (h) PCRE supports  named  capturing  substrings,  using  the  
      Python syntax.  
   
      (i) PCRE supports the  possessive  quantifier  "++"  syntax,  
      taken from Sun's Java package.  
2218    
2219       (j) The (R) condition, for  testing  recursion,  is  a  PCRE         1.  PCRE does not have full UTF-8 support. Details of what it does have
2220       extension.         are given in the section on UTF-8 support in the main pcre page.
2221    
2222       (k) The callout facility is PCRE-specific.         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    
 Last updated: 03 February 2003  
 Copyright (c) 1997-2003 University of Cambridge.  
 -----------------------------------------------------------------------------  
2327    
2328  NAME  NAME
2329       PCRE - Perl-compatible regular expressions         PCRE - Perl-compatible regular expressions
2330    
2331    
2332  PCRE REGULAR EXPRESSION DETAILS  PCRE REGULAR EXPRESSION DETAILS
2333    
2334       The syntax and semantics of  the  regular  expressions  sup-         The  syntax  and semantics of the regular expressions supported by PCRE
2335       ported  by PCRE are described below. Regular expressions are         are described below. Regular expressions are also described in the Perl
2336       also described in the Perl documentation and in a number  of         documentation  and  in  a  number  of books, some of which have copious
2337       other  books,  some  of which have copious examples. Jeffrey         examples.  Jeffrey Friedl's "Mastering Regular Expressions",  published
2338       Friedl's  "Mastering  Regular  Expressions",  published   by         by  O'Reilly, covers regular expressions in great detail. This descrip-
2339       O'Reilly,  covers them in great detail. The description here         tion of PCRE's regular expressions is intended as reference material.
2340       is intended as reference documentation.  
2341           The original operation of PCRE was on strings of  one-byte  characters.
2342       The basic operation of PCRE is on strings of bytes. However,         However,  there is now also support for UTF-8 character strings. To use
2343       there  is  also  support for UTF-8 character strings. To use         this, you must build PCRE to  include  UTF-8  support,  and  then  call
2344       this support you must build PCRE to include  UTF-8  support,         pcre_compile()  with  the  PCRE_UTF8  option.  How this affects pattern
2345       and  then call pcre_compile() with the PCRE_UTF8 option. How         matching is mentioned in several places below. There is also a  summary
2346       this affects the pattern matching is  mentioned  in  several         of  UTF-8  features  in  the  section on UTF-8 support in the main pcre
2347       places  below.  There is also a summary of UTF-8 features in         page.
2348       the section on UTF-8 support in the main pcre page.  
2349           The remainder of this document discusses the  patterns  that  are  sup-
2350       A regular expression is a pattern that is matched against  a         ported  by  PCRE when its main matching function, pcre_exec(), is used.
2351       subject string from left to right. Most characters stand for         From  release  6.0,   PCRE   offers   a   second   matching   function,
2352       themselves in a pattern, and match the corresponding charac-         pcre_dfa_exec(),  which matches using a different algorithm that is not
2353       ters in the subject. As a trivial example, the pattern         Perl-compatible. The advantages and disadvantages  of  the  alternative
2354           function, and how it differs from the normal function, are discussed in
2355         The quick brown fox         the pcrematching page.
2356    
2357       matches a portion of a subject string that is  identical  to         A regular expression is a pattern that is  matched  against  a  subject
2358       itself.  The  power  of  regular  expressions comes from the         string  from  left  to right. Most characters stand for themselves in a
2359       ability to include alternatives and repetitions in the  pat-         pattern, and match the corresponding characters in the  subject.  As  a
2360       tern.  These  are encoded in the pattern by the use of meta-         trivial example, the pattern
2361       characters, which do not stand for  themselves  but  instead  
2362       are interpreted in some special way.           The quick brown fox
2363    
2364       There are two different sets of meta-characters: those  that         matches a portion of a subject string that is identical to itself. When
2365       are  recognized anywhere in the pattern except within square         caseless matching is specified (the PCRE_CASELESS option), letters  are
2366       brackets, and those that are recognized in square  brackets.         matched  independently  of case. In UTF-8 mode, PCRE always understands
2367       Outside square brackets, the meta-characters are as follows:         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         \      general escape character with several uses         ues, the concept of case is supported if PCRE is compiled with  Unicode
2370         ^      assert start of string (or line, in multiline mode)         property  support,  but  not  otherwise.   If  you want to use caseless
2371         $      assert end of string (or line, in multiline mode)         matching for characters 128 and above, you must  ensure  that  PCRE  is
2372         .      match any character except newline (by default)         compiled with Unicode property support as well as with UTF-8 support.
2373         [      start character class definition  
2374         |      start of alternative branch         The  power  of  regular  expressions  comes from the ability to include
2375         (      start subpattern         alternatives and repetitions in the pattern. These are encoded  in  the
2376         )      end subpattern         pattern by the use of metacharacters, which do not stand for themselves
2377         ?      extends the meaning of (         but instead are interpreted in some special way.
2378                also 0 or 1 quantifier  
2379                also quantifier minimizer         There are two different sets of metacharacters: those that  are  recog-
2380         *      0 or more quantifier         nized  anywhere in the pattern except within square brackets, and those
2381         +      1 or more quantifier         that are recognized in square brackets. Outside  square  brackets,  the
2382                also "possessive quantifier"         metacharacters are as follows:
2383         {      start min/max quantifier  
2384             \      general escape character with several uses
2385       Part of a pattern that is in square  brackets  is  called  a           ^      assert start of string (or line, in multiline mode)
2386       "character  class".  In  a  character  class  the only meta-           $      assert end of string (or line, in multiline mode)
2387       characters are:           .      match any character except newline (by default)
2388             [      start character class definition
2389         \      general escape character           |      start of alternative branch
2390         ^      negate the class, but only if the first character           (      start subpattern
2391         -      indicates character range           )      end subpattern
2392         [      POSIX character class (only if followed by POSIX           ?      extends the meaning of (
2393                  syntax)                  also 0 or 1 quantifier
2394         ]      terminates the character class                  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         The following sections describe the use of each of the  metacharacters.
      meta-characters.  
2411    
2412    
2413  BACKSLASH  BACKSLASH
2414    
2415       The backslash character has several uses. Firstly, if it  is         The backslash character has several uses. Firstly, if it is followed by
2416       followed  by  a  non-alphameric character, it takes away any         a non-alphanumeric character, it takes away any  special  meaning  that
2417       special  meaning  that  character  may  have.  This  use  of         character  may  have.  This  use  of  backslash  as an escape character
2418       backslash  as  an  escape  character applies both inside and         applies both inside and outside character classes.
2419       outside character classes.  
2420           For example, if you want to match a * character, you write  \*  in  the
2421       For example, if you want to match a * character,  you  write         pattern.   This  escaping  action  applies whether or not the following
2422       \*  in the pattern.  This escaping action applies whether or         character would otherwise be interpreted as a metacharacter, so  it  is
2423       not the following character would otherwise  be  interpreted         always  safe  to  precede  a non-alphanumeric with backslash to specify
2424       as  a meta-character, so it is always safe to precede a non-         that it stands for itself. In particular, if you want to match a  back-
2425       alphameric with backslash to  specify  that  it  stands  for         slash, you write \\.
2426       itself. In particular, if you want to match a backslash, you  
2427       write \\.         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       If a pattern is compiled with the PCRE_EXTENDED option, whi-         # outside a character class and the next newline character are ignored.
2430       tespace in the pattern (other than in a character class) and         An escaping backslash can be used to include a whitespace or #  charac-
2431       characters between a # outside a  character  class  and  the         ter as part of the pattern.
2432       next  newline  character  are ignored. An escaping backslash  
2433       can be used to include a whitespace or # character  as  part         If  you  want  to remove the special meaning from a sequence of charac-
2434       of the pattern.         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       If you want to remove the special meaning from a sequence of         sequences in PCRE, whereas in Perl, $ and @ cause  variable  interpola-
2437       characters, you can do so by putting them between \Q and \E.         tion. Note the following examples:
2438       This is different from Perl in that $ and @ are  handled  as  
2439       literals  in  \Q...\E  sequences in PCRE, whereas in Perl, $           Pattern            PCRE matches   Perl matches
2440       and @ cause variable interpolation. Note the following exam-  
2441       ples:           \Qabc$xyz\E        abc$xyz        abc followed by the
2442                                                 contents of $xyz
2443         Pattern            PCRE matches   Perl matches           \Qabc\$xyz\E       abc\$xyz       abc\$xyz
2444             \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
2445         \Qabc$xyz\E        abc$xyz        abc followed by the  
2446           The  \Q...\E  sequence  is recognized both inside and outside character
2447                                             contents of $xyz         classes.
2448         \Qabc\$xyz\E       abc\$xyz       abc\$xyz  
2449         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz     Non-printing characters
2450    
2451       The \Q...\E sequence is recognized both inside  and  outside         A second use of backslash provides a way of encoding non-printing char-
2452       character classes.         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       A second use of backslash provides a way  of  encoding  non-         terminates  a  pattern,  but  when  a pattern is being prepared by text
2455       printing  characters  in patterns in a visible manner. There         editing, it is usually easier  to  use  one  of  the  following  escape
2456       is no restriction on the appearance of non-printing  charac-         sequences than the binary character it represents:
2457       ters,  apart from the binary zero that terminates a pattern,  
2458       but when a pattern is being prepared by text editing, it  is           \a        alarm, that is, the BEL character (hex 07)
2459       usually  easier to use one of the following escape sequences           \cx       "control-x", where x is any character
2460       than the binary character it represents:           \e        escape (hex 1B)
2461             \f        formfeed (hex 0C)
2462         \a        alarm, that is, the BEL character (hex 07)           \n        newline (hex 0A)
2463         \cx       "control-x", where x is any character           \r        carriage return (hex 0D)
2464         \e        escape (hex 1B)           \t        tab (hex 09)
2465         \f        formfeed (hex 0C)           \ddd      character with octal code ddd, or backreference
2466         \n        newline (hex 0A)           \xhh      character with hex code hh
2467         \r        carriage return (hex 0D)           \x{hhh..} character with hex code hhh... (UTF-8 mode only)
2468         \t        tab (hex 09)  
2469         \ddd      character with octal code ddd, or backreference         The  precise  effect of \cx is as follows: if x is a lower case letter,
2470         \xhh      character with hex code hh         it is converted to upper case. Then bit 6 of the character (hex 40)  is
2471         \x{hhh..} character with hex code hhh... (UTF-8 mode only)         inverted.   Thus  \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
2472           becomes hex 7B.
2473       The precise effect of \cx is as follows: if  x  is  a  lower  
2474       case  letter,  it  is converted to upper case. Then bit 6 of         After \x, from zero to two hexadecimal digits are read (letters can  be
2475       the character (hex 40) is inverted.  Thus  \cz  becomes  hex         in  upper or lower case). In UTF-8 mode, any number of hexadecimal dig-
2476       1A, but \c{ becomes hex 3B, while \c; becomes hex 7B.         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       After \x, from zero  to  two  hexadecimal  digits  are  read         7FFFFFFF). If characters other than hexadecimal digits  appear  between
2479       (letters  can be in upper or lower case). In UTF-8 mode, any         \x{  and }, or if there is no terminating }, this form of escape is not
2480       number of hexadecimal digits may appear between \x{  and  },         recognized. Instead, the initial \x will  be  interpreted  as  a  basic
2481       but  the value of the character code must be less than 2**31         hexadecimal  escape, with no following digits, giving a character whose
2482       (that is, the maximum hexadecimal  value  is  7FFFFFFF).  If         value is zero.
2483       characters  other than hexadecimal digits appear between \x{  
2484       and }, or if there is no terminating }, this form of  escape         Characters whose value is less than 256 can be defined by either of the
2485       is  not  recognized.  Instead, the initial \x will be inter-         two  syntaxes for \x when PCRE is in UTF-8 mode. There is no difference
2486       preted as a basic  hexadecimal  escape,  with  no  following         in the way they are handled. For example, \xdc is exactly the  same  as
2487       digits, giving a byte whose value is zero.         \x{dc}.
2488    
2489       Characters whose value is less than 256 can  be  defined  by         After  \0  up  to  two further octal digits are read. In both cases, if
2490       either  of  the  two  syntaxes  for \x when PCRE is in UTF-8         there are fewer than two digits, just those that are present are  used.
2491       mode. There is no difference in the way  they  are  handled.         Thus  the sequence \0\x\07 specifies two binary zeros followed by a BEL
2492       For example, \xdc is exactly the same as \x{dc}.         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       After \0 up to two further octal digits are  read.  In  both         digit.
2495       cases,  if  there are fewer than two digits, just those that  
2496       are present are used. Thus the  sequence  \0\x\07  specifies         The handling of a backslash followed by a digit other than 0 is compli-
2497       two binary zeros followed by a BEL character (code value 7).         cated.  Outside a character class, PCRE reads it and any following dig-
2498       Make sure you supply two digits after the  initial  zero  if         its as a decimal number. If the number is less than  10,  or  if  there
2499       the character that follows is itself an octal digit.         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       The handling of a backslash followed by a digit other than 0         description  of how this works is given later, following the discussion
2502       is  complicated.   Outside  a character class, PCRE reads it         of parenthesized subpatterns.
2503       and any following digits as a decimal number. If the  number  
2504       is  less  than  10, or if there have been at least that many         Inside a character class, or if the decimal number is  greater  than  9
2505       previous capturing left parentheses in the  expression,  the         and  there have not been that many capturing subpatterns, PCRE re-reads
2506       entire  sequence is taken as a back reference. A description         up to three octal digits following the backslash, and generates a  sin-
2507       of how this works is given later, following  the  discussion         gle byte from the least significant 8 bits of the value. Any subsequent
2508       of parenthesized subpatterns.         digits stand for themselves.  For example:
2509    
2510       Inside a character  class,  or  if  the  decimal  number  is           \040   is another way of writing a space
2511       greater  than  9 and there have not been that many capturing           \40    is the same, provided there are fewer than 40
2512       subpatterns, PCRE re-reads up to three octal digits  follow-                     previous capturing subpatterns
2513       ing  the  backslash,  and  generates  a single byte from the           \7     is always a back reference
2514       least significant 8 bits of the value. Any subsequent digits           \11    might be a back reference, or another way of
2515       stand for themselves.  For example:                     writing a tab
2516             \011   is always a tab
2517         \040   is another way of writing a space           \0113  is a tab followed by the character "3"
2518         \40    is the same, provided there are fewer than 40           \113   might be a back reference, otherwise the
2519                   previous capturing subpatterns                     character with octal code 113
2520         \7     is always a back reference           \377   might be a back reference, otherwise
2521         \11    might be a back reference, or another way of                     the byte consisting entirely of 1 bits
2522                   writing a tab           \81    is either a back reference, or a binary zero
2523         \011   is always a tab                     followed by the two characters "8" and "1"
2524         \0113  is a tab followed by the character "3"  
2525         \113   might be a back reference, otherwise the         Note that octal values of 100 or greater must not be  introduced  by  a
2526                   character with octal code 113         leading zero, because no more than three octal digits are ever read.
2527         \377   might be a back reference, otherwise  
2528                   the byte consisting entirely of 1 bits         All  the  sequences  that  define a single byte value or a single UTF-8
2529         \81    is either a back reference, or a binary zero         character (in UTF-8 mode) can be used both inside and outside character
2530                   followed by the two characters "8" and "1"         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       Note that octal values of 100 or greater must not be  intro-         interpreted  as  the  character  "X".  Outside a character class, these
2533       duced  by  a  leading zero, because no more than three octal         sequences have different meanings (see below).
2534       digits are ever read.  
2535       Generic character types
2536       All the sequences that define a single byte value or a  sin-  
2537       gle  UTF-8 character (in UTF-8 mode) can be used both inside         The third use of backslash is for specifying generic  character  types.
2538       and outside character classes. In addition, inside a charac-         The following are always recognized:
2539       ter  class,  the sequence \b is interpreted as the backspace  
2540       character (hex 08). Outside a character class it has a  dif-           \d     any decimal digit
2541       ferent meaning (see below).           \D     any character that is not a decimal digit
2542             \s     any whitespace character
2543       The third use of backslash is for specifying generic charac-           \S     any character that is not a whitespace character
2544       ter types:           \w     any "word" character
2545             \W     any "non-word" character
2546         \d     any decimal digit  
2547         \D     any character that is not a decimal digit         Each pair of escape sequences partitions the complete set of characters
2548         \s     any whitespace character         into two disjoint sets. Any given character matches one, and only  one,
2549         \S     any character that is not a whitespace character         of each pair.
2550         \w     any "word" character  
2551         W     any "non-word" character         These character type sequences can appear both inside and outside char-
2552           acter classes. They each match one character of the  appropriate  type.
2553       Each pair of escape sequences partitions the complete set of         If  the current matching point is at the end of the subject string, all
2554       characters  into  two  disjoint  sets.  Any  given character         of them fail, since there is no character to match.
2555       matches one, and only one, of each pair.  
2556           For compatibility with Perl, \s does not match the VT  character  (code
2557       In UTF-8 mode, characters with values greater than 255 never         11).   This makes it different from the the POSIX "space" class. The \s
2558       match \d, \s, or \w, and always match \D, \S, and \W.         characters are HT (9), LF (10), FF (12), CR (13), and space (32).
2559    
2560       For compatibility with Perl, \s does not match the VT  char-         A "word" character is an underscore or any character less than 256 that
2561       acter (code 11).  This makes it different from the the POSIX         is  a  letter  or  digit.  The definition of letters and digits is con-
2562       "space" class. The \s characters are HT  (9),  LF  (10),  FF         trolled by PCRE's low-valued character tables, and may vary if  locale-
2563       (12), CR (13), and space (32).         specific  matching is taking place (see "Locale support" in the pcreapi
2564           page). For example, in the  "fr_FR"  (French)  locale,  some  character
2565       A "word" character is any letter or digit or the  underscore         codes  greater  than  128  are used for accented letters, and these are
2566       character,  that  is,  any  character which can be part of a         matched by \w.
2567       Perl "word". The definition of letters and  digits  is  con-  
2568       trolled  by PCRE's character tables, and may vary if locale-         In UTF-8 mode, characters with values greater than 128 never match  \d,
2569       specific matching is taking place (see "Locale  support"  in         \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
2570       the pcreapi page). For example, in the "fr" (French) locale,         code character property support is available.
2571       some character codes greater than 128 are used for  accented  
2572       letters, and these are matched by \w.     Unicode character properties
2573    
2574       These character type sequences can appear  both  inside  and         When PCRE is built with Unicode character property support, three addi-
2575       outside  character classes. They each match one character of         tional  escape sequences to match generic character types are available
2576       the appropriate type. If the current matching  point  is  at         when UTF-8 mode is selected. They are:
2577       the end of the subject string, all of them fail, since there  
2578       is no character to match.          \p{xx}   a character with the xx property
2579            \P{xx}   a character without the xx property
2580       The fourth use of backslash is  for  certain  simple  asser-          \X       an extended Unicode sequence
2581       tions. An assertion specifies a condition that has to be met  
2582       at a particular point in  a  match,  without  consuming  any         The property names represented by xx above are limited to  the  Unicode
2583       characters  from  the subject string. The use of subpatterns         general  category properties. Each character has exactly one such prop-
2584       for more complicated  assertions  is  described  below.  The         erty, specified by a two-letter abbreviation.  For  compatibility  with
2585       backslashed assertions are         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         \b     matches at a word boundary         as \P{Lu}.
2588         \B     matches when not at a word boundary  
2589         \A     matches at start of subject         If  only  one  letter  is  specified with \p or \P, it includes all the
2590         \Z     matches at end of subject or before newline at end         properties that start with that letter. In this case, in the absence of
2591         \z     matches at end of subject         negation, the curly brackets in the escape sequence are optional; these
2592         \G     matches at first matching position in subject         two examples have the same effect:
2593    
2594       These assertions may not appear in  character  classes  (but           \p{L}
2595       note  that  \b has a different meaning, namely the backspace           \pL
2596       character, inside a character class).  
2597           The following property codes are supported:
2598       A word boundary is a position in the  subject  string  where  
2599       the current character and the previous character do not both           C     Other
2600       match \w or \W (i.e. one matches \w and  the  other  matches           Cc    Control
2601       \W),  or the start or end of the string if the first or last           Cf    Format
2602       character matches \w, respectively.           Cn    Unassigned
2603       The \A, \Z, and \z assertions differ  from  the  traditional           Co    Private use
2604       circumflex  and  dollar  (described below) in that they only           Cs    Surrogate
2605       ever match at the very start and end of the subject  string,  
2606       whatever options are set. Thus, they are independent of mul-           L     Letter
2607       tiline mode.           Ll    Lower case letter
2608             Lm    Modifier letter
2609       They are not affected  by  the  PCRE_NOTBOL  or  PCRE_NOTEOL           Lo    Other letter
2610       options.  If the startoffset argument of pcre_exec() is non-           Lt    Title case letter
2611       zero, indicating that matching is to start at a point  other           Lu    Upper case letter
2612       than  the  beginning of the subject, \A can never match. The  
2613       difference between \Z and \z is that  \Z  matches  before  a           M     Mark
2614       newline  that is the last character of the string as well as           Mc    Spacing mark
2615       at the end of the string, whereas \z  matches  only  at  the           Me    Enclosing mark
2616       end.           Mn    Non-spacing mark
2617    
2618       The \G assertion is true  only  when  the  current  matching           N     Number
2619       position is at the start point of the match, as specified by           Nd    Decimal number
2620       the startoffset argument of pcre_exec(). It differs from  \A           Nl    Letter number
2621       when  the  value  of  startoffset  is  non-zero.  By calling           No    Other number
2622       pcre_exec() multiple times with appropriate  arguments,  you  
2623       can mimic Perl's /g option, and it is in this kind of imple-           P     Punctuation
2624       mentation where \G can be useful.           Pc    Connector punctuation
2625             Pd    Dash punctuation
2626       Note, however, that PCRE's  interpretation  of  \G,  as  the           Pe    Close punctuation
2627       start of the current match, is subtly different from Perl's,           Pf    Final punctuation
2628       which defines it as the end of the previous match. In  Perl,           Pi    Initial punctuation
2629       these  can  be  different when the previously matched string           Po    Other punctuation
2630       was empty. Because PCRE does just one match at  a  time,  it           Ps    Open punctuation
2631       cannot reproduce this behaviour.  
2632             S     Symbol
2633       If all the alternatives of a  pattern  begin  with  \G,  the           Sc    Currency symbol
2634       expression  is  anchored to the starting match position, and           Sk    Modifier symbol
2635       the "anchored" flag is set in the compiled  regular  expres-           Sm    Mathematical symbol
2636       sion.           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.