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revision 464 by ph10, Sun Oct 18 19:50:34 2009 UTC revision 733 by ph10, Tue Oct 11 10:29:36 2011 UTC
# Line 32  Starting a pattern with this sequence is Line 32  Starting a pattern with this sequence is
32  option. This feature is not Perl-compatible. How setting UTF-8 mode affects  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  pattern matching is mentioned in several places below. There is also a summary
34  of UTF-8 features in the  of UTF-8 features in the
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
 .\"  
 in the main  
35  .\" HREF  .\" HREF
36  \fBpcre\fP  \fBpcreunicode\fP
37  .\"  .\"
38  page.  page.
39  .P  .P
40    Another special sequence that may appear at the start of a pattern or in
41    combination with (*UTF8) is:
42    .sp
43      (*UCP)
44    .sp
45    This has the same effect as setting the PCRE_UCP option: it causes sequences
46    such as \ed and \ew to use Unicode properties to determine character types,
47    instead of recognizing only characters with codes less than 128 via a lookup
48    table.
49    .P
50    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
51    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
52    also some more of these special sequences that are concerned with the handling
53    of newlines; they are described below.
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 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 169  The following sections describe the use Line 182  The following sections describe the use
182  .rs  .rs
183  .sp  .sp
184  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
185  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
186  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
187  outside character classes.  both inside and outside character classes.
188  .P  .P
189  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
190  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 179  otherwise be interpreted as a metacharac Line 192  otherwise be interpreted as a metacharac
192  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
193  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
194  .P  .P
195    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
196    backslash. All other characters (in particular, those whose codepoints are
197    greater than 127) are treated as literals.
198    .P
199  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
200  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
201  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 198  Perl, $ and @ cause variable interpolati Line 215  Perl, $ and @ cause variable interpolati
215    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
216  .sp  .sp
217  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
218    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
219    by \eE later in the pattern, the literal interpretation continues to the end of
220    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
221    a character class, this causes an error, because the character class is not
222    terminated.
223  .  .
224  .  .
225  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 211  but when a pattern is being prepared by Line 233  but when a pattern is being prepared by
233  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it represents:
234  .sp  .sp
235    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
236    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
237    \ee        escape (hex 1B)    \ee        escape (hex 1B)
238    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
239    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
240    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
241    \et        tab (hex 09)    \et        tab (hex 09)
242    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
243    \exhh      character with hex code hh    \exhh      character with hex code hh
244    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
245  .sp  .sp
246  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
247  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
248  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
249  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
250    than 127, a compile-time error occurs. This locks out non-ASCII characters in
251    both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
252    values are valid. A lower case letter is converted to upper case, and then the
253    0xc0 bits are flipped.)
254  .P  .P
255  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
256  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{
# Line 295  zero, because no more than three octal d Line 321  zero, because no more than three octal d
321  .P  .P
322  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
323  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
324  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
325  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
326  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
327  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
328  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
329  .  .
330  .  .
331  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 341  subroutine Line 364  subroutine
364  call.  call.
365  .  .
366  .  .
367    .\" HTML <a name="genericchartypes"></a>
368  .SS "Generic character types"  .SS "Generic character types"
369  .rs  .rs
370  .sp  .sp
371  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:  
372  .sp  .sp
373    \ed     any decimal digit    \ed     any decimal digit
374    \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 381  following are always recognized:
381    \ew     any "word" character    \ew     any "word" character
382    \eW     any "non-word" character    \eW     any "non-word" character
383  .sp  .sp
384  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.
385  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
386    .\" HTML <a href="#fullstopdot">
387    .\" </a>
388    the "." metacharacter
389    .\"
390    when PCRE_DOTALL is not set.
391  .P  .P
392  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
393    of characters into two disjoint sets. Any given character matches one, and only
394    one, of each pair. The sequences can appear both inside and outside character
395  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
396  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
397  there is no character to match.  there is no character to match.
398  .P  .P
399  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 402  are HT (9), LF (10), FF (12), CR (13), a
402  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
403  does.  does.
404  .P  .P
405  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.
406  \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
407  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
408  meanings from before UTF-8 support was available, mainly for efficiency  place (see
409  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" HTML <a href="pcreapi.html#localesupport">
410  and \eW.  .\" </a>
411  .P  "Locale support"
412  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  .\"
413  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  in the
414  The horizontal space characters are:  .\" HREF
415    \fBpcreapi\fP
416    .\"
417    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
418    or "french" in Windows, some character codes greater than 128 are used for
419    accented letters, and these are then matched by \ew. The use of locales with
420    Unicode is discouraged.
421    .P
422    By default, in UTF-8 mode, characters with values greater than 128 never match
423    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
424    their original meanings from before UTF-8 support was available, mainly for
425    efficiency reasons. However, if PCRE is compiled with Unicode property support,
426    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
427    properties are used to determine character types, as follows:
428    .sp
429      \ed  any character that \ep{Nd} matches (decimal digit)
430      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
431      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
432    .sp
433    The upper case escapes match the inverse sets of characters. Note that \ed
434    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
435    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
436    \eB because they are defined in terms of \ew and \eW. Matching these sequences
437    is noticeably slower when PCRE_UCP is set.
438    .P
439    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
440    release 5.10. In contrast to the other sequences, which match only ASCII
441    characters by default, these always match certain high-valued codepoints in
442    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
443    are:
444  .sp  .sp
445    U+0009     Horizontal tab    U+0009     Horizontal tab
446    U+0020     Space    U+0020     Space
# Line 412  The vertical space characters are: Line 471  The vertical space characters are:
471    U+0085     Next line    U+0085     Next line
472    U+2028     Line separator    U+2028     Line separator
473    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.  
474  .  .
475  .  .
476  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 436  is discouraged. Line 478  is discouraged.
478  .rs  .rs
479  .sp  .sp
480  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
481  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is  Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
 equivalent to the following:  
482  .sp  .sp
483    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
484  .sp  .sp
# Line 474  These override the default and the optio Line 515  These override the default and the optio
515  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
516  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
517  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
518  convention, for example, a pattern can start with:  convention; for example, a pattern can start with:
519  .sp  .sp
520    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
521  .sp  .sp
522  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
523    a character class, \eR is treated as an unrecognized escape sequence, and so
524    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
525  .  .
526  .  .
527  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 496  The extra escape sequences are: Line 539  The extra escape sequences are:
539    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
540  .sp  .sp
541  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
542  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
543  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
544  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
545  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
546    .\" </a>
547    next section).
548    .\"
549    Other Perl properties such as "InMusicalSymbols" are not currently supported by
550    PCRE. Note that \eP{Any} does not match any characters, so always causes a
551    match failure.
552  .P  .P
553  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
554  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 562  Those that are not part of an identified
562  .P  .P
563  Arabic,  Arabic,
564  Armenian,  Armenian,
565    Avestan,
566  Balinese,  Balinese,
567    Bamum,
568  Bengali,  Bengali,
569  Bopomofo,  Bopomofo,
570  Braille,  Braille,
571  Buginese,  Buginese,
572  Buhid,  Buhid,
573  Canadian_Aboriginal,  Canadian_Aboriginal,
574    Carian,
575    Cham,
576  Cherokee,  Cherokee,
577  Common,  Common,
578  Coptic,  Coptic,
# Line 528  Cypriot, Line 581  Cypriot,
581  Cyrillic,  Cyrillic,
582  Deseret,  Deseret,
583  Devanagari,  Devanagari,
584    Egyptian_Hieroglyphs,
585  Ethiopic,  Ethiopic,
586  Georgian,  Georgian,
587  Glagolitic,  Glagolitic,
# Line 540  Hangul, Line 594  Hangul,
594  Hanunoo,  Hanunoo,
595  Hebrew,  Hebrew,
596  Hiragana,  Hiragana,
597    Imperial_Aramaic,
598  Inherited,  Inherited,
599    Inscriptional_Pahlavi,
600    Inscriptional_Parthian,
601    Javanese,
602    Kaithi,
603  Kannada,  Kannada,
604  Katakana,  Katakana,
605    Kayah_Li,
606  Kharoshthi,  Kharoshthi,
607  Khmer,  Khmer,
608  Lao,  Lao,
609  Latin,  Latin,
610    Lepcha,
611  Limbu,  Limbu,
612  Linear_B,  Linear_B,
613    Lisu,
614    Lycian,
615    Lydian,
616  Malayalam,  Malayalam,
617    Meetei_Mayek,
618  Mongolian,  Mongolian,
619  Myanmar,  Myanmar,
620  New_Tai_Lue,  New_Tai_Lue,
# Line 557  Nko, Line 622  Nko,
622  Ogham,  Ogham,
623  Old_Italic,  Old_Italic,
624  Old_Persian,  Old_Persian,
625    Old_South_Arabian,
626    Old_Turkic,
627    Ol_Chiki,
628  Oriya,  Oriya,
629  Osmanya,  Osmanya,
630  Phags_Pa,  Phags_Pa,
631  Phoenician,  Phoenician,
632    Rejang,
633  Runic,  Runic,
634    Samaritan,
635    Saurashtra,
636  Shavian,  Shavian,
637  Sinhala,  Sinhala,
638    Sundanese,
639  Syloti_Nagri,  Syloti_Nagri,
640  Syriac,  Syriac,
641  Tagalog,  Tagalog,
642  Tagbanwa,  Tagbanwa,
643  Tai_Le,  Tai_Le,
644    Tai_Tham,
645    Tai_Viet,
646  Tamil,  Tamil,
647  Telugu,  Telugu,
648  Thaana,  Thaana,
# Line 576  Thai, Line 650  Thai,
650  Tibetan,  Tibetan,
651  Tifinagh,  Tifinagh,
652  Ugaritic,  Ugaritic,
653    Vai,
654  Yi.  Yi.
655  .P  .P
656  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
657  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
658  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
659  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
660  .P  .P
661  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
662  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 677  Characters with the "mark" property are Line 752  Characters with the "mark" property are
752  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
753  non-UTF-8 mode \eX matches any one character.  non-UTF-8 mode \eX matches any one character.
754  .P  .P
755    Note that recent versions of Perl have changed \eX to match what Unicode calls
756    an "extended grapheme cluster", which has a more complicated definition.
757    .P
758  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
759  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
760  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
761  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
762    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
763    (*UCP).
764    .
765    .
766    .\" HTML <a name="extraprops"></a>
767    .SS PCRE's additional properties
768    .rs
769    .sp
770    As well as the standard Unicode properties described in the previous
771    section, PCRE supports four more that make it possible to convert traditional
772    escape sequences such as \ew and \es and POSIX character classes to use Unicode
773    properties. PCRE uses these non-standard, non-Perl properties internally when
774    PCRE_UCP is set. They are:
775    .sp
776      Xan   Any alphanumeric character
777      Xps   Any POSIX space character
778      Xsp   Any Perl space character
779      Xwd   Any Perl "word" character
780    .sp
781    Xan matches characters that have either the L (letter) or the N (number)
782    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
783    carriage return, and any other character that has the Z (separator) property.
784    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
785    same characters as Xan, plus underscore.
786  .  .
787  .  .
788  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
789  .SS "Resetting the match start"  .SS "Resetting the match start"
790  .rs  .rs
791  .sp  .sp
792  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
793  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
794  .sp  .sp
795    foo\eKbar    foo\eKbar
796  .sp  .sp
# Line 711  For example, when the pattern Line 812  For example, when the pattern
812    (foo)\eKbar    (foo)\eKbar
813  .sp  .sp
814  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
815    .P
816    Perl documents that the use of \eK within assertions is "not well defined". In
817    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
818    ignored in negative assertions.
819  .  .
820  .  .
821  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 735  The backslashed assertions are: Line 840  The backslashed assertions are:
840    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
841    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
842  .sp  .sp
843  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
844  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
845    default it matches the corresponding literal character (for example, \eB
846    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
847    escape sequence" error is generated instead.
848  .P  .P
849  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
850  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
851  \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
852  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
853  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
854  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
855  \eba matches "a" at the start of a word.  of word" or "end of word" metasequence. However, whatever follows \eb normally
856    determines which it is. For example, the fragment \eba matches "a" at the start
857    of a word.
858  .P  .P
859  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
860  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 938  end of the subject in both modes, and if
938  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
939  .  .
940  .  .
941  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
942    .SH "FULL STOP (PERIOD, DOT) AND \eN"
943  .rs  .rs
944  .sp  .sp
945  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 961  to match it.
961  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
962  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
963  special meaning in a character class.  special meaning in a character class.
964    .P
965    The escape sequence \eN behaves like a dot, except that it is not affected by
966    the PCRE_DOTALL option. In other words, it matches any character except one
967    that signifies the end of a line.
968  .  .
969  .  .
970  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 858  special meaning in a character class. Line 973  special meaning in a character class.
973  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
974  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
975  characters. The feature is provided in Perl in order to match individual bytes  characters. The feature is provided in Perl in order to match individual bytes
976  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
977  what remains in the string may be a malformed UTF-8 string. For this reason,  rest of the string may start with a malformed UTF-8 character. For this reason,
978  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
979  .P  .P
980  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
# Line 944  characters in both cases. In UTF-8 mode, Line 1059  characters in both cases. In UTF-8 mode,
1059  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
1060  property support.  property support.
1061  .P  .P
1062  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1063  in a character class, and add the characters that they match to the class. For  \eV, \ew, and \eW may appear in a character class, and add the characters that
1064  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1065  conveniently be used with the upper case character types to specify a more  digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1066  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1067  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1068    .\" HTML <a href="#genericchartypes">
1069    .\" </a>
1070    "Generic character types"
1071    .\"
1072    above. The escape sequence \eb has a different meaning inside a character
1073    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1074    are not special inside a character class. Like any other unrecognized escape
1075    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1076    default, but cause an error if the PCRE_EXTRA option is set.
1077    .P
1078    A circumflex can conveniently be used with the upper case character types to
1079    specify a more restricted set of characters than the matching lower case type.
1080    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1081    whereas [\ew] includes underscore. A positive character class should be read as
1082    "something OR something OR ..." and a negative class as "NOT something AND NOT
1083    something AND NOT ...".
1084  .P  .P
1085  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1086  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 1100  this notation. For example,
1100    [01[:alpha:]%]    [01[:alpha:]%]
1101  .sp  .sp
1102  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1103  are  are:
1104  .sp  .sp
1105    alnum    letters and digits    alnum    letters and digits
1106    alpha    letters    alpha    letters
# Line 980  are Line 1111  are
1111    graph    printing characters, excluding space    graph    printing characters, excluding space
1112    lower    lower case letters    lower    lower case letters
1113    print    printing characters, including space    print    printing characters, including space
1114    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1115    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1116    upper    upper case letters    upper    upper case letters
1117    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 1001  matches "1", "2", or any non-digit. PCRE Line 1132  matches "1", "2", or any non-digit. PCRE
1132  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
1133  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1134  .P  .P
1135  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
1136  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1137    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1138    character properties are used. This is achieved by replacing the POSIX classes
1139    by other sequences, as follows:
1140    .sp
1141      [:alnum:]  becomes  \ep{Xan}
1142      [:alpha:]  becomes  \ep{L}
1143      [:blank:]  becomes  \eh
1144      [:digit:]  becomes  \ep{Nd}
1145      [:lower:]  becomes  \ep{Ll}
1146      [:space:]  becomes  \ep{Xps}
1147      [:upper:]  becomes  \ep{Lu}
1148      [:word:]   becomes  \ep{Xwd}
1149    .sp
1150    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1151    classes are unchanged, and match only characters with code points less than
1152    128.
1153  .  .
1154  .  .
1155  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1056  extracts it into the global options (and Line 1203  extracts it into the global options (and
1203  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1204  .P  .P
1205  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1206  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1207  .sp  .sp
1208    (a(?i)b)c    (a(?i)b)c
1209  .sp  .sp
# Line 1081  section entitled Line 1228  section entitled
1228  .\" </a>  .\" </a>
1229  "Newline sequences"  "Newline sequences"
1230  .\"  .\"
1231  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
1232  mode; this is equivalent to setting the PCRE_UTF8 option.  to set UTF-8 and Unicode property modes; they are equivalent to setting the
1233    PCRE_UTF8 and the PCRE_UCP options, respectively.
1234  .  .
1235  .  .
1236  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1096  Turning part of a pattern into a subpatt Line 1244  Turning part of a pattern into a subpatt
1244  .sp  .sp
1245    cat(aract|erpillar|)    cat(aract|erpillar|)
1246  .sp  .sp
1247  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1248  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1249  .sp  .sp
1250  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1251  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
1252  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
1253  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1254  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1255  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1256  .sp  .sp
1257    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1258  .sp  .sp
# Line 1154  at captured substring number one, whiche Line 1301  at captured substring number one, whiche
1301  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1302  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1303  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1304  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1305  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1306  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1307  .sp  .sp
1308    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1309    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1310    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1311  .sp  .sp
1312  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
1313  for that number by any subpattern. The following pattern matches "abcabc" or  set for that number by any subpattern. The following pattern matches "abcabc"
1314  "defdef":  or "defdef":
1315  .sp  .sp
1316    /(?|(abc)|(def))\e1/    /(?|(abc)|(def))\e1/
1317  .sp  .sp
1318  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a subroutine call to a numbered subpattern always refers to the
1319  refers to the first one in the pattern with the given number. The following  first one in the pattern with the given number. The following pattern matches
1320  pattern matches "abcabc" or "defabc":  "abcabc" or "defabc":
1321  .sp  .sp
1322    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1323  .sp  .sp
# Line 1204  In PCRE, a subpattern can be named in on Line 1350  In PCRE, a subpattern can be named in on
1350  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1351  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1352  .\" </a>  .\" </a>
1353  backreferences,  back references,
1354  .\"  .\"
1355  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1356  .\" </a>  .\" </a>
# Line 1246  The convenience function for extracting Line 1392  The convenience function for extracting
1392  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
1393  matched. This saves searching to find which numbered subpattern it was.  matched. This saves searching to find which numbered subpattern it was.
1394  .P  .P
1395  If you make a backreference to a non-unique named subpattern from elsewhere in  If you make a back reference to a non-unique named subpattern from elsewhere in
1396  the pattern, the one that corresponds to the first occurrence of the name is  the pattern, the one that corresponds to the first occurrence of the name is
1397  used. In the absence of duplicate numbers (see the previous section) this is  used. In the absence of duplicate numbers (see the previous section) this is
1398  the one with the lowest number. If you use a named reference in a condition  the one with the lowest number. If you use a named reference in a condition
# Line 1284  items: Line 1430  items:
1430    the \eC escape sequence    the \eC escape sequence
1431    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1432    the \eR escape sequence    the \eR escape sequence
1433    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1434    a character class    a character class
1435    a back reference (see next section)    a back reference (see next section)
1436    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1437    a recursive or "subroutine" call to a subpattern    a subroutine call to a subpattern (recursive or otherwise)
1438  .sp  .sp
1439  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1440  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1326  subpatterns that are referenced as Line 1472  subpatterns that are referenced as
1472  .\" </a>  .\" </a>
1473  subroutines  subroutines
1474  .\"  .\"
1475  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1476  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1477    .\" </a>
1478    "Defining subpatterns for use by reference only"
1479    .\"
1480    below). Items other than subpatterns that have a {0} quantifier are omitted
1481    from the compiled pattern.
1482  .P  .P
1483  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1484  abbreviations:  abbreviations:
# Line 1399  worth setting PCRE_DOTALL in order to ob Line 1550  worth setting PCRE_DOTALL in order to ob
1550  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1551  .P  .P
1552  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1553  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1554  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
1555  succeeds. Consider, for example:  succeeds. Consider, for example:
1556  .sp  .sp
# Line 1552  no such problem when named parentheses a Line 1703  no such problem when named parentheses a
1703  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1704  .P  .P
1705  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
1706  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence. This escape must be followed by an
1707  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1708  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1709  .sp  .sp
1710    (ring), \e1    (ring), \e1
1711    (ring), \eg1    (ring), \eg1
# Line 1568  example: Line 1719  example:
1719    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1720  .sp  .sp
1721  The sequence \eg{-1} is a reference to the most recently started capturing  The sequence \eg{-1} is a reference to the most recently started capturing
1722  subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}  subpattern before \eg, that is, is it equivalent to \e2 in this example.
1723  would be equivalent to \e1. The use of relative references can be helpful in  Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1724  long patterns, and also in patterns that are created by joining together  can be helpful in long patterns, and also in patterns that are created by
1725  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1726  .P  .P
1727  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1728  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1628  whitespace. Otherwise, the \eg{ syntax o Line 1779  whitespace. Otherwise, the \eg{ syntax o
1779  "Comments"  "Comments"
1780  .\"  .\"
1781  below) can be used.  below) can be used.
1782  .P  .
1783    .SS "Recursive back references"
1784    .rs
1785    .sp
1786  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
1787  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.
1788  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1642  to the previous iteration. In order for Line 1796  to the previous iteration. In order for
1796  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
1797  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
1798  minimum of zero.  minimum of zero.
1799    .P
1800    Back references of this type cause the group that they reference to be treated
1801    as an
1802    .\" HTML <a href="#atomicgroup">
1803    .\" </a>
1804    atomic group.
1805    .\"
1806    Once the whole group has been matched, a subsequent matching failure cannot
1807    cause backtracking into the middle of the group.
1808  .  .
1809  .  .
1810  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1661  those that look ahead of the current pos Line 1824  those that look ahead of the current pos
1824  that look behind it. An assertion subpattern is matched in the normal way,  that look behind it. An assertion subpattern is matched in the normal way,
1825  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
1826  .P  .P
1827  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1828  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
1829  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1830  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1831  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1832  because it does not make sense for negative assertions.  .P
1833    For compatibility with Perl, assertion subpatterns may be repeated; though
1834    it makes no sense to assert the same thing several times, the side effect of
1835    capturing parentheses may occasionally be useful. In practice, there only three
1836    cases:
1837    .sp
1838    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1839    However, it may contain internal capturing parenthesized groups that are called
1840    from elsewhere via the
1841    .\" HTML <a href="#subpatternsassubroutines">
1842    .\" </a>
1843    subroutine mechanism.
1844    .\"
1845    .sp
1846    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1847    were {0,1}. At run time, the rest of the pattern match is tried with and
1848    without the assertion, the order depending on the greediness of the quantifier.
1849    .sp
1850    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1851    The assertion is obeyed just once when encountered during matching.
1852  .  .
1853  .  .
1854  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1695  lookbehind assertion is needed to achiev Line 1877  lookbehind assertion is needed to achiev
1877  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
1878  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
1879  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.
1880  The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a  The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
 synonym for (?!).  
1881  .  .
1882  .  .
1883  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1721  is permitted, but Line 1902  is permitted, but
1902  .sp  .sp
1903  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1904  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
1905  extension compared with Perl (5.8 and 5.10), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1906  match the same length of string. An assertion such as  length of string. An assertion such as
1907  .sp  .sp
1908    (?<=ab(c|de))    (?<=ab(c|de))
1909  .sp  .sp
# Line 1732  branches: Line 1913  branches:
1913  .sp  .sp
1914    (?<=abc|abde)    (?<=abc|abde)
1915  .sp  .sp
1916  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1917  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1918  .\" </a>  .\" </a>
1919  (see above)  (see above)
# Line 1836  already been matched. The two possible f Line 2017  already been matched. The two possible f
2017  .sp  .sp
2018  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2019  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
2020  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2021    itself contain nested subpatterns of any form, including conditional
2022    subpatterns; the restriction to two alternatives applies only at the level of
2023    the condition. This pattern fragment is an example where the alternatives are
2024    complex:
2025    .sp
2026      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2027    .sp
2028  .P  .P
2029  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2030  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1853  matched. If there is more than one captu Line 2041  matched. If there is more than one captu
2041  .\" </a>  .\" </a>
2042  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2043  .\"  .\"
2044  the condition is true if any of them have been set. An alternative notation is  the condition is true if any of them have matched. An alternative notation is
2045  to precede the digits with a plus or minus sign. In this case, the subpattern  to precede the digits with a plus or minus sign. In this case, the subpattern
2046  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2047  can be referenced by (?(-1), the next most recent by (?(-2), and so on. In  can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2048  looping constructs it can also make sense to refer to subsequent groups with  loops it can also make sense to refer to subsequent groups. The next
2049  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2050    zero in any of these forms is not used; it provokes a compile-time error.)
2051  .P  .P
2052  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2053  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 1869  three parts for ease of discussion: Line 2058  three parts for ease of discussion:
2058  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2059  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
2060  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
2061  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2062  or not. If they did, that is, if subject started with an opening parenthesis,  matched. If they did, that is, if subject started with an opening parenthesis,
2063  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
2064  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2065  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 1926  The syntax for recursive patterns Line 2115  The syntax for recursive patterns
2115  .\"  .\"
2116  is described below.  is described below.
2117  .  .
2118    .\" HTML <a name="subdefine"></a>
2119  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2120  .rs  .rs
2121  .sp  .sp
# Line 1933  If the condition is the string (DEFINE), Line 2123  If the condition is the string (DEFINE),
2123  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
2124  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2125  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
2126  "subroutines" that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
2127  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2128  .\" </a>  .\" </a>
2129  "subroutines"  subroutines
2130  .\"  .\"
2131  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 such as
2132  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2133    breaks):
2134  .sp  .sp
2135    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2136    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1974  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2165  dd-aaa-dd or dd-dd-dd, where aaa are let
2165  .SH COMMENTS  .SH COMMENTS
2166  .rs  .rs
2167  .sp  .sp
2168  The sequence (?# marks the start of a comment that continues up to the next  There are two ways of including comments in patterns that are processed by
2169  closing parenthesis. Nested parentheses are not permitted. The characters  PCRE. In both cases, the start of the comment must not be in a character class,
2170  that make up a comment play no part in the pattern matching at all.  nor in the middle of any other sequence of related characters such as (?: or a
2171    subpattern name or number. The characters that make up a comment play no part
2172    in the pattern matching.
2173  .P  .P
2174  If the PCRE_EXTENDED option is set, an unescaped # character outside a  The sequence (?# marks the start of a comment that continues up to the next
2175  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2176  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2177    this case continues to immediately after the next newline character or
2178    character sequence in the pattern. Which characters are interpreted as newlines
2179    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2180    sequence at the start of the pattern, as described in the section entitled
2181    .\" HTML <a href="#newlines">
2182    .\" </a>
2183    "Newline conventions"
2184    .\"
2185    above. Note that the end of this type of comment is a literal newline sequence
2186    in the pattern; escape sequences that happen to represent a newline do not
2187    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2188    default newline convention is in force:
2189    .sp
2190      abc #comment \en still comment
2191    .sp
2192    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2193    a newline in the pattern. The sequence \en is still literal at this stage, so
2194    it does not terminate the comment. Only an actual character with the code value
2195    0x0a (the default newline) does so.
2196  .  .
2197  .  .
2198  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2009  individual subpattern recursion. After i Line 2221  individual subpattern recursion. After i
2221  this kind of recursion was subsequently introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2222  .P  .P
2223  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
2224  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive subroutine call of the subpattern of the
2225  provided that it occurs inside that subpattern. (If not, it is a  given number, provided that it occurs inside that subpattern. (If not, it is a
2226  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2227  .\" </a>  .\" </a>
2228  "subroutine"  non-recursive subroutine
2229  .\"  .\"
2230  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
2231  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
# Line 2038  We have put the pattern into parentheses Line 2250  We have put the pattern into parentheses
2250  them instead of the whole pattern.  them instead of the whole pattern.
2251  .P  .P
2252  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
2253  is made easier by the use of relative references (a Perl 5.10 feature).  is made easier by the use of relative references. Instead of (?1) in the
2254  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  pattern above you can write (?-2) to refer to the second most recently opened
2255  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2256  negative number counts capturing parentheses leftwards from the point at which  capturing parentheses leftwards from the point at which it is encountered.
 it is encountered.  
2257  .P  .P
2258  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2259  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2260  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2261  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2262  .\" </a>  .\" </a>
2263  "subroutine"  non-recursive subroutine
2264  .\"  .\"
2265  calls, as described in the next section.  calls, as described in the next section.
2266  .P  .P
# Line 2086  documentation). If the pattern above is Line 2297  documentation). If the pattern above is
2297  .sp  .sp
2298  the value for the inner capturing parentheses (numbered 2) is "ef", which is  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2299  the last value taken on at the top level. If a capturing subpattern is not  the last value taken on at the top level. If a capturing subpattern is not
2300  matched at the top level, its final value is unset, even if it is (temporarily)  matched at the top level, its final captured value is unset, even if it was
2301  set at a deeper level.  (temporarily) set at a deeper level during the matching process.
2302  .P  .P
2303  If there are more than 15 capturing parentheses in a pattern, PCRE has to  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2304  obtain extra memory to store data during a recursion, which it does by using  obtain extra memory to store data during a recursion, which it does by using
# Line 2107  is the actual recursive call. Line 2318  is the actual recursive call.
2318  .  .
2319  .  .
2320  .\" HTML <a name="recursiondifference"></a>  .\" HTML <a name="recursiondifference"></a>
2321  .SS "Recursion difference from Perl"  .SS "Differences in recursion processing between PCRE and Perl"
2322  .rs  .rs
2323  .sp  .sp
2324  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2325  treated as an atomic group. That is, once it has matched some of the subject  (like Python, but unlike Perl), a recursive subpattern call is always treated
2326  string, it is never re-entered, even if it contains untried alternatives and  as an atomic group. That is, once it has matched some of the subject string, it
2327  there is a subsequent matching failure. This can be illustrated by the  is never re-entered, even if it contains untried alternatives and there is a
2328  following pattern, which purports to match a palindromic string that contains  subsequent matching failure. This can be illustrated by the following pattern,
2329  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  which purports to match a palindromic string that contains an odd number of
2330    characters (for example, "a", "aba", "abcba", "abcdcba"):
2331  .sp  .sp
2332    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2333  .sp  .sp
# Line 2145  time we do have another alternative to t Line 2357  time we do have another alternative to t
2357  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2358  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2359  .P  .P
2360  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that it matches all palindromic strings, not just
2361  with an odd number of characters, it is tempting to change the pattern to this:  those with an odd number of characters, it is tempting to change the pattern to
2362    this:
2363  .sp  .sp
2364    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2365  .sp  .sp
# Line 2175  For example, although "abcba" is correct Line 2388  For example, although "abcba" is correct
2388  PCRE finds the palindrome "aba" at the start, then fails at top level because  PCRE finds the palindrome "aba" at the start, then fails at top level because
2389  the end of the string does not follow. Once again, it cannot jump back into the  the end of the string does not follow. Once again, it cannot jump back into the
2390  recursion to try other alternatives, so the entire match fails.  recursion to try other alternatives, so the entire match fails.
2391    .P
2392    The second way in which PCRE and Perl differ in their recursion processing is
2393    in the handling of captured values. In Perl, when a subpattern is called
2394    recursively or as a subpattern (see the next section), it has no access to any
2395    values that were captured outside the recursion, whereas in PCRE these values
2396    can be referenced. Consider this pattern:
2397    .sp
2398      ^(.)(\e1|a(?2))
2399    .sp
2400    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2401    then in the second group, when the back reference \e1 fails to match "b", the
2402    second alternative matches "a" and then recurses. In the recursion, \e1 does
2403    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2404    match because inside the recursive call \e1 cannot access the externally set
2405    value.
2406  .  .
2407  .  .
2408  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2409  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2410  .rs  .rs
2411  .sp  .sp
2412  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern call (either by number or by
2413  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
2414  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2415  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2416  relative, as in these examples:  relative, as in these examples:
2417  .sp  .sp
# Line 2203  matches "sense and sensibility" and "res Line 2431  matches "sense and sensibility" and "res
2431  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
2432  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2433  .P  .P
2434  Like recursive subpatterns, a subroutine call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2435  group. That is, once it has matched some of the subject string, it is never  groups. That is, once a subroutine has matched some of the subject string, it
2436  re-entered, even if it contains untried alternatives and there is a subsequent  is never re-entered, even if it contains untried alternatives and there is a
2437  matching failure. Any capturing parentheses that are set during the subroutine  subsequent matching failure. Any capturing parentheses that are set during the
2438  call revert to their previous values afterwards.  subroutine call revert to their previous values afterwards.
2439  .P  .P
2440  When a subpattern is used as a subroutine, processing options such as  Processing options such as case-independence are fixed when a subpattern is
2441  case-independence are fixed when the subpattern is defined. They cannot be  defined, so if it is used as a subroutine, such options cannot be changed for
2442  changed for different calls. For example, consider this pattern:  different calls. For example, consider this pattern:
2443  .sp  .sp
2444    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2445  .sp  .sp
# Line 2276  description of the interface to the call Line 2504  description of the interface to the call
2504  documentation.  documentation.
2505  .  .
2506  .  .
2507    .\" HTML <a name="backtrackcontrol"></a>
2508  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2509  .rs  .rs
2510  .sp  .sp
# Line 2291  a backtracking algorithm. With the excep Line 2520  a backtracking algorithm. With the excep
2520  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2521  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2522  .P  .P
2523  If any of these verbs are used in an assertion or subroutine subpattern  If any of these verbs are used in an assertion or in a subpattern that is
2524  (including recursive subpatterns), their effect is confined to that subpattern;  called as a subroutine (whether or not recursively), their effect is confined
2525  it does not extend to the surrounding pattern. Note that such subpatterns are  to that subpattern; it does not extend to the surrounding pattern, with one
2526  processed as anchored at the point where they are tested.  exception: a *MARK that is encountered in a positive assertion \fIis\fP passed
2527    back (compare capturing parentheses in assertions). Note that such subpatterns
2528    are processed as anchored at the point where they are tested. Note also that
2529    Perl's treatment of subroutines is different in some cases.
2530  .P  .P
2531  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
2532  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2533  (*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,
2534  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. A name is any sequence of
2535  are two kinds:  characters that does not include a closing parenthesis. If the name is empty,
2536    that is, if the closing parenthesis immediately follows the colon, the effect
2537    is as if the colon were not there. Any number of these verbs may occur in a
2538    pattern.
2539    .P
2540    PCRE contains some optimizations that are used to speed up matching by running
2541    some checks at the start of each match attempt. For example, it may know the
2542    minimum length of matching subject, or that a particular character must be
2543    present. When one of these optimizations suppresses the running of a match, any
2544    included backtracking verbs will not, of course, be processed. You can suppress
2545    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2546    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2547    pattern with (*NO_START_OPT).
2548    .
2549  .  .
2550  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2551  .rs  .rs
2552  .sp  .sp
2553  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2554    followed by a name.
2555  .sp  .sp
2556     (*ACCEPT)     (*ACCEPT)
2557  .sp  .sp
2558  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2559  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2560  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is  subroutine, only that subpattern is ended successfully. Matching then continues
2561  captured. (This feature was added to PCRE at release 8.00.) For example:  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2562    far is captured. For example:
2563  .sp  .sp
2564    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2565  .sp  .sp
# Line 2321  the outer parentheses. Line 2568  the outer parentheses.
2568  .sp  .sp
2569    (*FAIL) or (*F)    (*FAIL) or (*F)
2570  .sp  .sp
2571  This verb causes the match to fail, forcing backtracking to occur. It is  This verb causes a matching failure, forcing backtracking to occur. It is
2572  equivalent to (?!) but easier to read. The Perl documentation notes that it is  equivalent to (?!) but easier to read. The Perl documentation notes that it is
2573  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2574  Perl features that are not present in PCRE. The nearest equivalent is the  Perl features that are not present in PCRE. The nearest equivalent is the
# Line 2332  callout feature, as for example in this Line 2579  callout feature, as for example in this
2579  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
2580  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2581  .  .
2582    .
2583    .SS "Recording which path was taken"
2584    .rs
2585    .sp
2586    There is one verb whose main purpose is to track how a match was arrived at,
2587    though it also has a secondary use in conjunction with advancing the match
2588    starting point (see (*SKIP) below).
2589    .sp
2590      (*MARK:NAME) or (*:NAME)
2591    .sp
2592    A name is always required with this verb. There may be as many instances of
2593    (*MARK) as you like in a pattern, and their names do not have to be unique.
2594    .P
2595    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2596    to the caller via the \fIpcre_extra\fP data structure, as described in the
2597    .\" HTML <a href="pcreapi.html#extradata">
2598    .\" </a>
2599    section on \fIpcre_extra\fP
2600    .\"
2601    in the
2602    .\" HREF
2603    \fBpcreapi\fP
2604    .\"
2605    documentation. No data is returned for a partial match. Here is an example of
2606    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2607    outputting of (*MARK) data:
2608    .sp
2609      /X(*MARK:A)Y|X(*MARK:B)Z/K
2610      XY
2611       0: XY
2612      MK: A
2613      XZ
2614       0: XZ
2615      MK: B
2616    .sp
2617    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2618    indicates which of the two alternatives matched. This is a more efficient way
2619    of obtaining this information than putting each alternative in its own
2620    capturing parentheses.
2621    .P
2622    If (*MARK) is encountered in a positive assertion, its name is recorded and
2623    passed back if it is the last-encountered. This does not happen for negative
2624    assertions.
2625    .P
2626    A name may also be returned after a failed match if the final path through the
2627    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2628    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2629    starting point for matching is advanced, the final check is often with an empty
2630    string, causing a failure before (*MARK) is reached. For example:
2631    .sp
2632      /X(*MARK:A)Y|X(*MARK:B)Z/K
2633      XP
2634      No match
2635    .sp
2636    There are three potential starting points for this match (starting with X,
2637    starting with P, and with an empty string). If the pattern is anchored, the
2638    result is different:
2639    .sp
2640      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2641      XP
2642      No match, mark = B
2643    .sp
2644    PCRE's start-of-match optimizations can also interfere with this. For example,
2645    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2646    subject length for a match, a shorter subject will not be scanned at all.
2647    .P
2648    Note that similar anomalies (though different in detail) exist in Perl, no
2649    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2650    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2651    .
2652    .
2653  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2654  .rs  .rs
2655  .sp  .sp
2656  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2657  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
2658  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
2659    the verb. However, when one of these verbs appears inside an atomic group, its
2660    effect is confined to that group, because once the group has been matched,
2661    there is never any backtracking into it. In this situation, backtracking can
2662    "jump back" to the left of the entire atomic group. (Remember also, as stated
2663    above, that this localization also applies in subroutine calls and assertions.)
2664    .P
2665    These verbs differ in exactly what kind of failure occurs when backtracking
2666    reaches them.
2667  .sp  .sp
2668    (*COMMIT)    (*COMMIT)
2669  .sp  .sp
2670  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
2671  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
2672  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
2673  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
2674  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2675  .sp  .sp
2676    a+(*COMMIT)b    a+(*COMMIT)b
2677  .sp  .sp
2678  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
2679  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
2680  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2681    (*PRUNE)  match failure.
2682  .sp  .P
2683  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,
2684  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
2685  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2686  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2687  if there is no match to the right, backtracking cannot cross (*PRUNE).    /(*COMMIT)abc/
2688  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    xyzabc
2689  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2690  be expressed in any other way.    xyzabc\eY
2691      No match
2692    .sp
2693    PCRE knows that any match must start with "a", so the optimization skips along
2694    the subject to "a" before running the first match attempt, which succeeds. When
2695    the optimization is disabled by the \eY escape in the second subject, the match
2696    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2697    starting points.
2698    .sp
2699      (*PRUNE) or (*PRUNE:NAME)
2700    .sp
2701    This verb causes the match to fail at the current starting position in the
2702    subject if the rest of the pattern does not match. If the pattern is
2703    unanchored, the normal "bumpalong" advance to the next starting character then
2704    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2705    reached, or when matching to the right of (*PRUNE), but if there is no match to
2706    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2707    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2708    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2709    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2710    match fails completely; the name is passed back if this is the final attempt.
2711    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2712    pattern (*PRUNE) has the same effect as (*COMMIT).
2713  .sp  .sp
2714    (*SKIP)    (*SKIP)
2715  .sp  .sp
2716  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
2717  "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,
2718  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2719  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
2720    successful match. Consider:
2721  .sp  .sp
2722    a+(*SKIP)b    a+(*SKIP)b
2723  .sp  .sp
# Line 2379  effect as this example; although it woul Line 2728  effect as this example; although it woul
2728  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2729  instead of skipping on to "c".  instead of skipping on to "c".
2730  .sp  .sp
2731    (*THEN)    (*SKIP:NAME)
2732  .sp  .sp
2733  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
2734  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2735  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,
2736  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2737    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2738    matching name is found, normal "bumpalong" of one character happens (that is,
2739    the (*SKIP) is ignored).
2740    .sp
2741      (*THEN) or (*THEN:NAME)
2742    .sp
2743    This verb causes a skip to the next innermost alternative if the rest of the
2744    pattern does not match. That is, it cancels pending backtracking, but only
2745    within the current alternative. Its name comes from the observation that it can
2746    be used for a pattern-based if-then-else block:
2747  .sp  .sp
2748    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2749  .sp  .sp
2750  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
2751  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
2752  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2753  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
2754    overall match fails. If (*THEN) is not inside an alternation, it acts like
2755    (*PRUNE).
2756    .P
2757    Note that a subpattern that does not contain a | character is just a part of
2758    the enclosing alternative; it is not a nested alternation with only one
2759    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2760    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2761    pattern fragments that do not contain any | characters at this level:
2762    .sp
2763      A (B(*THEN)C) | D
2764    .sp
2765    If A and B are matched, but there is a failure in C, matching does not
2766    backtrack into A; instead it moves to the next alternative, that is, D.
2767    However, if the subpattern containing (*THEN) is given an alternative, it
2768    behaves differently:
2769    .sp
2770      A (B(*THEN)C | (*FAIL)) | D
2771    .sp
2772    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2773    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2774    because there are no more alternatives to try. In this case, matching does now
2775    backtrack into A.
2776    .P
2777    Note also that a conditional subpattern is not considered as having two
2778    alternatives, because only one is ever used. In other words, the | character in
2779    a conditional subpattern has a different meaning. Ignoring white space,
2780    consider:
2781    .sp
2782      ^.*? (?(?=a) a | b(*THEN)c )
2783    .sp
2784    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2785    it initially matches zero characters. The condition (?=a) then fails, the
2786    character "b" is matched, but "c" is not. At this point, matching does not
2787    backtrack to .*? as might perhaps be expected from the presence of the |
2788    character. The conditional subpattern is part of the single alternative that
2789    comprises the whole pattern, and so the match fails. (If there was a backtrack
2790    into .*?, allowing it to match "b", the match would succeed.)
2791    .P
2792    The verbs just described provide four different "strengths" of control when
2793    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2794    next alternative. (*PRUNE) comes next, failing the match at the current
2795    starting position, but allowing an advance to the next character (for an
2796    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2797    than one character. (*COMMIT) is the strongest, causing the entire match to
2798    fail.
2799    .P
2800    If more than one such verb is present in a pattern, the "strongest" one wins.
2801    For example, consider this pattern, where A, B, etc. are complex pattern
2802    fragments:
2803    .sp
2804      (A(*COMMIT)B(*THEN)C|D)
2805    .sp
2806    Once A has matched, PCRE is committed to this match, at the current starting
2807    position. If subsequently B matches, but C does not, the normal (*THEN) action
2808    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2809    overrides.
2810  .  .
2811  .  .
2812  .SH "SEE ALSO"  .SH "SEE ALSO"
# Line 2415  Cambridge CB2 3QH, England. Line 2830  Cambridge CB2 3QH, England.
2830  .rs  .rs
2831  .sp  .sp
2832  .nf  .nf
2833  Last updated: 18 October 2009  Last updated: 09 October 2011
2834  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2835  .fi  .fi

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