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# Line 4  PCRE - Perl-compatible regular expressio Line 4  PCRE - Perl-compatible regular expressio
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    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, you must  there is now also support for UTF-8 character strings. To use this,
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  PCRE must be built to include UTF-8 support, and you must call
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  places below. There is also a summary of UTF-8 features in the  is also a special sequence that can be given at the start of a pattern:
28    .sp
29      (*UTF8)
30    .sp
31    Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32    option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33    pattern matching is mentioned in several places below. There is also a summary
34    of UTF-8 features in the
35  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
36  .\" </a>  .\" </a>
37  section on UTF-8 support  section on UTF-8 support
# Line 40  discussed in the Line 56  discussed in the
56  page.  page.
57  .  .
58  .  .
59    .SH "NEWLINE CONVENTIONS"
60    .rs
61    .sp
62    PCRE supports five different conventions for indicating line breaks in
63    strings: a single CR (carriage return) character, a single LF (linefeed)
64    character, the two-character sequence CRLF, any of the three preceding, or any
65    Unicode newline sequence. The
66    .\" HREF
67    \fBpcreapi\fP
68    .\"
69    page has
70    .\" HTML <a href="pcreapi.html#newlines">
71    .\" </a>
72    further discussion
73    .\"
74    about newlines, and shows how to set the newline convention in the
75    \fIoptions\fP arguments for the compiling and matching functions.
76    .P
77    It is also possible to specify a newline convention by starting a pattern
78    string with one of the following five sequences:
79    .sp
80      (*CR)        carriage return
81      (*LF)        linefeed
82      (*CRLF)      carriage return, followed by linefeed
83      (*ANYCRLF)   any of the three above
84      (*ANY)       all Unicode newline sequences
85    .sp
86    These override the default and the options given to \fBpcre_compile()\fP or
87    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
88    newline sequence, the pattern
89    .sp
90      (*CR)a.b
91    .sp
92    changes the convention to CR. That pattern matches "a\enb" because LF is no
93    longer a newline. Note that these special settings, which are not
94    Perl-compatible, are recognized only at the very start of a pattern, and that
95    they must be in upper case. If more than one of them is present, the last one
96    is used.
97    .P
98    The newline convention affects the interpretation of the dot metacharacter when
99    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
100    affect what the \eR escape sequence matches. By default, this is any Unicode
101    newline sequence, for Perl compatibility. However, this can be changed; see the
102    description of \eR in the section entitled
103    .\" HTML <a href="#newlineseq">
104    .\" </a>
105    "Newline sequences"
106    .\"
107    below. A change of \eR setting can be combined with a change of newline
108    convention.
109    .
110    .
111  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
112  .rs  .rs
113  .sp  .sp
# Line 141  The \eQ...\eE sequence is recognized bot Line 209  The \eQ...\eE sequence is recognized bot
209  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
210  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
211  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
212  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
213  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
214  .sp  .sp
215    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
216    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
217    \ee        escape (hex 1B)    \ee        escape (hex 1B)
218    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
219    \en        newline (hex 0A)    \en        linefeed (hex 0A)
220    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
221    \et        tab (hex 09)    \et        tab (hex 09)
222    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
223    \exhh      character with hex code hh    \exhh      character with hex code hh
224    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
225  .sp  .sp
# Line 164  Thus \ecz becomes hex 1A, but \ec{ becom Line 231  Thus \ecz becomes hex 1A, but \ec{ becom
231  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
232  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
233  and }, but the value of the character code must be less than 256 in non-UTF-8  and }, but the value of the character code must be less than 256 in non-UTF-8
234  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
235  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
236  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
237  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
238  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
239    there is no terminating }, this form of escape is not recognized. Instead, the
240    initial \ex will be interpreted as a basic hexadecimal escape, with no
241    following digits, giving a character whose value is zero.
242  .P  .P
243  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
244  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex. There is no difference in the way they are handled. For
# Line 227  zero, because no more than three octal d Line 297  zero, because no more than three octal d
297  .P  .P
298  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
299  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
300  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
301  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
302  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
303  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
304  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
305  .  .
306  .  .
307  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
308  .rs  .rs
309  .sp  .sp
310  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
311  in braces, is an absolute or relative back reference. A named back reference  enclosed in braces, is an absolute or relative back reference. A named back
312  can be coded as \eg{name}. Back references are discussed  reference can be coded as \eg{name}. Back references are discussed
313  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
314  .\" </a>  .\" </a>
315  later,  later,
# Line 254  parenthesized subpatterns. Line 321  parenthesized subpatterns.
321  .\"  .\"
322  .  .
323  .  .
324    .SS "Absolute and relative subroutine calls"
325    .rs
326    .sp
327    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
328    a number enclosed either in angle brackets or single quotes, is an alternative
329    syntax for referencing a subpattern as a "subroutine". Details are discussed
330    .\" HTML <a href="#onigurumasubroutines">
331    .\" </a>
332    later.
333    .\"
334    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
335    synonymous. The former is a back reference; the latter is a
336    .\" HTML <a href="#subpatternsassubroutines">
337    .\" </a>
338    subroutine
339    .\"
340    call.
341    .
342    .
343  .SS "Generic character types"  .SS "Generic character types"
344  .rs  .rs
345  .sp  .sp
346  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:  
347  .sp  .sp
348    \ed     any decimal digit    \ed     any decimal digit
349    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
# Line 271  following are always recognized: Line 356  following are always recognized:
356    \ew     any "word" character    \ew     any "word" character
357    \eW     any "non-word" character    \eW     any "non-word" character
358  .sp  .sp
359  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.
360  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
361    .\" HTML <a href="#fullstopdot">
362    .\" </a>
363    the "." metacharacter
364    .\"
365    when PCRE_DOTALL is not set.
366    .P
367    Each pair of lower and upper case escape sequences partitions the complete set
368    of characters into two disjoint sets. Any given character matches one, and only
369    one, of each pair.
370  .P  .P
371  These character type sequences can appear both inside and outside character  These character type sequences can appear both inside and outside character
372  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
# Line 289  In UTF-8 mode, characters with values gr Line 383  In UTF-8 mode, characters with values gr
383  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
384  character property support is available. These sequences retain their original  character property support is available. These sequences retain their original
385  meanings from before UTF-8 support was available, mainly for efficiency  meanings from before UTF-8 support was available, mainly for efficiency
386  reasons.  reasons. Note that this also affects \eb, because it is defined in terms of \ew
387    and \eW.
388  .P  .P
389  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
390  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  other sequences, these do match certain high-valued codepoints in UTF-8 mode.
# Line 343  accented letters, and these are matched Line 438  accented letters, and these are matched
438  is discouraged.  is discouraged.
439  .  .
440  .  .
441    .\" HTML <a name="newlineseq"></a>
442  .SS "Newline sequences"  .SS "Newline sequences"
443  .rs  .rs
444  .sp  .sp
445  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
446  sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
447  the following:  equivalent to the following:
448  .sp  .sp
449    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
450  .sp  .sp
# Line 368  are added: LS (line separator, U+2028) a Line 464  are added: LS (line separator, U+2028) a
464  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
465  recognized.  recognized.
466  .P  .P
467  Inside a character class, \eR matches the letter "R".  It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
468    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
469    either at compile time or when the pattern is matched. (BSR is an abbrevation
470    for "backslash R".) This can be made the default when PCRE is built; if this is
471    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
472    It is also possible to specify these settings by starting a pattern string with
473    one of the following sequences:
474    .sp
475      (*BSR_ANYCRLF)   CR, LF, or CRLF only
476      (*BSR_UNICODE)   any Unicode newline sequence
477    .sp
478    These override the default and the options given to \fBpcre_compile()\fP or
479    \fBpcre_compile2()\fP, but they can be overridden by options given to
480    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
481    which are not Perl-compatible, are recognized only at the very start of a
482    pattern, and that they must be in upper case. If more than one of them is
483    present, the last one is used. They can be combined with a change of newline
484    convention, for example, a pattern can start with:
485    .sp
486      (*ANY)(*BSR_ANYCRLF)
487    .sp
488    Inside a character class, \eR is treated as an unrecognized escape sequence,
489    and so matches the letter "R" by default, but causes an error if PCRE_EXTRA is
490    set.
491  .  .
492  .  .
493  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 403  Those that are not part of an identified Line 522  Those that are not part of an identified
522  .P  .P
523  Arabic,  Arabic,
524  Armenian,  Armenian,
525    Avestan,
526  Balinese,  Balinese,
527    Bamum,
528  Bengali,  Bengali,
529  Bopomofo,  Bopomofo,
530  Braille,  Braille,
531  Buginese,  Buginese,
532  Buhid,  Buhid,
533  Canadian_Aboriginal,  Canadian_Aboriginal,
534    Carian,
535    Cham,
536  Cherokee,  Cherokee,
537  Common,  Common,
538  Coptic,  Coptic,
# Line 418  Cypriot, Line 541  Cypriot,
541  Cyrillic,  Cyrillic,
542  Deseret,  Deseret,
543  Devanagari,  Devanagari,
544    Egyptian_Hieroglyphs,
545  Ethiopic,  Ethiopic,
546  Georgian,  Georgian,
547  Glagolitic,  Glagolitic,
# Line 430  Hangul, Line 554  Hangul,
554  Hanunoo,  Hanunoo,
555  Hebrew,  Hebrew,
556  Hiragana,  Hiragana,
557    Imperial_Aramaic,
558  Inherited,  Inherited,
559    Inscriptional_Pahlavi,
560    Inscriptional_Parthian,
561    Javanese,
562    Kaithi,
563  Kannada,  Kannada,
564  Katakana,  Katakana,
565    Kayah_Li,
566  Kharoshthi,  Kharoshthi,
567  Khmer,  Khmer,
568  Lao,  Lao,
569  Latin,  Latin,
570    Lepcha,
571  Limbu,  Limbu,
572  Linear_B,  Linear_B,
573    Lisu,
574    Lycian,
575    Lydian,
576  Malayalam,  Malayalam,
577    Meetei_Mayek,
578  Mongolian,  Mongolian,
579  Myanmar,  Myanmar,
580  New_Tai_Lue,  New_Tai_Lue,
# Line 447  Nko, Line 582  Nko,
582  Ogham,  Ogham,
583  Old_Italic,  Old_Italic,
584  Old_Persian,  Old_Persian,
585    Old_South_Arabian,
586    Old_Turkic,
587    Ol_Chiki,
588  Oriya,  Oriya,
589  Osmanya,  Osmanya,
590  Phags_Pa,  Phags_Pa,
591  Phoenician,  Phoenician,
592    Rejang,
593  Runic,  Runic,
594    Samaritan,
595    Saurashtra,
596  Shavian,  Shavian,
597  Sinhala,  Sinhala,
598    Sundanese,
599  Syloti_Nagri,  Syloti_Nagri,
600  Syriac,  Syriac,
601  Tagalog,  Tagalog,
602  Tagbanwa,  Tagbanwa,
603  Tai_Le,  Tai_Le,
604    Tai_Tham,
605    Tai_Viet,
606  Tamil,  Tamil,
607  Telugu,  Telugu,
608  Thaana,  Thaana,
# Line 466  Thai, Line 610  Thai,
610  Tibetan,  Tibetan,
611  Tifinagh,  Tifinagh,
612  Ugaritic,  Ugaritic,
613    Vai,
614  Yi.  Yi.
615  .P  .P
616  Each character has exactly one general category property, specified by a  Each character has exactly one general category property, specified by a
# Line 531  The special property L& is also supporte Line 676  The special property L& is also supporte
676  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
677  a modifier or "other".  a modifier or "other".
678  .P  .P
679  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The Cs (Surrogate) property applies only to characters in the range U+D800 to
680    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
681    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
682    (see the discussion of PCRE_NO_UTF8_CHECK in the
683    .\" HREF
684    \fBpcreapi\fP
685    .\"
686    page). Perl does not support the Cs property.
687    .P
688    The long synonyms for property names that Perl supports (such as \ep{Letter})
689  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
690  properties with "Is".  properties with "Is".
691  .P  .P
# Line 555  atomic group Line 709  atomic group
709  (see below).  (see below).
710  .\"  .\"
711  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
712  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
713  non-UTF-8 mode \eX matches any one character.  non-UTF-8 mode \eX matches any one character.
714  .P  .P
715  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
# Line 592  For example, when the pattern Line 746  For example, when the pattern
746    (foo)\eKbar    (foo)\eKbar
747  .sp  .sp
748  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
749    .P
750    Perl documents that the use of \eK within assertions is "not well defined". In
751    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
752    ignored in negative assertions.
753  .  .
754  .  .
755  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 616  The backslashed assertions are: Line 774  The backslashed assertions are:
774    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
775    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
776  .sp  .sp
777  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
778  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
779    default it matches the corresponding literal character (for example, \eB
780    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
781    escape sequence" error is generated instead.
782  .P  .P
783  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
784  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
785  \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
786  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. Neither PCRE nor Perl has a
787    separte "start of word" or "end of word" metasequence. However, whatever
788    follows \eb normally determines which it is. For example, the fragment
789    \eba matches "a" at the start of a word.
790  .P  .P
791  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
792  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 706  end of the subject in both modes, and if Line 870  end of the subject in both modes, and if
870  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
871  .  .
872  .  .
873  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
874    .SH "FULL STOP (PERIOD, DOT) AND \eN"
875  .rs  .rs
876  .sp  .sp
877  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 728  to match it. Line 893  to match it.
893  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
894  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
895  special meaning in a character class.  special meaning in a character class.
896    .P
897    The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not
898    set. In other words, it matches any one character except one that signifies the
899    end of a line.
900  .  .
901  .  .
902  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 754  the lookbehind. Line 923  the lookbehind.
923  .rs  .rs
924  .sp  .sp
925  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
926  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
927  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
928  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
929  escaped with a backslash.  a member of the class, it should be the first data character in the class
930    (after an initial circumflex, if present) or escaped with a backslash.
931  .P  .P
932  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
933  character may occupy more than one byte. A matched character must be in the set  character may be more than one byte long. A matched character must be in the
934  of characters defined by the class, unless the first character in the class  set of characters defined by the class, unless the first character in the class
935  definition is a circumflex, in which case the subject character must not be in  definition is a circumflex, in which case the subject character must not be in
936  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
937  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
# Line 771  For example, the character class [aeiou] Line 941  For example, the character class [aeiou]
941  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
942  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
943  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
944  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
945  string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
946  string.  string.
947  .P  .P
# Line 785  caseful version would. In UTF-8 mode, PC Line 955  caseful version would. In UTF-8 mode, PC
955  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
956  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
957  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
958  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in UTF8-mode for characters 128 and above,
959  ensure that PCRE is compiled with Unicode property support as well as with  you must ensure that PCRE is compiled with Unicode property support as well as
960  UTF-8 support.  with UTF-8 support.
961  .P  .P
962  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
963  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 906  alternative in the subpattern. Line 1076  alternative in the subpattern.
1076  .rs  .rs
1077  .sp  .sp
1078  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1079  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1080  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1081    The option letters are
1082  .sp  .sp
1083    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1084    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 921  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1092  PCRE_MULTILINE while unsetting PCRE_DOTA
1092  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
1093  unset.  unset.
1094  .P  .P
1095  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1096  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1097  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1098  the global options (and it will therefore show up in data extracted by the  .P
1099  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1100    subpattern parentheses), the change applies to the remainder of the pattern
1101    that follows. If the change is placed right at the start of a pattern, PCRE
1102    extracts it into the global options (and it will therefore show up in data
1103    extracted by the \fBpcre_fullinfo()\fP function).
1104  .P  .P
1105  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1106  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
# Line 944  branch is abandoned before the option se Line 1119  branch is abandoned before the option se
1119  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1120  behaviour otherwise.  behaviour otherwise.
1121  .P  .P
1122  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  \fBNote:\fP There are other PCRE-specific options that can be set by the
1123  changed in the same way as the Perl-compatible options by using the characters  application when the compile or match functions are called. In some cases the
1124  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1125    the application has set or what has been defaulted. Details are given in the
1126    section entitled
1127    .\" HTML <a href="#newlineseq">
1128    .\" </a>
1129    "Newline sequences"
1130    .\"
1131    above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1132    mode; this is equivalent to setting the PCRE_UTF8 option.
1133  .  .
1134  .  .
1135  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1001  is reached, an option setting in one bra Line 1184  is reached, an option setting in one bra
1184  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1185  .  .
1186  .  .
1187    .\" HTML <a name="dupsubpatternnumber"></a>
1188  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1189  .rs  .rs
1190  .sp  .sp
# Line 1026  stored. Line 1210  stored.
1210    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1211    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1212  .sp  .sp
1213  A backreference or a recursive call to a numbered subpattern always refers to  A back reference to a numbered subpattern uses the most recent value that is
1214  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1215    or "defdef":
1216    .sp
1217      /(?|(abc)|(def))\e1/
1218    .sp
1219    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1220    refers to the first one in the pattern with the given number. The following
1221    pattern matches "abcabc" or "defabc":
1222    .sp
1223      /(?|(abc)|(def))(?1)/
1224    .sp
1225    If a
1226    .\" HTML <a href="#conditions">
1227    .\" </a>
1228    condition test
1229    .\"
1230    for a subpattern's having matched refers to a non-unique number, the test is
1231    true if any of the subpatterns of that number have matched.
1232  .P  .P
1233  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1234  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1042  if an expression is modified, the number Line 1243  if an expression is modified, the number
1243  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1244  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
1245  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
1246  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1247    have different names, but PCRE does not.
1248  .P  .P
1249  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
1250  (?'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
1251  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1252  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1253  .\" </a>  .\" </a>
1254  backreferences,  back references,
1255  .\"  .\"
1256  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1257  .\" </a>  .\" </a>
# Line 1069  extracting the name-to-number translatio Line 1271  extracting the name-to-number translatio
1271  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1272  .P  .P
1273  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
1274  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
1275  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
1276  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
1277  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
1278  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
1279    name, and in both cases you want to extract the abbreviation. This pattern
1280    (ignoring the line breaks) does the job:
1281  .sp  .sp
1282    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1283    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1087  subpattern, as described in the previous Line 1291  subpattern, as described in the previous
1291  .P  .P
1292  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1293  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
1294  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1295  make a reference to a non-unique named subpattern from elsewhere in the  .P
1296  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
1297  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1298    used. In the absence of duplicate numbers (see the previous section) this is
1299    the one with the lowest number. If you use a named reference in a condition
1300    test (see the
1301    .\"
1302    .\" HTML <a href="#conditions">
1303    .\" </a>
1304    section about conditions
1305    .\"
1306    below), either to check whether a subpattern has matched, or to check for
1307    recursion, all subpatterns with the same name are tested. If the condition is
1308    true for any one of them, the overall condition is true. This is the same
1309    behaviour as testing by number. For further details of the interfaces for
1310    handling named subpatterns, see the
1311  .\" HREF  .\" HREF
1312  \fBpcreapi\fP  \fBpcreapi\fP
1313  .\"  .\"
1314  documentation.  documentation.
1315    .P
1316    \fBWarning:\fP You cannot use different names to distinguish between two
1317    subpatterns with the same number because PCRE uses only the numbers when
1318    matching. For this reason, an error is given at compile time if different names
1319    are given to subpatterns with the same number. However, you can give the same
1320    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1321  .  .
1322  .  .
1323  .SH REPETITION  .SH REPETITION
# Line 1112  items: Line 1335  items:
1335    a character class    a character class
1336    a back reference (see next section)    a back reference (see next section)
1337    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1338      a recursive or "subroutine" call to a subpattern
1339  .sp  .sp
1340  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1341  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1143  support is available, \eX{3} matches thr Line 1367  support is available, \eX{3} matches thr
1367  which may be several bytes long (and they may be of different lengths).  which may be several bytes long (and they may be of different lengths).
1368  .P  .P
1369  The quantifier {0} is permitted, causing the expression to behave as if the  The quantifier {0} is permitted, causing the expression to behave as if the
1370  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1371    subpatterns that are referenced as
1372    .\" HTML <a href="#subpatternsassubroutines">
1373    .\" </a>
1374    subroutines
1375    .\"
1376    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1377    quantifier are omitted from the compiled pattern.
1378  .P  .P
1379  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1380  abbreviations:  abbreviations:
# Line 1215  worth setting PCRE_DOTALL in order to ob Line 1446  worth setting PCRE_DOTALL in order to ob
1446  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1447  .P  .P
1448  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1449  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1450  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
1451  succeeds. Consider, for example:  succeeds. Consider, for example:
1452  .sp  .sp
# Line 1290  previous example can be rewritten as Line 1521  previous example can be rewritten as
1521  .sp  .sp
1522    \ed++foo    \ed++foo
1523  .sp  .sp
1524    Note that a possessive quantifier can be used with an entire group, for
1525    example:
1526    .sp
1527      (abc|xyz){2,3}+
1528    .sp
1529  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1530  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1531  atomic group. However, there is no difference in the meaning of a possessive  atomic group. However, there is no difference in the meaning of a possessive
# Line 1364  subpattern is possible using named paren Line 1600  subpattern is possible using named paren
1600  .P  .P
1601  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1602  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence, which is a feature introduced in
1603  Perl 5.10. This escape must be followed by a positive or a negative number,  Perl 5.10. This escape must be followed by an unsigned number or a negative
1604  optionally enclosed in braces. These examples are all identical:  number, optionally enclosed in braces. These examples are all identical:
1605  .sp  .sp
1606    (ring), \e1    (ring), \e1
1607    (ring), \eg1    (ring), \eg1
1608    (ring), \eg{1}    (ring), \eg{1}
1609  .sp  .sp
1610  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1611  present in the older syntax. It is also useful when literal digits follow the  is present in the older syntax. It is also useful when literal digits follow
1612  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1613    example:
1614  .sp  .sp
1615    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1616  .sp  .sp
# Line 1420  after the reference. Line 1657  after the reference.
1657  .P  .P
1658  There may be more than one back reference to the same subpattern. If a  There may be more than one back reference to the same subpattern. If a
1659  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1660  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1661  .sp  .sp
1662    (a|(bc))\e2    (a|(bc))\e2
1663  .sp  .sp
1664  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". However, if the
1665  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1666  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1667  with a digit character, some delimiter must be used to terminate the back  .P
1668  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1669  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1670    If the pattern continues with a digit character, some delimiter must be used to
1671    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1672    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1673  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1674  .\" </a>  .\" </a>
1675  "Comments"  "Comments"
1676  .\"  .\"
1677  below) can be used.  below) can be used.
1678  .P  .
1679    .SS "Recursive back references"
1680    .rs
1681    .sp
1682  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
1683  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.
1684  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1449  to the previous iteration. In order for Line 1692  to the previous iteration. In order for
1692  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
1693  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
1694  minimum of zero.  minimum of zero.
1695    .P
1696    Back references of this type cause the group that they reference to be treated
1697    as an
1698    .\" HTML <a href="#atomicgroup">
1699    .\" </a>
1700    atomic group.
1701    .\"
1702    Once the whole group has been matched, a subsequent matching failure cannot
1703    cause backtracking into the middle of the group.
1704  .  .
1705  .  .
1706  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1502  lookbehind assertion is needed to achiev Line 1754  lookbehind assertion is needed to achiev
1754  If you want to force a matching failure at some point in a pattern, the most  If you want to force a matching failure at some point in a pattern, the most
1755  convenient way to do it is with (?!) because an empty string always matches, so  convenient way to do it is with (?!) because an empty string always matches, so
1756  an assertion that requires there not to be an empty string must always fail.  an assertion that requires there not to be an empty string must always fail.
1757    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1758    synonym for (?!).
1759  .  .
1760  .  .
1761  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1526  is permitted, but Line 1780  is permitted, but
1780  .sp  .sp
1781  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1782  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1783  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (5.8 and 5.10), which requires all branches to
1784  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1785  .sp  .sp
1786    (?<=ab(c|de))    (?<=ab(c|de))
1787  .sp  .sp
1788  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1789  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1790    branches:
1791  .sp  .sp
1792    (?<=abc|abde)    (?<=abc|abde)
1793  .sp  .sp
# Line 1541  In some cases, the Perl 5.10 escape sequ Line 1796  In some cases, the Perl 5.10 escape sequ
1796  .\" </a>  .\" </a>
1797  (see above)  (see above)
1798  .\"  .\"
1799  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
1800  fixed-length.  restriction.
1801  .P  .P
1802  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1803  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
# Line 1554  to appear in lookbehind assertions, beca Line 1809  to appear in lookbehind assertions, beca
1809  the length of the lookbehind. The \eX and \eR escapes, which can match  the length of the lookbehind. The \eX and \eR escapes, which can match
1810  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1811  .P  .P
1812    .\" HTML <a href="#subpatternsassubroutines">
1813    .\" </a>
1814    "Subroutine"
1815    .\"
1816    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1817    as the subpattern matches a fixed-length string.
1818    .\" HTML <a href="#recursion">
1819    .\" </a>
1820    Recursion,
1821    .\"
1822    however, is not supported.
1823    .P
1824  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1825  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching of fixed-length strings at the end of subject
1826  pattern such as  strings. Consider a simple pattern such as
1827  .sp  .sp
1828    abcd$    abcd$
1829  .sp  .sp
# Line 1620  characters that are not "999". Line 1887  characters that are not "999".
1887  .sp  .sp
1888  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1889  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1890  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1891  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1892  .sp  .sp
1893    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1894    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1637  recursion, a pseudo-condition called DEF Line 1904  recursion, a pseudo-condition called DEF
1904  .rs  .rs
1905  .sp  .sp
1906  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
1907  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1908  matched. An alternative notation is to precede the digits with a plus or minus  matched. If there is more than one capturing subpattern with the same number
1909  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
1910  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
1911  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
1912  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
1913    section about duplicate subpattern numbers),
1914    .\"
1915    the condition is true if any of them have been set. An alternative notation is
1916    to precede the digits with a plus or minus sign. In this case, the subpattern
1917    number is relative rather than absolute. The most recently opened parentheses
1918    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1919    looping constructs it can also make sense to refer to subsequent groups with
1920    constructs such as (?(+2).
1921  .P  .P
1922  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1923  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1683  Rewriting the above example to use a nam Line 1958  Rewriting the above example to use a nam
1958  .sp  .sp
1959    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1960  .sp  .sp
1961    If the name used in a condition of this kind is a duplicate, the test is
1962    applied to all subpatterns of the same name, and is true if any one of them has
1963    matched.
1964  .  .
1965  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
1966  .rs  .rs
# Line 1694  letter R, for example: Line 1972  letter R, for example:
1972  .sp  .sp
1973    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
1974  .sp  .sp
1975  the condition is true if the most recent recursion is into the subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
1976  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
1977  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
1978    applied to all subpatterns of the same name, and is true if any one of them is
1979    the most recent recursion.
1980  .P  .P
1981  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
1982  patterns are described below.  .\" HTML <a href="#recursion">
1983    .\" </a>
1984    The syntax for recursive patterns
1985    .\"
1986    is described below.
1987  .  .
1988  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
1989  .rs  .rs
# Line 1708  If the condition is the string (DEFINE), Line 1992  If the condition is the string (DEFINE),
1992  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
1993  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
1994  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
1995  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
1996    .\" HTML <a href="#subpatternsassubroutines">
1997    .\" </a>
1998    "subroutines"
1999    .\"
2000  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address could be
2001  written like this (ignore whitespace and line breaks):  written like this (ignore whitespace and line breaks):
2002  .sp  .sp
# Line 1718  written like this (ignore whitespace and Line 2006  written like this (ignore whitespace and
2006  The first part of the pattern is a DEFINE group inside which a another group  The first part of the pattern is a DEFINE group inside which a another group
2007  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2008  address (a number less than 256). When matching takes place, this part of the  address (a number less than 256). When matching takes place, this part of the
2009  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2010  .P  pattern uses references to the named group to match the four dot-separated
2011  The rest of the pattern uses references to the named group to match the four  components of an IPv4 address, insisting on a word boundary at each end.
 dot-separated components of an IPv4 address, insisting on a word boundary at  
 each end.  
2012  .  .
2013  .SS "Assertion conditions"  .SS "Assertion conditions"
2014  .rs  .rs
# Line 1779  recursively to the pattern in which it a Line 2065  recursively to the pattern in which it a
2065  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2066  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2067  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2068  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2069  .P  .P
2070  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
2071  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,
2072  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  provided that it occurs inside that subpattern. (If not, it is a
2073    .\" HTML <a href="#subpatternsassubroutines">
2074    .\" </a>
2075    "subroutine"
2076    .\"
2077  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2078  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2079  .P  .P
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 .P  
2080  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2081  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2082  .sp  .sp
2083    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2084  .sp  .sp
2085  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2086  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
2087  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2088  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2089    to avoid backtracking into sequences of non-parentheses.
2090  .P  .P
2091  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
2092  pattern, so instead you could use this:  pattern, so instead you could use this:
2093  .sp  .sp
2094    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2095  .sp  .sp
2096  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
2097  them instead of the whole pattern.  them instead of the whole pattern.
2098  .P  .P
2099  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2100  is made easier by the use of relative references. (A Perl 5.10 feature.)  is made easier by the use of relative references (a Perl 5.10 feature).
2101  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2102  most recently opened parentheses preceding the recursion. In other words, a  most recently opened parentheses preceding the recursion. In other words, a
2103  negative number counts capturing parentheses leftwards from the point at which  negative number counts capturing parentheses leftwards from the point at which
# Line 1820  it is encountered. Line 2106  it is encountered.
2106  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2107  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2108  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2109  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2110    .\" </a>
2111    "subroutine"
2112    .\"
2113    calls, as described in the next section.
2114  .P  .P
2115  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2116  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2117  could rewrite the above example as follows:  could rewrite the above example as follows:
2118  .sp  .sp
2119    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2120  .sp  .sp
2121  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
2122  used.  used.
2123  .P  .P
2124  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2125  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of a possessive quantifier for matching
2126  non-parentheses is important when applying the pattern to strings that do not  strings of non-parentheses is important when applying the pattern to strings
2127  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2128  .sp  .sp
2129    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2130  .sp  .sp
2131  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2132  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
2133  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
2134  before failure can be reported.  before failure can be reported.
2135  .P  .P
2136  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
2137  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
2138  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  
2139  .\" HREF  .\" HREF
2140  \fBpcrecallout\fP  \fBpcrecallout\fP
2141  .\"  .\"
# Line 1854  documentation). If the pattern above is Line 2143  documentation). If the pattern above is
2143  .sp  .sp
2144    (ab(cd)ef)    (ab(cd)ef)
2145  .sp  .sp
2146  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
2147  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
2148  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2149    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2150       ^                        ^  .P
2151       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2152  .sp  obtain extra memory to store data during a recursion, which it does by using
2153  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
2154  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.  
2155  .P  .P
2156  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.
2157  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1879  different alternatives for the recursive Line 2165  different alternatives for the recursive
2165  is the actual recursive call.  is the actual recursive call.
2166  .  .
2167  .  .
2168    .\" HTML <a name="recursiondifference"></a>
2169    .SS "Recursion difference from Perl"
2170    .rs
2171    .sp
2172    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2173    treated as an atomic group. That is, once it has matched some of the subject
2174    string, it is never re-entered, even if it contains untried alternatives and
2175    there is a subsequent matching failure. This can be illustrated by the
2176    following pattern, which purports to match a palindromic string that contains
2177    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2178    .sp
2179      ^(.|(.)(?1)\e2)$
2180    .sp
2181    The idea is that it either matches a single character, or two identical
2182    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2183    it does not if the pattern is longer than three characters. Consider the
2184    subject string "abcba":
2185    .P
2186    At the top level, the first character is matched, but as it is not at the end
2187    of the string, the first alternative fails; the second alternative is taken
2188    and the recursion kicks in. The recursive call to subpattern 1 successfully
2189    matches the next character ("b"). (Note that the beginning and end of line
2190    tests are not part of the recursion).
2191    .P
2192    Back at the top level, the next character ("c") is compared with what
2193    subpattern 2 matched, which was "a". This fails. Because the recursion is
2194    treated as an atomic group, there are now no backtracking points, and so the
2195    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2196    try the second alternative.) However, if the pattern is written with the
2197    alternatives in the other order, things are different:
2198    .sp
2199      ^((.)(?1)\e2|.)$
2200    .sp
2201    This time, the recursing alternative is tried first, and continues to recurse
2202    until it runs out of characters, at which point the recursion fails. But this
2203    time we do have another alternative to try at the higher level. That is the big
2204    difference: in the previous case the remaining alternative is at a deeper
2205    recursion level, which PCRE cannot use.
2206    .P
2207    To change the pattern so that matches all palindromic strings, not just those
2208    with an odd number of characters, it is tempting to change the pattern to this:
2209    .sp
2210      ^((.)(?1)\e2|.?)$
2211    .sp
2212    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2213    deeper recursion has matched a single character, it cannot be entered again in
2214    order to match an empty string. The solution is to separate the two cases, and
2215    write out the odd and even cases as alternatives at the higher level:
2216    .sp
2217      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2218    .sp
2219    If you want to match typical palindromic phrases, the pattern has to ignore all
2220    non-word characters, which can be done like this:
2221    .sp
2222      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2223    .sp
2224    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2225    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2226    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2227    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2228    more) to match typical phrases, and Perl takes so long that you think it has
2229    gone into a loop.
2230    .P
2231    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2232    string does not start with a palindrome that is shorter than the entire string.
2233    For example, although "abcba" is correctly matched, if the subject is "ababa",
2234    PCRE finds the palindrome "aba" at the start, then fails at top level because
2235    the end of the string does not follow. Once again, it cannot jump back into the
2236    recursion to try other alternatives, so the entire match fails.
2237    .
2238    .
2239  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2240  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2241  .rs  .rs
# Line 1905  matches "sense and sensibility" and "res Line 2262  matches "sense and sensibility" and "res
2262  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
2263  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2264  .P  .P
2265  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2266  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
2267  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
2268  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2269    call revert to their previous values afterwards.
2270  .P  .P
2271  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2272  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 1920  It matches "abcabc". It does not match " Line 2278  It matches "abcabc". It does not match "
2278  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2279  .  .
2280  .  .
2281    .\" HTML <a name="onigurumasubroutines"></a>
2282    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2283    .rs
2284    .sp
2285    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2286    a number enclosed either in angle brackets or single quotes, is an alternative
2287    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2288    are two of the examples used above, rewritten using this syntax:
2289    .sp
2290      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2291      (sens|respons)e and \eg'1'ibility
2292    .sp
2293    PCRE supports an extension to Oniguruma: if a number is preceded by a
2294    plus or a minus sign it is taken as a relative reference. For example:
2295    .sp
2296      (abc)(?i:\eg<-1>)
2297    .sp
2298    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2299    synonymous. The former is a back reference; the latter is a subroutine call.
2300    .
2301    .
2302  .SH CALLOUTS  .SH CALLOUTS
2303  .rs  .rs
2304  .sp  .sp
# Line 1956  description of the interface to the call Line 2335  description of the interface to the call
2335  documentation.  documentation.
2336  .  .
2337  .  .
2338    .\" HTML <a name="backtrackcontrol"></a>
2339    .SH "BACKTRACKING CONTROL"
2340    .rs
2341    .sp
2342    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2343    are described in the Perl documentation as "experimental and subject to change
2344    or removal in a future version of Perl". It goes on to say: "Their usage in
2345    production code should be noted to avoid problems during upgrades." The same
2346    remarks apply to the PCRE features described in this section.
2347    .P
2348    Since these verbs are specifically related to backtracking, most of them can be
2349    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2350    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2351    failing negative assertion, they cause an error if encountered by
2352    \fBpcre_dfa_exec()\fP.
2353    .P
2354    If any of these verbs are used in an assertion or subroutine subpattern
2355    (including recursive subpatterns), their effect is confined to that subpattern;
2356    it does not extend to the surrounding pattern. Note that such subpatterns are
2357    processed as anchored at the point where they are tested.
2358    .P
2359    The new verbs make use of what was previously invalid syntax: an opening
2360    parenthesis followed by an asterisk. They are generally of the form
2361    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2362    depending on whether or not an argument is present. An name is a sequence of
2363    letters, digits, and underscores. If the name is empty, that is, if the closing
2364    parenthesis immediately follows the colon, the effect is as if the colon were
2365    not there. Any number of these verbs may occur in a pattern.
2366    .P
2367    PCRE contains some optimizations that are used to speed up matching by running
2368    some checks at the start of each match attempt. For example, it may know the
2369    minimum length of matching subject, or that a particular character must be
2370    present. When one of these optimizations suppresses the running of a match, any
2371    included backtracking verbs will not, of course, be processed. You can suppress
2372    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2373    when calling \fBpcre_exec()\fP.
2374    .
2375    .
2376    .SS "Verbs that act immediately"
2377    .rs
2378    .sp
2379    The following verbs act as soon as they are encountered. They may not be
2380    followed by a name.
2381    .sp
2382       (*ACCEPT)
2383    .sp
2384    This verb causes the match to end successfully, skipping the remainder of the
2385    pattern. When inside a recursion, only the innermost pattern is ended
2386    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2387    captured. (This feature was added to PCRE at release 8.00.) For example:
2388    .sp
2389      A((?:A|B(*ACCEPT)|C)D)
2390    .sp
2391    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2392    the outer parentheses.
2393    .sp
2394      (*FAIL) or (*F)
2395    .sp
2396    This verb causes the match to fail, forcing backtracking to occur. It is
2397    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2398    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2399    Perl features that are not present in PCRE. The nearest equivalent is the
2400    callout feature, as for example in this pattern:
2401    .sp
2402      a+(?C)(*FAIL)
2403    .sp
2404    A match with the string "aaaa" always fails, but the callout is taken before
2405    each backtrack happens (in this example, 10 times).
2406    .
2407    .
2408    .SS "Recording which path was taken"
2409    .rs
2410    .sp
2411    There is one verb whose main purpose is to track how a match was arrived at,
2412    though it also has a secondary use in conjunction with advancing the match
2413    starting point (see (*SKIP) below).
2414    .sp
2415      (*MARK:NAME) or (*:NAME)
2416    .sp
2417    A name is always required with this verb. There may be as many instances of
2418    (*MARK) as you like in a pattern, and their names do not have to be unique.
2419    .P
2420    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2421    to the caller via the \fIpcre_extra\fP data structure, as described in the
2422    .\" HTML <a href="pcreapi.html#extradata">
2423    .\" </a>
2424    section on \fIpcre_extra\fP
2425    .\"
2426    in the
2427    .\" HREF
2428    \fBpcreapi\fP
2429    .\"
2430    documentation. No data is returned for a partial match. Here is an example of
2431    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2432    outputting of (*MARK) data:
2433    .sp
2434      /X(*MARK:A)Y|X(*MARK:B)Z/K
2435      XY
2436       0: XY
2437      MK: A
2438      XZ
2439       0: XZ
2440      MK: B
2441    .sp
2442    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2443    indicates which of the two alternatives matched. This is a more efficient way
2444    of obtaining this information than putting each alternative in its own
2445    capturing parentheses.
2446    .P
2447    A name may also be returned after a failed match if the final path through the
2448    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2449    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2450    starting point for matching is advanced, the final check is often with an empty
2451    string, causing a failure before (*MARK) is reached. For example:
2452    .sp
2453      /X(*MARK:A)Y|X(*MARK:B)Z/K
2454      XP
2455      No match
2456    .sp
2457    There are three potential starting points for this match (starting with X,
2458    starting with P, and with an empty string). If the pattern is anchored, the
2459    result is different:
2460    .sp
2461      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2462      XP
2463      No match, mark = B
2464    .sp
2465    PCRE's start-of-match optimizations can also interfere with this. For example,
2466    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2467    subject length for a match, a shorter subject will not be scanned at all.
2468    .P
2469    Note that similar anomalies (though different in detail) exist in Perl, no
2470    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2471    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2472    .
2473    .
2474    .SS "Verbs that act after backtracking"
2475    .rs
2476    .sp
2477    The following verbs do nothing when they are encountered. Matching continues
2478    with what follows, but if there is no subsequent match, causing a backtrack to
2479    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2480    the verb. However, when one of these verbs appears inside an atomic group, its
2481    effect is confined to that group, because once the group has been matched,
2482    there is never any backtracking into it. In this situation, backtracking can
2483    "jump back" to the left of the entire atomic group. (Remember also, as stated
2484    above, that this localization also applies in subroutine calls and assertions.)
2485    .P
2486    These verbs differ in exactly what kind of failure occurs when backtracking
2487    reaches them.
2488    .sp
2489      (*COMMIT)
2490    .sp
2491    This verb, which may not be followed by a name, causes the whole match to fail
2492    outright if the rest of the pattern does not match. Even if the pattern is
2493    unanchored, no further attempts to find a match by advancing the starting point
2494    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2495    finding a match at the current starting point, or not at all. For example:
2496    .sp
2497      a+(*COMMIT)b
2498    .sp
2499    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2500    dynamic anchor, or "I've started, so I must finish." The name of the most
2501    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2502    match failure.
2503    .P
2504    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2505    unless PCRE's start-of-match optimizations are turned off, as shown in this
2506    \fBpcretest\fP example:
2507    .sp
2508      /(*COMMIT)abc/
2509      xyzabc
2510       0: abc
2511      xyzabc\eY
2512      No match
2513    .sp
2514    PCRE knows that any match must start with "a", so the optimization skips along
2515    the subject to "a" before running the first match attempt, which succeeds. When
2516    the optimization is disabled by the \eY escape in the second subject, the match
2517    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2518    starting points.
2519    .sp
2520      (*PRUNE) or (*PRUNE:NAME)
2521    .sp
2522    This verb causes the match to fail at the current starting position in the
2523    subject if the rest of the pattern does not match. If the pattern is
2524    unanchored, the normal "bumpalong" advance to the next starting character then
2525    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2526    reached, or when matching to the right of (*PRUNE), but if there is no match to
2527    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2528    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2529    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2530    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2531    match fails completely; the name is passed back if this is the final attempt.
2532    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2533    pattern (*PRUNE) has the same effect as (*COMMIT).
2534    .sp
2535      (*SKIP)
2536    .sp
2537    This verb, when given without a name, is like (*PRUNE), except that if the
2538    pattern is unanchored, the "bumpalong" advance is not to the next character,
2539    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2540    signifies that whatever text was matched leading up to it cannot be part of a
2541    successful match. Consider:
2542    .sp
2543      a+(*SKIP)b
2544    .sp
2545    If the subject is "aaaac...", after the first match attempt fails (starting at
2546    the first character in the string), the starting point skips on to start the
2547    next attempt at "c". Note that a possessive quantifer does not have the same
2548    effect as this example; although it would suppress backtracking during the
2549    first match attempt, the second attempt would start at the second character
2550    instead of skipping on to "c".
2551    .sp
2552      (*SKIP:NAME)
2553    .sp
2554    When (*SKIP) has an associated name, its behaviour is modified. If the
2555    following pattern fails to match, the previous path through the pattern is
2556    searched for the most recent (*MARK) that has the same name. If one is found,
2557    the "bumpalong" advance is to the subject position that corresponds to that
2558    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2559    matching name is found, normal "bumpalong" of one character happens (the
2560    (*SKIP) is ignored).
2561    .sp
2562      (*THEN) or (*THEN:NAME)
2563    .sp
2564    This verb causes a skip to the next alternation if the rest of the pattern does
2565    not match. That is, it cancels pending backtracking, but only within the
2566    current alternation. Its name comes from the observation that it can be used
2567    for a pattern-based if-then-else block:
2568    .sp
2569      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2570    .sp
2571    If the COND1 pattern matches, FOO is tried (and possibly further items after
2572    the end of the group if FOO succeeds); on failure the matcher skips to the
2573    second alternative and tries COND2, without backtracking into COND1. The
2574    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2575    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2576    like (*PRUNE).
2577    .
2578    .
2579  .SH "SEE ALSO"  .SH "SEE ALSO"
2580  .rs  .rs
2581  .sp  .sp
2582  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2583    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2584  .  .
2585  .  .
2586  .SH AUTHOR  .SH AUTHOR
# Line 1976  Cambridge CB2 3QH, England. Line 2597  Cambridge CB2 3QH, England.
2597  .rs  .rs
2598  .sp  .sp
2599  .nf  .nf
2600  Last updated: 19 June 2007  Last updated: 03 May 2010
2601  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2602  .fi  .fi

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