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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 416 by ph10, Sat Apr 11 14:34:02 2009 UTC
# 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, you must
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  the PCRE_UTF8 option. There is also a special sequence that can be given at the
27  places below. There is also a summary of UTF-8 features in 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 30  The remainder of this document discusses Line 46  The remainder of this document discusses
46  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
47  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
48  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
49  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
50  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
51    alternative function, and how it differs from the normal function, are
52    discussed in the
53  .\" HREF  .\" HREF
54  \fBpcrematching\fP  \fBpcrematching\fP
55  .\"  .\"
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  .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. For
87    example, on a Unix system where LF is the default newline sequence, the pattern
88    .sp
89      (*CR)a.b
90    .sp
91    changes the convention to CR. That pattern matches "a\enb" because LF is no
92    longer a newline. Note that these special settings, which are not
93    Perl-compatible, are recognized only at the very start of a pattern, and that
94    they must be in upper case. If more than one of them is present, the last one
95    is used.
96    .P
97    The newline convention does not affect what the \eR escape sequence matches. By
98    default, this is any Unicode newline sequence, for Perl compatibility. However,
99    this can be changed; see the description of \eR in the section entitled
100    .\" HTML <a href="#newlineseq">
101    .\" </a>
102    "Newline sequences"
103    .\"
104    below. A change of \eR setting can be combined with a change of newline
105    convention.
106    .
107    .
108    .SH "CHARACTERS AND METACHARACTERS"
109    .rs
110    .sp
111  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
112  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
113  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
# Line 60  interpreted in some special way. Line 131  interpreted in some special way.
131  .P  .P
132  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
133  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
134  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
135  as follows:  are as follows:
136  .sp  .sp
137    \e      general escape character with several uses    \e      general escape character with several uses
138    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 92  a character class the only metacharacter Line 163  a character class the only metacharacter
163  .sp  .sp
164  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
165  .  .
166    .
167  .SH BACKSLASH  .SH BACKSLASH
168  .rs  .rs
169  .sp  .sp
# Line 142  represents: Line 214  represents:
214    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
215    \ee        escape (hex 1B)    \ee        escape (hex 1B)
216    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
217    \en        newline (hex 0A)    \en        linefeed (hex 0A)
218    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
219    \et        tab (hex 09)    \et        tab (hex 09)
220    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
# Line 157  Thus \ecz becomes hex 1A, but \ec{ becom Line 229  Thus \ecz becomes hex 1A, but \ec{ becom
229  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
230  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{
231  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
232  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
233  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
234  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
235  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
236  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
237    there is no terminating }, this form of escape is not recognized. Instead, the
238    initial \ex will be interpreted as a basic hexadecimal escape, with no
239    following digits, giving a character whose value is zero.
240  .P  .P
241  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
242  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 190  parenthesized subpatterns. Line 265  parenthesized subpatterns.
265  .P  .P
266  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
267  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
268  digits following the backslash, ane uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
269  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
270  character specified in octal must be less than \e400. In UTF-8 mode, values up  character specified in octal must be less than \e400. In UTF-8 mode, values up
271  to \e777 are permitted. For example:  to \e777 are permitted. For example:
# Line 221  zero, because no more than three octal d Line 296  zero, because no more than three octal d
296  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
297  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
298  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08), and the
299  sequence \eX is interpreted as the character "X". Outside a character class,  sequences \eR and \eX are interpreted as the characters "R" and "X",
300  these sequences have different meanings  respectively. Outside a character class, these sequences have different
301    meanings
302  .\" HTML <a href="#uniextseq">  .\" HTML <a href="#uniextseq">
303  .\" </a>  .\" </a>
304  (see below).  (see below).
305  .\"  .\"
306  .  .
307  .  .
308    .SS "Absolute and relative back references"
309    .rs
310    .sp
311    The sequence \eg followed by an unsigned or a negative number, optionally
312    enclosed in braces, is an absolute or relative back reference. A named back
313    reference can be coded as \eg{name}. Back references are discussed
314    .\" HTML <a href="#backreferences">
315    .\" </a>
316    later,
317    .\"
318    following the discussion of
319    .\" HTML <a href="#subpattern">
320    .\" </a>
321    parenthesized subpatterns.
322    .\"
323    .
324    .
325    .SS "Absolute and relative subroutine calls"
326    .rs
327    .sp
328    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
329    a number enclosed either in angle brackets or single quotes, is an alternative
330    syntax for referencing a subpattern as a "subroutine". Details are discussed
331    .\" HTML <a href="#onigurumasubroutines">
332    .\" </a>
333    later.
334    .\"
335    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
336    synonymous. The former is a back reference; the latter is a subroutine call.
337    .
338    .
339  .SS "Generic character types"  .SS "Generic character types"
340  .rs  .rs
341  .sp  .sp
342  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
343  following are always recognized:  following are always recognized:
344  .sp  .sp
345    \ed     any decimal digit    \ed     any decimal digit
346    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
347      \eh     any horizontal whitespace character
348      \eH     any character that is not a horizontal whitespace character
349    \es     any whitespace character    \es     any whitespace character
350    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
351      \ev     any vertical whitespace character
352      \eV     any character that is not a vertical whitespace character
353    \ew     any "word" character    \ew     any "word" character
354    \eW     any "non-word" character    \eW     any "non-word" character
355  .sp  .sp
# Line 252  there is no character to match. Line 363  there is no character to match.
363  .P  .P
364  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).
365  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
366  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
367  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
368  does.)  does.
369    .P
370    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
371    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
372    character property support is available. These sequences retain their original
373    meanings from before UTF-8 support was available, mainly for efficiency
374    reasons. Note that this also affects \eb, because it is defined in terms of \ew
375    and \eW.
376    .P
377    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
378    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
379    The horizontal space characters are:
380    .sp
381      U+0009     Horizontal tab
382      U+0020     Space
383      U+00A0     Non-break space
384      U+1680     Ogham space mark
385      U+180E     Mongolian vowel separator
386      U+2000     En quad
387      U+2001     Em quad
388      U+2002     En space
389      U+2003     Em space
390      U+2004     Three-per-em space
391      U+2005     Four-per-em space
392      U+2006     Six-per-em space
393      U+2007     Figure space
394      U+2008     Punctuation space
395      U+2009     Thin space
396      U+200A     Hair space
397      U+202F     Narrow no-break space
398      U+205F     Medium mathematical space
399      U+3000     Ideographic space
400    .sp
401    The vertical space characters are:
402    .sp
403      U+000A     Linefeed
404      U+000B     Vertical tab
405      U+000C     Formfeed
406      U+000D     Carriage return
407      U+0085     Next line
408      U+2028     Line separator
409      U+2029     Paragraph separator
410  .P  .P
411  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character less than 256 that is a
412  letter or digit. The definition of letters and digits is controlled by PCRE's  letter or digit. The definition of letters and digits is controlled by PCRE's
# Line 268  in the Line 420  in the
420  .\" HREF  .\" HREF
421  \fBpcreapi\fP  \fBpcreapi\fP
422  .\"  .\"
423  page). For example, in the "fr_FR" (French) locale, some character codes  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
424  greater than 128 are used for accented letters, and these are matched by \ew.  or "french" in Windows, some character codes greater than 128 are used for
425  .P  accented letters, and these are matched by \ew. The use of locales with Unicode
426  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  is discouraged.
427  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .
428  character property support is available. The use of locales with Unicode is  .
429  discouraged.  .\" HTML <a name="newlineseq"></a>
430    .SS "Newline sequences"
431    .rs
432    .sp
433    Outside a character class, by default, the escape sequence \eR matches any
434    Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
435    equivalent to the following:
436    .sp
437      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
438    .sp
439    This is an example of an "atomic group", details of which are given
440    .\" HTML <a href="#atomicgroup">
441    .\" </a>
442    below.
443    .\"
444    This particular group matches either the two-character sequence CR followed by
445    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
446    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
447    line, U+0085). The two-character sequence is treated as a single unit that
448    cannot be split.
449    .P
450    In UTF-8 mode, two additional characters whose codepoints are greater than 255
451    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
452    Unicode character property support is not needed for these characters to be
453    recognized.
454    .P
455    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
456    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
457    either at compile time or when the pattern is matched. (BSR is an abbrevation
458    for "backslash R".) This can be made the default when PCRE is built; if this is
459    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
460    It is also possible to specify these settings by starting a pattern string with
461    one of the following sequences:
462    .sp
463      (*BSR_ANYCRLF)   CR, LF, or CRLF only
464      (*BSR_UNICODE)   any Unicode newline sequence
465    .sp
466    These override the default and the options given to \fBpcre_compile()\fP, but
467    they can be overridden by options given to \fBpcre_exec()\fP. Note that these
468    special settings, which are not Perl-compatible, are recognized only at the
469    very start of a pattern, and that they must be in upper case. If more than one
470    of them is present, the last one is used. They can be combined with a change of
471    newline convention, for example, a pattern can start with:
472    .sp
473      (*ANY)(*BSR_ANYCRLF)
474    .sp
475    Inside a character class, \eR matches the letter "R".
476  .  .
477  .  .
478  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 282  discouraged. Line 480  discouraged.
480  .rs  .rs
481  .sp  .sp
482  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
483  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
484  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
485    characters whose codepoints are less than 256, but they do work in this mode.
486    The extra escape sequences are:
487  .sp  .sp
488    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
489    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
# Line 307  Those that are not part of an identified Line 507  Those that are not part of an identified
507  .P  .P
508  Arabic,  Arabic,
509  Armenian,  Armenian,
510    Balinese,
511  Bengali,  Bengali,
512  Bopomofo,  Bopomofo,
513  Braille,  Braille,
# Line 316  Canadian_Aboriginal, Line 517  Canadian_Aboriginal,
517  Cherokee,  Cherokee,
518  Common,  Common,
519  Coptic,  Coptic,
520    Cuneiform,
521  Cypriot,  Cypriot,
522  Cyrillic,  Cyrillic,
523  Deseret,  Deseret,
# Line 345  Malayalam, Line 547  Malayalam,
547  Mongolian,  Mongolian,
548  Myanmar,  Myanmar,
549  New_Tai_Lue,  New_Tai_Lue,
550    Nko,
551  Ogham,  Ogham,
552  Old_Italic,  Old_Italic,
553  Old_Persian,  Old_Persian,
554  Oriya,  Oriya,
555  Osmanya,  Osmanya,
556    Phags_Pa,
557    Phoenician,
558  Runic,  Runic,
559  Shavian,  Shavian,
560  Sinhala,  Sinhala,
# Line 430  The special property L& is also supporte Line 635  The special property L& is also supporte
635  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
636  a modifier or "other".  a modifier or "other".
637  .P  .P
638    The Cs (Surrogate) property applies only to characters in the range U+D800 to
639    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
640    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
641    (see the discussion of PCRE_NO_UTF8_CHECK in the
642    .\" HREF
643    \fBpcreapi\fP
644    .\"
645    page).
646    .P
647  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for these properties that Perl supports (such as \ep{Letter})
648  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
649  properties with "Is".  properties with "Is".
# Line 454  atomic group Line 668  atomic group
668  (see below).  (see below).
669  .\"  .\"
670  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
671  preceding character.  preceding character. None of them have codepoints less than 256, so in
672    non-UTF-8 mode \eX matches any one character.
673  .P  .P
674  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
675  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
# Line 462  why the traditional escape sequences suc Line 677  why the traditional escape sequences suc
677  properties in PCRE.  properties in PCRE.
678  .  .
679  .  .
680    .\" HTML <a name="resetmatchstart"></a>
681    .SS "Resetting the match start"
682    .rs
683    .sp
684    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
685    matched characters not to be included in the final matched sequence. For
686    example, the pattern:
687    .sp
688      foo\eKbar
689    .sp
690    matches "foobar", but reports that it has matched "bar". This feature is
691    similar to a lookbehind assertion
692    .\" HTML <a href="#lookbehind">
693    .\" </a>
694    (described below).
695    .\"
696    However, in this case, the part of the subject before the real match does not
697    have to be of fixed length, as lookbehind assertions do. The use of \eK does
698    not interfere with the setting of
699    .\" HTML <a href="#subpattern">
700    .\" </a>
701    captured substrings.
702    .\"
703    For example, when the pattern
704    .sp
705      (foo)\eKbar
706    .sp
707    matches "foobar", the first substring is still set to "foo".
708    .
709    .
710  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
711  .SS "Simple assertions"  .SS "Simple assertions"
712  .rs  .rs
713  .sp  .sp
714  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
715  specifies a condition that has to be met at a particular point in a match,  specifies a condition that has to be met at a particular point in a match,
716  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
717  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 723  The backslashed assertions are:
723  .sp  .sp
724    \eb     matches at a word boundary    \eb     matches at a word boundary
725    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
726    \eA     matches at start of subject    \eA     matches at the start of the subject
727    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
728    \ez     matches at end of subject            also matches before a newline at the end of the subject
729    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
730      \eG     matches at the first matching position in the subject
731  .sp  .sp
732  These assertions may not appear in character classes (but note that \eb has a  These assertions may not appear in character classes (but note that \eb has a
733  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
# Line 578  end of the subject in both modes, and if Line 824  end of the subject in both modes, and if
824  .sp  .sp
825  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
826  the subject string except (by default) a character that signifies the end of a  the subject string except (by default) a character that signifies the end of a
827  line. In UTF-8 mode, the matched character may be more than one byte long. When  line. In UTF-8 mode, the matched character may be more than one byte long.
828  a line ending is defined as a single character (CR or LF), dot never matches  .P
829  that character; when the two-character sequence CRLF is used, dot does not  When a line ending is defined as a single character, dot never matches that
830  match CR if it is immediately followed by LF, but otherwise it matches all  character; when the two-character sequence CRLF is used, dot does not match CR
831  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
832    (including isolated CRs and LFs). When any Unicode line endings are being
833    recognized, dot does not match CR or LF or any of the other line ending
834    characters.
835  .P  .P
836  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
837  option is set, a dot matches any one character, without exception. If newline  option is set, a dot matches any one character, without exception. If the
838  is defined as the two-character sequence CRLF, it takes two dots to match it.  two-character sequence CRLF is present in the subject string, it takes two dots
839    to match it.
840  .P  .P
841  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
842  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
# Line 597  special meaning in a character class. Line 847  special meaning in a character class.
847  .rs  .rs
848  .sp  .sp
849  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
850  in and out of UTF-8 mode. Unlike a dot, it always matches CR and LF. The  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
851  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  characters. The feature is provided in Perl in order to match individual bytes
852  Because it breaks up UTF-8 characters into individual bytes, what remains in  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
853  the string may be a malformed UTF-8 string. For this reason, the \eC escape  what remains in the string may be a malformed UTF-8 string. For this reason,
854  sequence is best avoided.  the \eC escape sequence is best avoided.
855  .P  .P
856  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
857  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 652  If you want to use caseless matching for Line 902  If you want to use caseless matching for
902  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
903  UTF-8 support.  UTF-8 support.
904  .P  .P
905  Characters that might indicate line breaks (CR and LF) are never treated in any  Characters that might indicate line breaks are never treated in any special way
906  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
907  in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
908  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
909  .P  .P
910  The minus (hyphen) character can be used to specify a range of characters in a  The minus (hyphen) character can be used to specify a range of characters in a
911  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
# Line 679  example [\ex{100}-\ex{2ff}]. Line 929  example [\ex{100}-\ex{2ff}].
929  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
930  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
931  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
932  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
933  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
934  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
935  property support.  property support.
# Line 769  alternative in the subpattern. Line 1019  alternative in the subpattern.
1019  .rs  .rs
1020  .sp  .sp
1021  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1022  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
1023  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1024    The option letters are
1025  .sp  .sp
1026    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1027    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 784  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1035  PCRE_MULTILINE while unsetting PCRE_DOTA
1035  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
1036  unset.  unset.
1037  .P  .P
1038  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
1039  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
1040  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1041  the global options (and it will therefore show up in data extracted by the  .P
1042  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1043    subpattern parentheses), the change applies to the remainder of the pattern
1044    that follows. If the change is placed right at the start of a pattern, PCRE
1045    extracts it into the global options (and it will therefore show up in data
1046    extracted by the \fBpcre_fullinfo()\fP function).
1047  .P  .P
1048  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
1049  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
1050  .sp  .sp
1051    (a(?i)b)c    (a(?i)b)c
1052  .sp  .sp
# Line 807  branch is abandoned before the option se Line 1062  branch is abandoned before the option se
1062  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1063  behaviour otherwise.  behaviour otherwise.
1064  .P  .P
1065  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
1066  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
1067  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1068    the application has set or what has been defaulted. Details are given in the
1069    section entitled
1070    .\" HTML <a href="#newlineseq">
1071    .\" </a>
1072    "Newline sequences"
1073    .\"
1074    above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1075    mode; this is equivalent to setting the PCRE_UTF8 option.
1076  .  .
1077  .  .
1078  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 824  Turning part of a pattern into a subpatt Line 1087  Turning part of a pattern into a subpatt
1087    cat(aract|erpillar|)    cat(aract|erpillar|)
1088  .sp  .sp
1089  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1090  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1091  .sp  .sp
1092  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
1093  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
# Line 849  the string "the white queen" is matched Line 1112  the string "the white queen" is matched
1112    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1113  .sp  .sp
1114  the captured substrings are "white queen" and "queen", and are numbered 1 and  the captured substrings are "white queen" and "queen", and are numbered 1 and
1115  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
 of nesting of all subpatterns, both capturing and non-capturing, is 200.  
1116  .P  .P
1117  As a convenient shorthand, if any option settings are required at the start of  As a convenient shorthand, if any option settings are required at the start of
1118  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
# Line 865  is reached, an option setting in one bra Line 1127  is reached, an option setting in one bra
1127  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1128  .  .
1129  .  .
1130    .SH "DUPLICATE SUBPATTERN NUMBERS"
1131    .rs
1132    .sp
1133    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1134    the same numbers for its capturing parentheses. Such a subpattern starts with
1135    (?| and is itself a non-capturing subpattern. For example, consider this
1136    pattern:
1137    .sp
1138      (?|(Sat)ur|(Sun))day
1139    .sp
1140    Because the two alternatives are inside a (?| group, both sets of capturing
1141    parentheses are numbered one. Thus, when the pattern matches, you can look
1142    at captured substring number one, whichever alternative matched. This construct
1143    is useful when you want to capture part, but not all, of one of a number of
1144    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1145    number is reset at the start of each branch. The numbers of any capturing
1146    buffers that follow the subpattern start after the highest number used in any
1147    branch. The following example is taken from the Perl documentation.
1148    The numbers underneath show in which buffer the captured content will be
1149    stored.
1150    .sp
1151      # before  ---------------branch-reset----------- after
1152      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1153      # 1            2         2  3        2     3     4
1154    .sp
1155    A backreference or a recursive call to a numbered subpattern always refers to
1156    the first one in the pattern with the given number.
1157    .P
1158    An alternative approach to using this "branch reset" feature is to use
1159    duplicate named subpatterns, as described in the next section.
1160    .
1161    .
1162  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1163  .rs  .rs
1164  .sp  .sp
1165  Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
1166  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1167  if an expression is modified, the numbers may change. To help with this  if an expression is modified, the numbers may change. To help with this
1168  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1169  not provide. The Python syntax (?P<name>...) is used. References to capturing  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1170    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1171    the Perl and the Python syntax.
1172    .P
1173    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1174    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1175  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1176  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1177  .\" </a>  .\" </a>
# Line 890  conditions, Line 1189  conditions,
1189  can be made by name as well as by number.  can be made by name as well as by number.
1190  .P  .P
1191  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1192  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1193  API provides function calls for extracting the name-to-number translation table  if the names were not present. The PCRE API provides function calls for
1194  from a compiled pattern. There is also a convenience function for extracting a  extracting the name-to-number translation table from a compiled pattern. There
1195  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1196  .P  .P
1197  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
1198  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. This can
# Line 902  match. Suppose you want to match the nam Line 1201  match. Suppose you want to match the nam
1201  abbreviation or as the full name, and in both cases you want to extract the  abbreviation or as the full name, and in both cases you want to extract the
1202  abbreviation. This pattern (ignoring the line breaks) does the job:  abbreviation. This pattern (ignoring the line breaks) does the job:
1203  .sp  .sp
1204    (?P<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1205    (?P<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
1206    (?P<DN>Wed)(?:nesday)?|    (?<DN>Wed)(?:nesday)?|
1207    (?P<DN>Thu)(?:rsday)?|    (?<DN>Thu)(?:rsday)?|
1208    (?P<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1209  .sp  .sp
1210  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1211    (An alternative way of solving this problem is to use a "branch reset"
1212    subpattern, as described in the previous section.)
1213    .P
1214  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1215  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
1216  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was. If you
1217  make a reference to a non-unique named subpattern from elsewhere in the  make a reference to a non-unique named subpattern from elsewhere in the
1218  pattern, the one that corresponds to the lowest number is used. For further  pattern, the one that corresponds to the lowest number is used. For further
# Line 919  details of the interfaces for handling n Line 1221  details of the interfaces for handling n
1221  \fBpcreapi\fP  \fBpcreapi\fP
1222  .\"  .\"
1223  documentation.  documentation.
1224    .P
1225    \fBWarning:\fP You cannot use different names to distinguish between two
1226    subpatterns with the same number (see the previous section) because PCRE uses
1227    only the numbers when matching.
1228  .  .
1229  .  .
1230  .SH REPETITION  .SH REPETITION
# Line 928  Repetition is specified by quantifiers, Line 1234  Repetition is specified by quantifiers,
1234  items:  items:
1235  .sp  .sp
1236    a literal data character    a literal data character
1237    the . metacharacter    the dot metacharacter
1238    the \eC escape sequence    the \eC escape sequence
1239    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1240      the \eR escape sequence
1241    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1242    a character class    a character class
1243    a back reference (see next section)    a back reference (see next section)
# Line 966  support is available, \eX{3} matches thr Line 1273  support is available, \eX{3} matches thr
1273  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).
1274  .P  .P
1275  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
1276  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1277    subpatterns that are referenced as
1278    .\" HTML <a href="#subpatternsassubroutines">
1279    .\" </a>
1280    subroutines
1281    .\"
1282    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1283    quantifier are omitted from the compiled pattern.
1284  .P  .P
1285  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1286  quantifiers have single-character abbreviations:  abbreviations:
1287  .sp  .sp
1288    *    is equivalent to {0,}    *    is equivalent to {0,}
1289    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1331  own right. Because it has two uses, it c
1331  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
1332  way the rest of the pattern matches.  way the rest of the pattern matches.
1333  .P  .P
1334  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1335  the quantifiers are not greedy by default, but individual ones can be made  the quantifiers are not greedy by default, but individual ones can be made
1336  greedy by following them with a question mark. In other words, it inverts the  greedy by following them with a question mark. In other words, it inverts the
1337  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1341  is greater than 1 or with a limited maxi
1341  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1342  .P  .P
1343  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1344  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1345  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1346  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1347  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
# Line 1039  alternatively using ^ to indicate anchor Line 1353  alternatively using ^ to indicate anchor
1353  .P  .P
1354  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1355  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1356  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail where a later one
1357  succeed. Consider, for example:  succeeds. Consider, for example:
1358  .sp  .sp
1359    (.*)abc\e1    (.*)abc\e1
1360  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1380  matches "aba" the value of the second ca
1380  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1381  .rs  .rs
1382  .sp  .sp
1383  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1384  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1385  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1386  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1387  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1388  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1389  .P  .P
1390  Consider, for example, the pattern \ed+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1391  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1397  item, and then with 4, and so on, before
1397  (a term taken from Jeffrey Friedl's book) provides the means for specifying  (a term taken from Jeffrey Friedl's book) provides the means for specifying
1398  that once a subpattern has matched, it is not to be re-evaluated in this way.  that once a subpattern has matched, it is not to be re-evaluated in this way.
1399  .P  .P
1400  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher gives up
1401  immediately on failing to match "foo" the first time. The notation is a kind of  immediately on failing to match "foo" the first time. The notation is a kind of
1402  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1403  .sp  .sp
# Line 1113  previous example can be rewritten as Line 1427  previous example can be rewritten as
1427  .sp  .sp
1428    \ed++foo    \ed++foo
1429  .sp  .sp
1430    Note that a possessive quantifier can be used with an entire group, for
1431    example:
1432    .sp
1433      (abc|xyz){2,3}+
1434    .sp
1435  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1436  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
1437  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1438  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1439  .P  difference; possessive quantifiers should be slightly faster.
1440  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1441  Friedl originated the idea (and the name) in the first edition of his book.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1442  Mike McCloskey liked it, so implemented it when he built Sun's Java package,  Jeffrey Friedl originated the idea (and the name) in the first edition of his
1443  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1444    package, and PCRE copied it from there. It ultimately found its way into Perl
1445    at release 5.10.
1446    .P
1447    PCRE has an optimization that automatically "possessifies" certain simple
1448    pattern constructs. For example, the sequence A+B is treated as A++B because
1449    there is no point in backtracking into a sequence of A's when B must follow.
1450  .P  .P
1451  When a pattern contains an unlimited repeat inside a subpattern that can itself  When a pattern contains an unlimited repeat inside a subpattern that can itself
1452  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
# Line 1167  numbers less than 10. A "forward back re Line 1492  numbers less than 10. A "forward back re
1492  when a repetition is involved and the subpattern to the right has participated  when a repetition is involved and the subpattern to the right has participated
1493  in an earlier iteration.  in an earlier iteration.
1494  .P  .P
1495  It is not possible to have a numerical "forward back reference" to subpattern  It is not possible to have a numerical "forward back reference" to a subpattern
1496  whose number is 10 or more. However, a back reference to any subpattern is  whose number is 10 or more using this syntax because a sequence such as \e50 is
1497  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1498  "Non-printing characters"  "Non-printing characters"
1499  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1500  .\" </a>  .\" </a>
1501  above  above
1502  .\"  .\"
1503  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1504    no such problem when named parentheses are used. A back reference to any
1505    subpattern is possible using named parentheses (see below).
1506    .P
1507    Another way of avoiding the ambiguity inherent in the use of digits following a
1508    backslash is to use the \eg escape sequence, which is a feature introduced in
1509    Perl 5.10. This escape must be followed by an unsigned number or a negative
1510    number, optionally enclosed in braces. These examples are all identical:
1511    .sp
1512      (ring), \e1
1513      (ring), \eg1
1514      (ring), \eg{1}
1515    .sp
1516    An unsigned number specifies an absolute reference without the ambiguity that
1517    is present in the older syntax. It is also useful when literal digits follow
1518    the reference. A negative number is a relative reference. Consider this
1519    example:
1520    .sp
1521      (abc(def)ghi)\eg{-1}
1522    .sp
1523    The sequence \eg{-1} is a reference to the most recently started capturing
1524    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1525    would be equivalent to \e1. The use of relative references can be helpful in
1526    long patterns, and also in patterns that are created by joining together
1527    fragments that contain references within themselves.
1528  .P  .P
1529  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1530  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1197  back reference, the case of letters is r Line 1546  back reference, the case of letters is r
1546  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1547  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1548  .P  .P
1549  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1550  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1551    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1552    back reference syntax, in which \eg can be used for both numeric and named
1553    references, is also supported. We could rewrite the above example in any of
1554    the following ways:
1555  .sp  .sp
1556      (?<p1>(?i)rah)\es+\ek<p1>
1557      (?'p1'(?i)rah)\es+\ek{p1}
1558    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1559      (?<p1>(?i)rah)\es+\eg{p1}
1560  .sp  .sp
1561  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
1562  after the reference.  after the reference.
# Line 1323  lengths, but it is acceptable if rewritt Line 1679  lengths, but it is acceptable if rewritt
1679  .sp  .sp
1680    (?<=abc|abde)    (?<=abc|abde)
1681  .sp  .sp
1682    In some cases, the Perl 5.10 escape sequence \eK
1683    .\" HTML <a href="#resetmatchstart">
1684    .\" </a>
1685    (see above)
1686    .\"
1687    can be used instead of a lookbehind assertion; this is not restricted to a
1688    fixed-length.
1689    .P
1690  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1691  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
1692  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1693  match is deemed to fail.  assertion fails.
1694  .P  .P
1695  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1696  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1697  the length of the lookbehind. The \eX escape, which can match different numbers  the length of the lookbehind. The \eX and \eR escapes, which can match
1698  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1699  .P  .P
1700  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1701  efficient matching at the end of the subject string. Consider a simple pattern  specify efficient matching at the end of the subject string. Consider a simple
1702  such as  pattern such as
1703  .sp  .sp
1704    abcd$    abcd$
1705  .sp  .sp
# Line 1351  then all but the last two characters, an Line 1715  then all but the last two characters, an
1715  covers the entire string, from right to left, so we are no better off. However,  covers the entire string, from right to left, so we are no better off. However,
1716  if the pattern is written as  if the pattern is written as
1717  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1718    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1719  .sp  .sp
1720  there can be no backtracking for the .* item; it can match only the entire  there can be no backtracking for the .*+ item; it can match only the entire
1721  string. The subsequent lookbehind assertion does a single test on the last four  string. The subsequent lookbehind assertion does a single test on the last four
1722  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1723  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1413  If the condition is satisfied, the yes-p Line 1773  If the condition is satisfied, the yes-p
1773  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
1774  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1775  .P  .P
1776  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1777  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1778  underscores, the condition is satisfied if the capturing subpattern of that  .
1779  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
1780  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
1781  named subpattern; if it cannot find one and the text consists entirely of  .sp
1782  digits, it looks for a subpattern of that number, which must be greater than  If the text between the parentheses consists of a sequence of digits, the
1783  zero. Using subpattern names that consist entirely of digits is not  condition is true if the capturing subpattern of that number has previously
1784  recommended.  matched. An alternative notation is to precede the digits with a plus or minus
1785    sign. In this case, the subpattern number is relative rather than absolute.
1786    The most recently opened parentheses can be referenced by (?(-1), the next most
1787    recent by (?(-2), and so on. In looping constructs it can also make sense to
1788    refer to subsequent groups with constructs such as (?(+2).
1789  .P  .P
1790  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1791  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 1437  or not. If they did, that is, if subject Line 1801  or not. If they did, that is, if subject
1801  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
1802  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1803  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1804  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
1805  named subpattern gives this:  .P
1806    If you were embedding this pattern in a larger one, you could use a relative
1807    reference:
1808    .sp
1809      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1810    .sp
1811    This makes the fragment independent of the parentheses in the larger pattern.
1812    .
1813    .SS "Checking for a used subpattern by name"
1814    .rs
1815    .sp
1816    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1817    subpattern by name. For compatibility with earlier versions of PCRE, which had
1818    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1819    there is a possible ambiguity with this syntax, because subpattern names may
1820    consist entirely of digits. PCRE looks first for a named subpattern; if it
1821    cannot find one and the name consists entirely of digits, PCRE looks for a
1822    subpattern of that number, which must be greater than zero. Using subpattern
1823    names that consist entirely of digits is not recommended.
1824    .P
1825    Rewriting the above example to use a named subpattern gives this:
1826  .sp  .sp
1827    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1828    .sp
1829    .
1830    .SS "Checking for pattern recursion"
1831    .rs
1832  .sp  .sp
1833  If the condition is the string (R), and there is no subpattern with the name R,  If the condition is the string (R), and there is no subpattern with the name R,
1834  the condition is satisfied if a recursive call to the pattern or subpattern has  the condition is true if a recursive call to the whole pattern or any
1835  been made. At "top level", the condition is false. This is a PCRE extension.  subpattern has been made. If digits or a name preceded by ampersand follow the
1836  Recursive patterns are described in the next section.  letter R, for example:
1837    .sp
1838      (?(R3)...) or (?(R&name)...)
1839    .sp
1840    the condition is true if the most recent recursion is into the subpattern whose
1841    number or name is given. This condition does not check the entire recursion
1842    stack.
1843    .P
1844    At "top level", all these recursion test conditions are false. Recursive
1845    patterns are described below.
1846    .
1847    .SS "Defining subpatterns for use by reference only"
1848    .rs
1849    .sp
1850    If the condition is the string (DEFINE), and there is no subpattern with the
1851    name DEFINE, the condition is always false. In this case, there may be only one
1852    alternative in the subpattern. It is always skipped if control reaches this
1853    point in the pattern; the idea of DEFINE is that it can be used to define
1854    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1855    is described below.) For example, a pattern to match an IPv4 address could be
1856    written like this (ignore whitespace and line breaks):
1857    .sp
1858      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1859      \eb (?&byte) (\e.(?&byte)){3} \eb
1860    .sp
1861    The first part of the pattern is a DEFINE group inside which a another group
1862    named "byte" is defined. This matches an individual component of an IPv4
1863    address (a number less than 256). When matching takes place, this part of the
1864    pattern is skipped because DEFINE acts like a false condition.
1865  .P  .P
1866  If the condition is not a sequence of digits or (R), it must be an assertion.  The rest of the pattern uses references to the named group to match the four
1867    dot-separated components of an IPv4 address, insisting on a word boundary at
1868    each end.
1869    .
1870    .SS "Assertion conditions"
1871    .rs
1872    .sp
1873    If the condition is not in any of the above formats, it must be an assertion.
1874  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1875  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1876  alternatives on the second line:  alternatives on the second line:
# Line 1483  next newline in the pattern. Line 1906  next newline in the pattern.
1906  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1907  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1908  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
1909  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1910  that allows regular expressions to recurse (amongst other things). It does this  .P
1911  by interpolating Perl code in the expression at run time, and the code can  For some time, Perl has provided a facility that allows regular expressions to
1912  refer to the expression itself. A Perl pattern to solve the parentheses problem  recurse (amongst other things). It does this by interpolating Perl code in the
1913  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1914    pattern using code interpolation to solve the parentheses problem can be
1915    created like this:
1916  .sp  .sp
1917    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1918  .sp  .sp
1919  The (?p{...}) item interpolates Perl code at run time, and in this case refers  The (?p{...}) item interpolates Perl code at run time, and in this case refers
1920  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1921  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1922  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1923  .P  supports special syntax for recursion of the entire pattern, and also for
1924  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1925  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was introduced into Perl at release 5.10.
1926  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1927  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
1928  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1929  .P  provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1930  A recursive subpattern call is always treated as an atomic group. That is, once  call, which is described in the next section.) The special item (?R) or (?0) is
1931  it has matched some of the subject string, it is never re-entered, even if  a recursive call of the entire regular expression.
1932  it contains untried alternatives and there is a subsequent matching failure.  .P
1933    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1934    treated as an atomic group. That is, once it has matched some of the subject
1935    string, it is never re-entered, even if it contains untried alternatives and
1936    there is a subsequent matching failure.
1937  .P  .P
1938  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1939  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
# Line 1522  pattern, so instead you could use this: Line 1951  pattern, so instead you could use this:
1951    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1952  .sp  .sp
1953  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
1954  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1955  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1956  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1957  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references. (A Perl 5.10 feature.)
1958  named parentheses). We could rewrite the above example as follows:  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1959  .sp  most recently opened parentheses preceding the recursion. In other words, a
1960    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
1961  .sp  it is encountered.
1962  This particular example pattern contains nested unlimited repeats, and so the  .P
1963  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1964  when applying the pattern to strings that do not match. For example, when this  references such as (?+2). However, these cannot be recursive because the
1965  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1966    "subroutine" calls, as described in the next section.
1967    .P
1968    An alternative approach is to use named parentheses instead. The Perl syntax
1969    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1970    could rewrite the above example as follows:
1971    .sp
1972      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1973    .sp
1974    If there is more than one subpattern with the same name, the earliest one is
1975    used.
1976    .P
1977    This particular example pattern that we have been looking at contains nested
1978    unlimited repeats, and so the use of atomic grouping for matching strings of
1979    non-parentheses is important when applying the pattern to strings that do not
1980    match. For example, when this pattern is applied to
1981  .sp  .sp
1982    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1983  .sp  .sp
# Line 1545  before failure can be reported. Line 1989  before failure can be reported.
1989  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values set for any capturing subpatterns are those
1990  from the outermost level of the recursion at which the subpattern value is set.  from the outermost level of the recursion at which the subpattern value is set.
1991  If you want to obtain intermediate values, a callout function can be used (see  If you want to obtain intermediate values, a callout function can be used (see
1992  the next section and the  below and the
1993  .\" HREF  .\" HREF
1994  \fBpcrecallout\fP  \fBpcrecallout\fP
1995  .\"  .\"
# Line 1584  is the actual recursive call. Line 2028  is the actual recursive call.
2028  .sp  .sp
2029  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern reference (either by number or by
2030  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
2031  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2032  pattern  before or after the reference. A numbered reference can be absolute or
2033    relative, as in these examples:
2034    .sp
2035      (...(absolute)...)...(?2)...
2036      (...(relative)...)...(?-1)...
2037      (...(?+1)...(relative)...
2038    .sp
2039    An earlier example pointed out that the pattern
2040  .sp  .sp
2041    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2042  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 2046  matches "sense and sensibility" and "res
2046    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2047  .sp  .sp
2048  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
2049  strings. Such references, if given numerically, must follow the subpattern to  strings. Another example is given in the discussion of DEFINE above.
 which they refer. However, named references can refer to later subpatterns.  
2050  .P  .P
2051  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2052  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
2053  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
2054  matching failure.  matching failure.
2055    .P
2056    When a subpattern is used as a subroutine, processing options such as
2057    case-independence are fixed when the subpattern is defined. They cannot be
2058    changed for different calls. For example, consider this pattern:
2059    .sp
2060      (abc)(?i:(?-1))
2061    .sp
2062    It matches "abcabc". It does not match "abcABC" because the change of
2063    processing option does not affect the called subpattern.
2064    .
2065    .
2066    .\" HTML <a name="onigurumasubroutines"></a>
2067    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2068    .rs
2069    .sp
2070    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2071    a number enclosed either in angle brackets or single quotes, is an alternative
2072    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2073    are two of the examples used above, rewritten using this syntax:
2074    .sp
2075      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2076      (sens|respons)e and \eg'1'ibility
2077    .sp
2078    PCRE supports an extension to Oniguruma: if a number is preceded by a
2079    plus or a minus sign it is taken as a relative reference. For example:
2080    .sp
2081      (abc)(?i:\eg<-1>)
2082    .sp
2083    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2084    synonymous. The former is a back reference; the latter is a subroutine call.
2085  .  .
2086  .  .
2087  .SH CALLOUTS  .SH CALLOUTS
# Line 1622  function is to be called. If you want to Line 2102  function is to be called. If you want to
2102  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
2103  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2104  .sp  .sp
2105    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2106  .sp  .sp
2107  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2108  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
# Line 1638  description of the interface to the call Line 2118  description of the interface to the call
2118  \fBpcrecallout\fP  \fBpcrecallout\fP
2119  .\"  .\"
2120  documentation.  documentation.
2121    .
2122    .
2123    .SH "BACKTRACKING CONTROL"
2124    .rs
2125    .sp
2126    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2127    are described in the Perl documentation as "experimental and subject to change
2128    or removal in a future version of Perl". It goes on to say: "Their usage in
2129    production code should be noted to avoid problems during upgrades." The same
2130    remarks apply to the PCRE features described in this section.
2131    .P
2132    Since these verbs are specifically related to backtracking, most of them can be
2133    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2134    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2135    failing negative assertion, they cause an error if encountered by
2136    \fBpcre_dfa_exec()\fP.
2137  .P  .P
2138  .in 0  The new verbs make use of what was previously invalid syntax: an opening
2139  Last updated: 06 June 2006  parenthesis followed by an asterisk. In Perl, they are generally of the form
2140  .br  (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2141  Copyright (c) 1997-2006 University of Cambridge.  form is just (*VERB). Any number of these verbs may occur in a pattern. There
2142    are two kinds:
2143    .
2144    .SS "Verbs that act immediately"
2145    .rs
2146    .sp
2147    The following verbs act as soon as they are encountered:
2148    .sp
2149       (*ACCEPT)
2150    .sp
2151    This verb causes the match to end successfully, skipping the remainder of the
2152    pattern. When inside a recursion, only the innermost pattern is ended
2153    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2154    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2155    captured. For example:
2156    .sp
2157      A(A|B(*ACCEPT)|C)D
2158    .sp
2159    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2160    captured.
2161    .sp
2162      (*FAIL) or (*F)
2163    .sp
2164    This verb causes the match to fail, forcing backtracking to occur. It is
2165    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2166    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2167    Perl features that are not present in PCRE. The nearest equivalent is the
2168    callout feature, as for example in this pattern:
2169    .sp
2170      a+(?C)(*FAIL)
2171    .sp
2172    A match with the string "aaaa" always fails, but the callout is taken before
2173    each backtrack happens (in this example, 10 times).
2174    .
2175    .SS "Verbs that act after backtracking"
2176    .rs
2177    .sp
2178    The following verbs do nothing when they are encountered. Matching continues
2179    with what follows, but if there is no subsequent match, a failure is forced.
2180    The verbs differ in exactly what kind of failure occurs.
2181    .sp
2182      (*COMMIT)
2183    .sp
2184    This verb causes the whole match to fail outright if the rest of the pattern
2185    does not match. Even if the pattern is unanchored, no further attempts to find
2186    a match by advancing the start point take place. Once (*COMMIT) has been
2187    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2188    starting point, or not at all. For example:
2189    .sp
2190      a+(*COMMIT)b
2191    .sp
2192    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2193    dynamic anchor, or "I've started, so I must finish."
2194    .sp
2195      (*PRUNE)
2196    .sp
2197    This verb causes the match to fail at the current position if the rest of the
2198    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2199    advance to the next starting character then happens. Backtracking can occur as
2200    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2201    if there is no match to the right, backtracking cannot cross (*PRUNE).
2202    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2203    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2204    be expressed in any other way.
2205    .sp
2206      (*SKIP)
2207    .sp
2208    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2209    "bumpalong" advance is not to the next character, but to the position in the
2210    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2211    was matched leading up to it cannot be part of a successful match. Consider:
2212    .sp
2213      a+(*SKIP)b
2214    .sp
2215    If the subject is "aaaac...", after the first match attempt fails (starting at
2216    the first character in the string), the starting point skips on to start the
2217    next attempt at "c". Note that a possessive quantifer does not have the same
2218    effect in this example; although it would suppress backtracking during the
2219    first match attempt, the second attempt would start at the second character
2220    instead of skipping on to "c".
2221    .sp
2222      (*THEN)
2223    .sp
2224    This verb causes a skip to the next alternation if the rest of the pattern does
2225    not match. That is, it cancels pending backtracking, but only within the
2226    current alternation. Its name comes from the observation that it can be used
2227    for a pattern-based if-then-else block:
2228    .sp
2229      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2230    .sp
2231    If the COND1 pattern matches, FOO is tried (and possibly further items after
2232    the end of the group if FOO succeeds); on failure the matcher skips to the
2233    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2234    is used outside of any alternation, it acts exactly like (*PRUNE).
2235    .
2236    .
2237    .SH "SEE ALSO"
2238    .rs
2239    .sp
2240    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
2241    .
2242    .
2243    .SH AUTHOR
2244    .rs
2245    .sp
2246    .nf
2247    Philip Hazel
2248    University Computing Service
2249    Cambridge CB2 3QH, England.
2250    .fi
2251    .
2252    .
2253    .SH REVISION
2254    .rs
2255    .sp
2256    .nf
2257    Last updated: 11 April 2009
2258    Copyright (c) 1997-2009 University of Cambridge.
2259    .fi

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