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revision 456 by ph10, Fri Oct 2 08:53:31 2009 UTC revision 556 by ph10, Tue Oct 26 11:06:44 2010 UTC
# Line 21  published by O'Reilly, covers regular ex Line 21  published by O'Reilly, covers regular ex
21  description of PCRE's regular expressions is intended as reference material.  description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this,  there is now also support for UTF-8 character strings. To use this,
25  PCRE must be built to include UTF-8 support, and you must call  PCRE must be built to include UTF-8 support, and you must call
26  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  is also a special sequence that can be given at the start of a pattern:  is also a special sequence that can be given at the start of a pattern:
# Line 42  in the main Line 42  in the main
42  .\"  .\"
43  page.  page.
44  .P  .P
45    Another special sequence that may appear at the start of a pattern or in
46    combination with (*UTF8) is:
47    .sp
48      (*UCP)
49    .sp
50    This has the same effect as setting the PCRE_UCP option: it causes sequences
51    such as \ed and \ew to use Unicode properties to determine character types,
52    instead of recognizing only characters with codes less than 128 via a lookup
53    table.
54    .P
55  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
56  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
# Line 56  discussed in the Line 66  discussed in the
66  page.  page.
67  .  .
68  .  .
69    .\" HTML <a name="newlines"></a>
70  .SH "NEWLINE CONVENTIONS"  .SH "NEWLINE CONVENTIONS"
71  .rs  .rs
72  .sp  .sp
# Line 83  string with one of the following five se Line 94  string with one of the following five se
94    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
95    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
96  .sp  .sp
97  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to \fBpcre_compile()\fP or
98  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
99  newline sequence, the pattern  newline sequence, the pattern
100  .sp  .sp
# Line 95  Perl-compatible, are recognized only at Line 106  Perl-compatible, are recognized only at
106  they must be in upper case. If more than one of them is present, the last one  they must be in upper case. If more than one of them is present, the last one
107  is used.  is used.
108  .P  .P
109  The newline convention does not affect what the \eR escape sequence matches. By  The newline convention affects the interpretation of the dot metacharacter when
110  default, this is any Unicode newline sequence, for Perl compatibility. However,  PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
111  this can be changed; see the description of \eR in the section entitled  affect what the \eR escape sequence matches. By default, this is any Unicode
112    newline sequence, for Perl compatibility. However, this can be changed; see the
113    description of \eR in the section entitled
114  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
115  .\" </a>  .\" </a>
116  "Newline sequences"  "Newline sequences"
# Line 198  Perl, $ and @ cause variable interpolati Line 211  Perl, $ and @ cause variable interpolati
211    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
212  .sp  .sp
213  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
214    An isolated \eE that is not preceded by \eQ is ignored.
215  .  .
216  .  .
217  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 217  one of the following escape sequences th Line 231  one of the following escape sequences th
231    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
232    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
233    \et        tab (hex 09)    \et        tab (hex 09)
234    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
235    \exhh      character with hex code hh    \exhh      character with hex code hh
236    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
237  .sp  .sp
# Line 295  zero, because no more than three octal d Line 309  zero, because no more than three octal d
309  .P  .P
310  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
311  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
312  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
313  sequences \eR and \eX are interpreted as the characters "R" and "X",  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
314  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
315  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
316  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
317  .  .
318  .  .
319  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 333  syntax for referencing a subpattern as a Line 344  syntax for referencing a subpattern as a
344  later.  later.
345  .\"  .\"
346  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
347  synonymous. The former is a back reference; the latter is a  synonymous. The former is a back reference; the latter is a
348  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
349  .\" </a>  .\" </a>
350  subroutine  subroutine
# Line 341  subroutine Line 352  subroutine
352  call.  call.
353  .  .
354  .  .
355    .\" HTML <a name="genericchartypes"></a>
356  .SS "Generic character types"  .SS "Generic character types"
357  .rs  .rs
358  .sp  .sp
359  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
360  .sp  .sp
361    \ed     any decimal digit    \ed     any decimal digit
362    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
# Line 358  following are always recognized: Line 369  following are always recognized:
369    \ew     any "word" character    \ew     any "word" character
370    \eW     any "non-word" character    \eW     any "non-word" character
371  .sp  .sp
372  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
373  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
374    .\" HTML <a href="#fullstopdot">
375    .\" </a>
376    the "." metacharacter
377    .\"
378    when PCRE_DOTALL is not set.
379  .P  .P
380  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
381    of characters into two disjoint sets. Any given character matches one, and only
382    one, of each pair. The sequences can appear both inside and outside character
383  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
384  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
385  there is no character to match.  there is no character to match.
386  .P  .P
387  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
# Line 372  are HT (9), LF (10), FF (12), CR (13), a Line 390  are HT (9), LF (10), FF (12), CR (13), a
390  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
391  does.  does.
392  .P  .P
393  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  A "word" character is an underscore or any character that is a letter or digit.
394  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  By default, the definition of letters and digits is controlled by PCRE's
395  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
396  meanings from before UTF-8 support was available, mainly for efficiency  place (see
397  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" HTML <a href="pcreapi.html#localesupport">
398  and \eW.  .\" </a>
399    "Locale support"
400    .\"
401    in the
402    .\" HREF
403    \fBpcreapi\fP
404    .\"
405    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
406    or "french" in Windows, some character codes greater than 128 are used for
407    accented letters, and these are then matched by \ew. The use of locales with
408    Unicode is discouraged.
409    .P
410    By default, in UTF-8 mode, characters with values greater than 128 never match
411    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
412    their original meanings from before UTF-8 support was available, mainly for
413    efficiency reasons. However, if PCRE is compiled with Unicode property support,
414    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
415    properties are used to determine character types, as follows:
416    .sp
417      \ed  any character that \ep{Nd} matches (decimal digit)
418      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
419      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
420    .sp
421    The upper case escapes match the inverse sets of characters. Note that \ed
422    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
423    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
424    \eB because they are defined in terms of \ew and \eW. Matching these sequences
425    is noticeably slower when PCRE_UCP is set.
426  .P  .P
427  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
428  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  other sequences, which match only ASCII characters by default, these always
429  The horizontal space characters are:  match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is
430    set. The horizontal space characters are:
431  .sp  .sp
432    U+0009     Horizontal tab    U+0009     Horizontal tab
433    U+0020     Space    U+0020     Space
# Line 412  The vertical space characters are: Line 458  The vertical space characters are:
458    U+0085     Next line    U+0085     Next line
459    U+2028     Line separator    U+2028     Line separator
460    U+2029     Paragraph separator    U+2029     Paragraph separator
 .P  
 A "word" character is an underscore or any character less than 256 that is a  
 letter or digit. The definition of letters and digits is controlled by PCRE's  
 low-valued character tables, and may vary if locale-specific matching is taking  
 place (see  
 .\" HTML <a href="pcreapi.html#localesupport">  
 .\" </a>  
 "Locale support"  
 .\"  
 in the  
 .\" HREF  
 \fBpcreapi\fP  
 .\"  
 page). For example, in a French locale such as "fr_FR" in Unix-like systems,  
 or "french" in Windows, some character codes greater than 128 are used for  
 accented letters, and these are matched by \ew. The use of locales with Unicode  
 is discouraged.  
461  .  .
462  .  .
463  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 468  one of the following sequences: Line 497  one of the following sequences:
497    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
498    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
499  .sp  .sp
500  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to \fBpcre_compile()\fP or
501  \fBpcre_compile2()\fP, but they can be overridden by options given to  \fBpcre_compile2()\fP, but they can be overridden by options given to
502  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
503  which are not Perl-compatible, are recognized only at the very start of a  which are not Perl-compatible, are recognized only at the very start of a
504  pattern, and that they must be in upper case. If more than one of them is  pattern, and that they must be in upper case. If more than one of them is
505  present, the last one is used. They can be combined with a change of newline  present, the last one is used. They can be combined with a change of newline
506  convention, for example, a pattern can start with:  convention; for example, a pattern can start with:
507  .sp  .sp
508    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
509  .sp  .sp
510  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
511    a character class, \eR is treated as an unrecognized escape sequence, and so
512    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
513  .  .
514  .  .
515  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 496  The extra escape sequences are: Line 527  The extra escape sequences are:
527    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
528  .sp  .sp
529  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
530  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
531  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
532  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
533  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
534    .\" </a>
535    next section).
536    .\"
537    Other Perl properties such as "InMusicalSymbols" are not currently supported by
538    PCRE. Note that \eP{Any} does not match any characters, so always causes a
539    match failure.
540  .P  .P
541  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
542  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 513  Those that are not part of an identified Line 550  Those that are not part of an identified
550  .P  .P
551  Arabic,  Arabic,
552  Armenian,  Armenian,
553    Avestan,
554  Balinese,  Balinese,
555    Bamum,
556  Bengali,  Bengali,
557  Bopomofo,  Bopomofo,
558  Braille,  Braille,
559  Buginese,  Buginese,
560  Buhid,  Buhid,
561  Canadian_Aboriginal,  Canadian_Aboriginal,
562    Carian,
563    Cham,
564  Cherokee,  Cherokee,
565  Common,  Common,
566  Coptic,  Coptic,
# Line 528  Cypriot, Line 569  Cypriot,
569  Cyrillic,  Cyrillic,
570  Deseret,  Deseret,
571  Devanagari,  Devanagari,
572    Egyptian_Hieroglyphs,
573  Ethiopic,  Ethiopic,
574  Georgian,  Georgian,
575  Glagolitic,  Glagolitic,
# Line 540  Hangul, Line 582  Hangul,
582  Hanunoo,  Hanunoo,
583  Hebrew,  Hebrew,
584  Hiragana,  Hiragana,
585    Imperial_Aramaic,
586  Inherited,  Inherited,
587    Inscriptional_Pahlavi,
588    Inscriptional_Parthian,
589    Javanese,
590    Kaithi,
591  Kannada,  Kannada,
592  Katakana,  Katakana,
593    Kayah_Li,
594  Kharoshthi,  Kharoshthi,
595  Khmer,  Khmer,
596  Lao,  Lao,
597  Latin,  Latin,
598    Lepcha,
599  Limbu,  Limbu,
600  Linear_B,  Linear_B,
601    Lisu,
602    Lycian,
603    Lydian,
604  Malayalam,  Malayalam,
605    Meetei_Mayek,
606  Mongolian,  Mongolian,
607  Myanmar,  Myanmar,
608  New_Tai_Lue,  New_Tai_Lue,
# Line 557  Nko, Line 610  Nko,
610  Ogham,  Ogham,
611  Old_Italic,  Old_Italic,
612  Old_Persian,  Old_Persian,
613    Old_South_Arabian,
614    Old_Turkic,
615    Ol_Chiki,
616  Oriya,  Oriya,
617  Osmanya,  Osmanya,
618  Phags_Pa,  Phags_Pa,
619  Phoenician,  Phoenician,
620    Rejang,
621  Runic,  Runic,
622    Samaritan,
623    Saurashtra,
624  Shavian,  Shavian,
625  Sinhala,  Sinhala,
626    Sundanese,
627  Syloti_Nagri,  Syloti_Nagri,
628  Syriac,  Syriac,
629  Tagalog,  Tagalog,
630  Tagbanwa,  Tagbanwa,
631  Tai_Le,  Tai_Le,
632    Tai_Tham,
633    Tai_Viet,
634  Tamil,  Tamil,
635  Telugu,  Telugu,
636  Thaana,  Thaana,
# Line 576  Thai, Line 638  Thai,
638  Tibetan,  Tibetan,
639  Tifinagh,  Tifinagh,
640  Ugaritic,  Ugaritic,
641    Vai,
642  Yi.  Yi.
643  .P  .P
644  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
645  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
646  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
647  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
648  .P  .P
649  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
650  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 680  non-UTF-8 mode \eX matches any one chara Line 743  non-UTF-8 mode \eX matches any one chara
743  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
744  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
745  why the traditional escape sequences such as \ed and \ew do not use Unicode  why the traditional escape sequences such as \ed and \ew do not use Unicode
746  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
747    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
748    (*UCP).
749    .
750    .
751    .\" HTML <a name="extraprops"></a>
752    .SS PCRE's additional properties
753    .rs
754    .sp
755    As well as the standard Unicode properties described in the previous
756    section, PCRE supports four more that make it possible to convert traditional
757    escape sequences such as \ew and \es and POSIX character classes to use Unicode
758    properties. PCRE uses these non-standard, non-Perl properties internally when
759    PCRE_UCP is set. They are:
760    .sp
761      Xan   Any alphanumeric character
762      Xps   Any POSIX space character
763      Xsp   Any Perl space character
764      Xwd   Any Perl "word" character
765    .sp
766    Xan matches characters that have either the L (letter) or the N (number)
767    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
768    carriage return, and any other character that has the Z (separator) property.
769    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
770    same characters as Xan, plus underscore.
771  .  .
772  .  .
773  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
# Line 711  For example, when the pattern Line 798  For example, when the pattern
798    (foo)\eKbar    (foo)\eKbar
799  .sp  .sp
800  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
801    .P
802    Perl documents that the use of \eK within assertions is "not well defined". In
803    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
804    ignored in negative assertions.
805  .  .
806  .  .
807  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 735  The backslashed assertions are: Line 826  The backslashed assertions are:
826    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
827    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
828  .sp  .sp
829  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
830  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
831    default it matches the corresponding literal character (for example, \eB
832    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
833    escape sequence" error is generated instead.
834  .P  .P
835  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
836  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
837  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
838  first or last character matches \ew, respectively. Neither PCRE nor Perl has a  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
839  separte "start of word" or "end of word" metasequence. However, whatever  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
840  follows \eb normally determines which it is. For example, the fragment  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
841  \eba matches "a" at the start of a word.  of word" or "end of word" metasequence. However, whatever follows \eb normally
842    determines which it is. For example, the fragment \eba matches "a" at the start
843    of a word.
844  .P  .P
845  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
846  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 828  end of the subject in both modes, and if Line 924  end of the subject in both modes, and if
924  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
925  .  .
926  .  .
927  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
928    .SH "FULL STOP (PERIOD, DOT) AND \eN"
929  .rs  .rs
930  .sp  .sp
931  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
# Line 850  to match it. Line 947  to match it.
947  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
948  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
949  special meaning in a character class.  special meaning in a character class.
950    .P
951    The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not
952    set. In other words, it matches any one character except one that signifies the
953    end of a line.
954  .  .
955  .  .
956  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 876  the lookbehind. Line 977  the lookbehind.
977  .rs  .rs
978  .sp  .sp
979  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
980  square bracket. A closing square bracket on its own is not special by default.  square bracket. A closing square bracket on its own is not special by default.
981  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
982  bracket causes a compile-time error. If a closing square bracket is required as  bracket causes a compile-time error. If a closing square bracket is required as
983  a member of the class, it should be the first data character in the class  a member of the class, it should be the first data character in the class
984  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
# Line 944  characters in both cases. In UTF-8 mode, Line 1045  characters in both cases. In UTF-8 mode,
1045  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1046  property support.  property support.
1047  .P  .P
1048  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and
1049  in a character class, and add the characters that they match to the class. For  \eW may also appear in a character class, and add the characters that they
1050  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
1051  conveniently be used with the upper case character types to specify a more  circumflex can conveniently be used with the upper case character types to
1052  restricted set of characters than the matching lower case type. For example,  specify a more restricted set of characters than the matching lower case type.
1053  the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore.
1054  .P  .P
1055  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1056  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 969  this notation. For example, Line 1070  this notation. For example,
1070    [01[:alpha:]%]    [01[:alpha:]%]
1071  .sp  .sp
1072  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1073  are  are:
1074  .sp  .sp
1075    alnum    letters and digits    alnum    letters and digits
1076    alpha    letters    alpha    letters
# Line 980  are Line 1081  are
1081    graph    printing characters, excluding space    graph    printing characters, excluding space
1082    lower    lower case letters    lower    lower case letters
1083    print    printing characters, including space    print    printing characters, including space
1084    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1085    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1086    upper    upper case letters    upper    upper case letters
1087    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 1001  matches "1", "2", or any non-digit. PCRE Line 1102  matches "1", "2", or any non-digit. PCRE
1102  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1103  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1104  .P  .P
1105  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF-8 mode, characters with values greater than 128 do not match
1106  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1107    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1108    character properties are used. This is achieved by replacing the POSIX classes
1109    by other sequences, as follows:
1110    .sp
1111      [:alnum:]  becomes  \ep{Xan}
1112      [:alpha:]  becomes  \ep{L}
1113      [:blank:]  becomes  \eh
1114      [:digit:]  becomes  \ep{Nd}
1115      [:lower:]  becomes  \ep{Ll}
1116      [:space:]  becomes  \ep{Xps}
1117      [:upper:]  becomes  \ep{Lu}
1118      [:word:]   becomes  \ep{Xwd}
1119    .sp
1120    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1121    classes are unchanged, and match only characters with code points less than
1122    128.
1123  .  .
1124  .  .
1125  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1081  section entitled Line 1198  section entitled
1198  .\" </a>  .\" </a>
1199  "Newline sequences"  "Newline sequences"
1200  .\"  .\"
1201  above. There is also the (*UTF8) leading sequence that can be used to set UTF-8  above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1202  mode; this is equivalent to setting the PCRE_UTF8 option.  to set UTF-8 and Unicode property modes; they are equivalent to setting the
1203    PCRE_UTF8 and the PCRE_UCP options, respectively.
1204  .  .
1205  .  .
1206  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1163  stored. Line 1281  stored.
1281    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1282    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1283  .sp  .sp
1284  A backreference to a numbered subpattern uses the most recent value that is set  A back reference to a numbered subpattern uses the most recent value that is
1285  for that number by any subpattern. The following pattern matches "abcabc" or  set for that number by any subpattern. The following pattern matches "abcabc"
1286  "defdef":  or "defdef":
1287  .sp  .sp
1288    /(?|(abc)|(def))\1/    /(?|(abc)|(def))\e1/
1289  .sp  .sp
1290  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a recursive or "subroutine" call to a numbered subpattern always
1291  refers to the first one in the pattern with the given number. The following  refers to the first one in the pattern with the given number. The following
1292  pattern matches "abcabc" or "defabc":  pattern matches "abcabc" or "defabc":
1293  .sp  .sp
1294    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1295  .sp  .sp
1296    If a
1297    .\" HTML <a href="#conditions">
1298    .\" </a>
1299    condition test
1300    .\"
1301    for a subpattern's having matched refers to a non-unique number, the test is
1302    true if any of the subpatterns of that number have matched.
1303  .P  .P
1304  An alternative approach to using the "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1305  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
1306  .  .
1307  .  .
# Line 1189  if an expression is modified, the number Line 1314  if an expression is modified, the number
1314  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1315  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1316  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1317  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1318    have different names, but PCRE does not.
1319  .P  .P
1320  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1321  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1322  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1323  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1324  .\" </a>  .\" </a>
1325  backreferences,  back references,
1326  .\"  .\"
1327  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1328  .\" </a>  .\" </a>
# Line 1216  extracting the name-to-number translatio Line 1342  extracting the name-to-number translatio
1342  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1343  .P  .P
1344  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
1345  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1346  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1347  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1348  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1349  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1350    name, and in both cases you want to extract the abbreviation. This pattern
1351    (ignoring the line breaks) does the job:
1352  .sp  .sp
1353    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1354    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1234  subpattern, as described in the previous Line 1362  subpattern, as described in the previous
1362  .P  .P
1363  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1364  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1365  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1366  make a reference to a non-unique named subpattern from elsewhere in the  .P
1367  pattern, the one that corresponds to the lowest number is used. For further  If you make a back reference to a non-unique named subpattern from elsewhere in
1368  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1369    used. In the absence of duplicate numbers (see the previous section) this is
1370    the one with the lowest number. If you use a named reference in a condition
1371    test (see the
1372    .\"
1373    .\" HTML <a href="#conditions">
1374    .\" </a>
1375    section about conditions
1376    .\"
1377    below), either to check whether a subpattern has matched, or to check for
1378    recursion, all subpatterns with the same name are tested. If the condition is
1379    true for any one of them, the overall condition is true. This is the same
1380    behaviour as testing by number. For further details of the interfaces for
1381    handling named subpatterns, see the
1382  .\" HREF  .\" HREF
1383  \fBpcreapi\fP  \fBpcreapi\fP
1384  .\"  .\"
1385  documentation.  documentation.
1386  .P  .P
1387  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1388  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1389  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1390    are given to subpatterns with the same number. However, you can give the same
1391    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1392  .  .
1393  .  .
1394  .SH REPETITION  .SH REPETITION
# Line 1263  items: Line 1406  items:
1406    a character class    a character class
1407    a back reference (see next section)    a back reference (see next section)
1408    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1409    a recursive or "subroutine" call to a subpattern    a recursive or "subroutine" call to a subpattern
1410  .sp  .sp
1411  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1412  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1374  worth setting PCRE_DOTALL in order to ob Line 1517  worth setting PCRE_DOTALL in order to ob
1517  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1518  .P  .P
1519  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1520  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1521  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1522  succeeds. Consider, for example:  succeeds. Consider, for example:
1523  .sp  .sp
# Line 1589  references to it always fail by default. Line 1732  references to it always fail by default.
1732  .sp  .sp
1733    (a|(bc))\e2    (a|(bc))\e2
1734  .sp  .sp
1735  always fails if it starts to match "a" rather than "bc". However, if the  always fails if it starts to match "a" rather than "bc". However, if the
1736  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1737  unset value matches an empty string.  unset value matches an empty string.
1738  .P  .P
1739  Because there may be many capturing parentheses in a pattern, all digits  Because there may be many capturing parentheses in a pattern, all digits
# Line 1603  whitespace. Otherwise, the \eg{ syntax o Line 1746  whitespace. Otherwise, the \eg{ syntax o
1746  "Comments"  "Comments"
1747  .\"  .\"
1748  below) can be used.  below) can be used.
1749  .P  .
1750    .SS "Recursive back references"
1751    .rs
1752    .sp
1753  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1754  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1755  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1617  to the previous iteration. In order for Line 1763  to the previous iteration. In order for
1763  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1764  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1765  minimum of zero.  minimum of zero.
1766    .P
1767    Back references of this type cause the group that they reference to be treated
1768    as an
1769    .\" HTML <a href="#atomicgroup">
1770    .\" </a>
1771    atomic group.
1772    .\"
1773    Once the whole group has been matched, a subsequent matching failure cannot
1774    cause backtracking into the middle of the group.
1775  .  .
1776  .  .
1777  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1712  In some cases, the Perl 5.10 escape sequ Line 1867  In some cases, the Perl 5.10 escape sequ
1867  .\" </a>  .\" </a>
1868  (see above)  (see above)
1869  .\"  .\"
1870  can be used instead of a lookbehind assertion to get round the fixed-length  can be used instead of a lookbehind assertion to get round the fixed-length
1871  restriction.  restriction.
1872  .P  .P
1873  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
# Line 1730  different numbers of bytes, are also not Line 1885  different numbers of bytes, are also not
1885  "Subroutine"  "Subroutine"
1886  .\"  .\"
1887  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1888  as the subpattern matches a fixed-length string.  as the subpattern matches a fixed-length string.
1889  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1890  .\" </a>  .\" </a>
1891  Recursion,  Recursion,
# Line 1803  characters that are not "999". Line 1958  characters that are not "999".
1958  .sp  .sp
1959  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1960  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1961  the result of an assertion, or whether a specific capturing subpattern has  the result of an assertion, or whether a specific capturing subpattern has
1962  already been matched. The two possible forms of conditional subpattern are:  already been matched. The two possible forms of conditional subpattern are:
1963  .sp  .sp
1964    (?(condition)yes-pattern)    (?(condition)yes-pattern)
# Line 1821  recursion, a pseudo-condition called DEF Line 1976  recursion, a pseudo-condition called DEF
1976  .sp  .sp
1977  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
1978  condition is true if a capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1979  matched. If there is more than one capturing subpattern with the same number  matched. If there is more than one capturing subpattern with the same number
1980  (see the earlier  (see the earlier
1981  .\"  .\"
1982  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1983  .\" </a>  .\" </a>
# Line 1874  Rewriting the above example to use a nam Line 2029  Rewriting the above example to use a nam
2029  .sp  .sp
2030    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2031  .sp  .sp
2032    If the name used in a condition of this kind is a duplicate, the test is
2033    applied to all subpatterns of the same name, and is true if any one of them has
2034    matched.
2035  .  .
2036  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2037  .rs  .rs
# Line 1887  letter R, for example: Line 2045  letter R, for example:
2045  .sp  .sp
2046  the condition is true if the most recent recursion is into a subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2047  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
2048  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2049    applied to all subpatterns of the same name, and is true if any one of them is
2050    the most recent recursion.
2051  .P  .P
2052  At "top level", all these recursion test conditions are false.  At "top level", all these recursion test conditions are false.
2053  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
2054  .\" </a>  .\" </a>
2055  Recursive patterns  The syntax for recursive patterns
2056  .\"  .\"
2057  are described below.  is described below.
2058  .  .
2059  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2060  .rs  .rs
# Line 1903  If the condition is the string (DEFINE), Line 2063  If the condition is the string (DEFINE),
2063  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
2064  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2065  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
2066  "subroutines" that can be referenced from elsewhere. (The use of  "subroutines" that can be referenced from elsewhere. (The use of
2067  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2068  .\" </a>  .\" </a>
2069  "subroutines"  "subroutines"
# Line 1950  that make up a comment play no part in t Line 2110  that make up a comment play no part in t
2110  .P  .P
2111  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
2112  character class introduces a comment that continues to immediately after the  character class introduces a comment that continues to immediately after the
2113  next newline in the pattern.  next newline character or character sequence in the pattern. Which characters
2114    are interpreted as newlines is controlled by the options passed to
2115    \fBpcre_compile()\fP or by a special sequence at the start of the pattern, as
2116    described in the section entitled
2117    .\" HTML <a href="#recursion">
2118    .\" </a>
2119    "Newline conventions"
2120    .\"
2121    above. Note that end of a comment is a literal newline sequence in the pattern;
2122    escape sequences that happen to represent a newline do not terminate a comment.
2123    For example, consider this pattern when PCRE_EXTENDED is set, and the default
2124    newline convention is in force:
2125    .sp
2126      abc #comment \en still comment
2127    .sp
2128    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2129    a newline in the pattern. The sequence \en is still literal at this stage, so
2130    it does not terminate the comment. Only an actual character with the code value
2131    0x0a does so.
2132  .  .
2133  .  .
2134  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1980  this kind of recursion was subsequently Line 2158  this kind of recursion was subsequently
2158  .P  .P
2159  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2160  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive call of the subpattern of the given number,
2161  provided that it occurs inside that subpattern. (If not, it is a  provided that it occurs inside that subpattern. (If not, it is a
2162  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2163  .\" </a>  .\" </a>
2164  "subroutine"  "subroutine"
# Line 1996  PCRE_EXTENDED option is set so that whit Line 2174  PCRE_EXTENDED option is set so that whit
2174  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2175  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
2176  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2177  Finally there is a closing parenthesis. Note the use of a possessive quantifier  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2178  to avoid backtracking into sequences of non-parentheses.  to avoid backtracking into sequences of non-parentheses.
2179  .P  .P
2180  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
# Line 2044  the match runs for a very long time inde Line 2222  the match runs for a very long time inde
2222  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
2223  before failure can be reported.  before failure can be reported.
2224  .P  .P
2225  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2226  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2227  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2228  .\" HREF  .\" HREF
2229  \fBpcrecallout\fP  \fBpcrecallout\fP
2230  .\"  .\"
# Line 2055  documentation). If the pattern above is Line 2232  documentation). If the pattern above is
2232  .sp  .sp
2233    (ab(cd)ef)    (ab(cd)ef)
2234  .sp  .sp
2235  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2236  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2237  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2238    \e( ( ( [^()]++ | (?R) )* ) \e)  set at a deeper level.
2239       ^                        ^  .P
2240       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2241  .sp  obtain extra memory to store data during a recursion, which it does by using
2242  the string they capture is "ab(cd)ef", the contents of the top level  \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2243  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 has to obtain extra memory to store data during a recursion, which it does by  
 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
2244  .P  .P
2245  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
2246  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 2087  is the actual recursive call. Line 2261  is the actual recursive call.
2261  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2262  treated as an atomic group. That is, once it has matched some of the subject  treated as an atomic group. That is, once it has matched some of the subject
2263  string, it is never re-entered, even if it contains untried alternatives and  string, it is never re-entered, even if it contains untried alternatives and
2264  there is a subsequent matching failure. This can be illustrated by the  there is a subsequent matching failure. This can be illustrated by the
2265  following pattern, which purports to match a palindromic string that contains  following pattern, which purports to match a palindromic string that contains
2266  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2267  .sp  .sp
2268    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2269  .sp  .sp
2270  The idea is that it either matches a single character, or two identical  The idea is that it either matches a single character, or two identical
2271  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2272  it does not if the pattern is longer than three characters. Consider the  it does not if the pattern is longer than three characters. Consider the
2273  subject string "abcba":  subject string "abcba":
2274  .P  .P
2275  At the top level, the first character is matched, but as it is not at the end  At the top level, the first character is matched, but as it is not at the end
2276  of the string, the first alternative fails; the second alternative is taken  of the string, the first alternative fails; the second alternative is taken
2277  and the recursion kicks in. The recursive call to subpattern 1 successfully  and the recursion kicks in. The recursive call to subpattern 1 successfully
2278  matches the next character ("b"). (Note that the beginning and end of line  matches the next character ("b"). (Note that the beginning and end of line
2279  tests are not part of the recursion).  tests are not part of the recursion).
2280  .P  .P
2281  Back at the top level, the next character ("c") is compared with what  Back at the top level, the next character ("c") is compared with what
2282  subpattern 2 matched, which was "a". This fails. Because the recursion is  subpattern 2 matched, which was "a". This fails. Because the recursion is
2283  treated as an atomic group, there are now no backtracking points, and so the  treated as an atomic group, there are now no backtracking points, and so the
2284  entire match fails. (Perl is able, at this point, to re-enter the recursion and  entire match fails. (Perl is able, at this point, to re-enter the recursion and
2285  try the second alternative.) However, if the pattern is written with the  try the second alternative.) However, if the pattern is written with the
# Line 2113  alternatives in the other order, things Line 2287  alternatives in the other order, things
2287  .sp  .sp
2288    ^((.)(?1)\e2|.)$    ^((.)(?1)\e2|.)$
2289  .sp  .sp
2290  This time, the recursing alternative is tried first, and continues to recurse  This time, the recursing alternative is tried first, and continues to recurse
2291  until it runs out of characters, at which point the recursion fails. But this  until it runs out of characters, at which point the recursion fails. But this
2292  time we do have another alternative to try at the higher level. That is the big  time we do have another alternative to try at the higher level. That is the big
2293  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2294  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2295  .P  .P
2296  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that matches all palindromic strings, not just those
2297  with an odd number of characters, it is tempting to change the pattern to this:  with an odd number of characters, it is tempting to change the pattern to this:
2298  .sp  .sp
2299    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2300  .sp  .sp
2301  Again, this works in Perl, but not in PCRE, and for the same reason. When a  Again, this works in Perl, but not in PCRE, and for the same reason. When a
2302  deeper recursion has matched a single character, it cannot be entered again in  deeper recursion has matched a single character, it cannot be entered again in
2303  order to match an empty string. The solution is to separate the two cases, and  order to match an empty string. The solution is to separate the two cases, and
2304  write out the odd and even cases as alternatives at the higher level:  write out the odd and even cases as alternatives at the higher level:
2305  .sp  .sp
2306    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2307  .sp  .sp
2308  If you want to match typical palindromic phrases, the pattern has to ignore all  If you want to match typical palindromic phrases, the pattern has to ignore all
2309  non-word characters, which can be done like this:  non-word characters, which can be done like this:
2310  .sp  .sp
2311    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2312  .sp  .sp
2313  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2314  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2315  the use of the possessive quantifier *+ to avoid backtracking into sequences of  the use of the possessive quantifier *+ to avoid backtracking into sequences of
2316  non-word characters. Without this, PCRE takes a great deal longer (ten times or  non-word characters. Without this, PCRE takes a great deal longer (ten times or
2317  more) to match typical phrases, and Perl takes so long that you think it has  more) to match typical phrases, and Perl takes so long that you think it has
2318  gone into a loop.  gone into a loop.
# Line 2177  matches "sense and sensibility" and "res Line 2351  matches "sense and sensibility" and "res
2351  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
2352  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2353  .P  .P
2354  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2355  group. That is, once it has matched some of the subject string, it is never  group. That is, once it has matched some of the subject string, it is never
2356  re-entered, even if it contains untried alternatives and there is a subsequent  re-entered, even if it contains untried alternatives and there is a subsequent
2357  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2358    call revert to their previous values afterwards.
2359  .P  .P
2360  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2361  case-independence are fixed when the subpattern is defined. They cannot be  case-independence are fixed when the subpattern is defined. They cannot be
# Line 2249  description of the interface to the call Line 2424  description of the interface to the call
2424  documentation.  documentation.
2425  .  .
2426  .  .
2427    .\" HTML <a name="backtrackcontrol"></a>
2428  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2429  .rs  .rs
2430  .sp  .sp
# Line 2264  a backtracking algorithm. With the excep Line 2440  a backtracking algorithm. With the excep
2440  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2441  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2442  .P  .P
2443  If any of these verbs are used in an assertion subpattern, their effect is  If any of these verbs are used in an assertion or subroutine subpattern
2444  confined to that subpattern; it does not extend to the surrounding pattern.  (including recursive subpatterns), their effect is confined to that subpattern;
2445  Note that assertion subpatterns are processed as anchored at the point where  it does not extend to the surrounding pattern. Note that such subpatterns are
2446  they are tested.  processed as anchored at the point where they are tested.
2447  .P  .P
2448  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2449  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2450  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2451  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. An name is a sequence of
2452  are two kinds:  letters, digits, and underscores. If the name is empty, that is, if the closing
2453    parenthesis immediately follows the colon, the effect is as if the colon were
2454    not there. Any number of these verbs may occur in a pattern.
2455    .P
2456    PCRE contains some optimizations that are used to speed up matching by running
2457    some checks at the start of each match attempt. For example, it may know the
2458    minimum length of matching subject, or that a particular character must be
2459    present. When one of these optimizations suppresses the running of a match, any
2460    included backtracking verbs will not, of course, be processed. You can suppress
2461    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2462    when calling \fBpcre_exec()\fP.
2463    .
2464  .  .
2465  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2466  .rs  .rs
2467  .sp  .sp
2468  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2469    followed by a name.
2470  .sp  .sp
2471     (*ACCEPT)     (*ACCEPT)
2472  .sp  .sp
# Line 2289  captured. (This feature was added to PCR Line 2477  captured. (This feature was added to PCR
2477  .sp  .sp
2478    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2479  .sp  .sp
2480  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2481  the outer parentheses.  the outer parentheses.
2482  .sp  .sp
2483    (*FAIL) or (*F)    (*FAIL) or (*F)
# Line 2305  callout feature, as for example in this Line 2493  callout feature, as for example in this
2493  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2494  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2495  .  .
2496    .
2497    .SS "Recording which path was taken"
2498    .rs
2499    .sp
2500    There is one verb whose main purpose is to track how a match was arrived at,
2501    though it also has a secondary use in conjunction with advancing the match
2502    starting point (see (*SKIP) below).
2503    .sp
2504      (*MARK:NAME) or (*:NAME)
2505    .sp
2506    A name is always required with this verb. There may be as many instances of
2507    (*MARK) as you like in a pattern, and their names do not have to be unique.
2508    .P
2509    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2510    to the caller via the \fIpcre_extra\fP data structure, as described in the
2511    .\" HTML <a href="pcreapi.html#extradata">
2512    .\" </a>
2513    section on \fIpcre_extra\fP
2514    .\"
2515    in the
2516    .\" HREF
2517    \fBpcreapi\fP
2518    .\"
2519    documentation. No data is returned for a partial match. Here is an example of
2520    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2521    outputting of (*MARK) data:
2522    .sp
2523      /X(*MARK:A)Y|X(*MARK:B)Z/K
2524      XY
2525       0: XY
2526      MK: A
2527      XZ
2528       0: XZ
2529      MK: B
2530    .sp
2531    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2532    indicates which of the two alternatives matched. This is a more efficient way
2533    of obtaining this information than putting each alternative in its own
2534    capturing parentheses.
2535    .P
2536    A name may also be returned after a failed match if the final path through the
2537    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2538    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2539    starting point for matching is advanced, the final check is often with an empty
2540    string, causing a failure before (*MARK) is reached. For example:
2541    .sp
2542      /X(*MARK:A)Y|X(*MARK:B)Z/K
2543      XP
2544      No match
2545    .sp
2546    There are three potential starting points for this match (starting with X,
2547    starting with P, and with an empty string). If the pattern is anchored, the
2548    result is different:
2549    .sp
2550      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2551      XP
2552      No match, mark = B
2553    .sp
2554    PCRE's start-of-match optimizations can also interfere with this. For example,
2555    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2556    subject length for a match, a shorter subject will not be scanned at all.
2557    .P
2558    Note that similar anomalies (though different in detail) exist in Perl, no
2559    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2560    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2561    .
2562    .
2563  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2564  .rs  .rs
2565  .sp  .sp
2566  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2567  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2568  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2569    the verb. However, when one of these verbs appears inside an atomic group, its
2570    effect is confined to that group, because once the group has been matched,
2571    there is never any backtracking into it. In this situation, backtracking can
2572    "jump back" to the left of the entire atomic group. (Remember also, as stated
2573    above, that this localization also applies in subroutine calls and assertions.)
2574    .P
2575    These verbs differ in exactly what kind of failure occurs when backtracking
2576    reaches them.
2577  .sp  .sp
2578    (*COMMIT)    (*COMMIT)
2579  .sp  .sp
2580  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2581  does not match. Even if the pattern is unanchored, no further attempts to find  outright if the rest of the pattern does not match. Even if the pattern is
2582  a match by advancing the starting point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2583  passed, \fBpcre_exec()\fP is committed to finding a match at the current  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2584  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2585  .sp  .sp
2586    a+(*COMMIT)b    a+(*COMMIT)b
2587  .sp  .sp
2588  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2589  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2590  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2591    (*PRUNE)  match failure.
2592  .sp  .P
2593  This verb causes the match to fail at the current position if the rest of the  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2594  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  unless PCRE's start-of-match optimizations are turned off, as shown in this
2595  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2596  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2597  if there is no match to the right, backtracking cannot cross (*PRUNE).    /(*COMMIT)abc/
2598  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    xyzabc
2599  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2600  be expressed in any other way.    xyzabc\eY
2601      No match
2602    .sp
2603    PCRE knows that any match must start with "a", so the optimization skips along
2604    the subject to "a" before running the first match attempt, which succeeds. When
2605    the optimization is disabled by the \eY escape in the second subject, the match
2606    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2607    starting points.
2608    .sp
2609      (*PRUNE) or (*PRUNE:NAME)
2610    .sp
2611    This verb causes the match to fail at the current starting position in the
2612    subject if the rest of the pattern does not match. If the pattern is
2613    unanchored, the normal "bumpalong" advance to the next starting character then
2614    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2615    reached, or when matching to the right of (*PRUNE), but if there is no match to
2616    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2617    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2618    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2619    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2620    match fails completely; the name is passed back if this is the final attempt.
2621    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2622    pattern (*PRUNE) has the same effect as (*COMMIT).
2623  .sp  .sp
2624    (*SKIP)    (*SKIP)
2625  .sp  .sp
2626  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
2627  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
2628  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2629  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
2630    successful match. Consider:
2631  .sp  .sp
2632    a+(*SKIP)b    a+(*SKIP)b
2633  .sp  .sp
# Line 2352  effect as this example; although it woul Line 2638  effect as this example; although it woul
2638  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2639  instead of skipping on to "c".  instead of skipping on to "c".
2640  .sp  .sp
2641    (*THEN)    (*SKIP:NAME)
2642  .sp  .sp
2643  This verb causes a skip to the next alternation if the rest of the pattern does  When (*SKIP) has an associated name, its behaviour is modified. If the
2644  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2645  current alternation. Its name comes from the observation that it can be used  searched for the most recent (*MARK) that has the same name. If one is found,
2646  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2647    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2648    matching name is found, normal "bumpalong" of one character happens (the
2649    (*SKIP) is ignored).
2650    .sp
2651      (*THEN) or (*THEN:NAME)
2652    .sp
2653    This verb causes a skip to the next alternation in the innermost enclosing
2654    group if the rest of the pattern does not match. That is, it cancels pending
2655    backtracking, but only within the current alternation. Its name comes from the
2656    observation that it can be used for a pattern-based if-then-else block:
2657  .sp  .sp
2658    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2659  .sp  .sp
2660  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2661  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure the matcher skips to the
2662  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2663  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2664    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2665    like (*PRUNE).
2666    .
2667    .P
2668    The above verbs provide four different "strengths" of control when subsequent
2669    matching fails. (*THEN) is the weakest, carrying on the match at the next
2670    alternation. (*PRUNE) comes next, failing the match at the current starting
2671    position, but allowing an advance to the next character (for an unanchored
2672    pattern). (*SKIP) is similar, except that the advance may be more than one
2673    character. (*COMMIT) is the strongest, causing the entire match to fail.
2674    .P
2675    If more than one is present in a pattern, the "stongest" one wins. For example,
2676    consider this pattern, where A, B, etc. are complex pattern fragments:
2677    .sp
2678      (A(*COMMIT)B(*THEN)C|D)
2679    .sp
2680    Once A has matched, PCRE is committed to this match, at the current starting
2681    position. If subsequently B matches, but C does not, the normal (*THEN) action
2682    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2683    overrides.
2684  .  .
2685  .  .
2686  .SH "SEE ALSO"  .SH "SEE ALSO"
2687  .rs  .rs
2688  .sp  .sp
2689  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2690  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3).
2691  .  .
2692  .  .
# Line 2388  Cambridge CB2 3QH, England. Line 2704  Cambridge CB2 3QH, England.
2704  .rs  .rs
2705  .sp  .sp
2706  .nf  .nf
2707  Last updated: 30 September 2009  Last updated: 26 October 2010
2708  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2709  .fi  .fi

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