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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 341 by ph10, Sat Apr 19 16:41:04 2008 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
# Line 30  The remainder of this document discusses Line 39  The remainder of this document discusses
39  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
40  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
41  \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
42  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
43  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
44    alternative function, and how it differs from the normal function, are
45    discussed in the
46  .\" HREF  .\" HREF
47  \fBpcrematching\fP  \fBpcrematching\fP
48  .\"  .\"
49  page.  page.
50    .
51    .
52    .SH "NEWLINE CONVENTIONS"
53    .rs
54    .sp
55    PCRE supports five different conventions for indicating line breaks in
56    strings: a single CR (carriage return) character, a single LF (linefeed)
57    character, the two-character sequence CRLF, any of the three preceding, or any
58    Unicode newline sequence. The
59    .\" HREF
60    \fBpcreapi\fP
61    .\"
62    page has
63    .\" HTML <a href="pcreapi.html#newlines">
64    .\" </a>
65    further discussion
66    .\"
67    about newlines, and shows how to set the newline convention in the
68    \fIoptions\fP arguments for the compiling and matching functions.
69  .P  .P
70    It is also possible to specify a newline convention by starting a pattern
71    string with one of the following five sequences:
72    .sp
73      (*CR)        carriage return
74      (*LF)        linefeed
75      (*CRLF)      carriage return, followed by linefeed
76      (*ANYCRLF)   any of the three above
77      (*ANY)       all Unicode newline sequences
78    .sp
79    These override the default and the options given to \fBpcre_compile()\fP. For
80    example, on a Unix system where LF is the default newline sequence, the pattern
81    .sp
82      (*CR)a.b
83    .sp
84    changes the convention to CR. That pattern matches "a\enb" because LF is no
85    longer a newline. Note that these special settings, which are not
86    Perl-compatible, are recognized only at the very start of a pattern, and that
87    they must be in upper case. If more than one of them is present, the last one
88    is used.
89    .P
90    The newline convention does not affect what the \eR escape sequence matches. By
91    default, this is any Unicode newline sequence, for Perl compatibility. However,
92    this can be changed; see the description of \eR in the section entitled
93    .\" HTML <a href="#newlineseq">
94    .\" </a>
95    "Newline sequences"
96    .\"
97    below. A change of \eR setting can be combined with a change of newline
98    convention.
99    .
100    .
101    .SH "CHARACTERS AND METACHARACTERS"
102    .rs
103    .sp
104  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
105  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
106  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 124  interpreted in some special way.
124  .P  .P
125  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
126  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
127  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
128  as follows:  are as follows:
129  .sp  .sp
130    \e      general escape character with several uses    \e      general escape character with several uses
131    ^      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 156  a character class the only metacharacter
156  .sp  .sp
157  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
158  .  .
159    .
160  .SH BACKSLASH  .SH BACKSLASH
161  .rs  .rs
162  .sp  .sp
# Line 142  represents: Line 207  represents:
207    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
208    \ee        escape (hex 1B)    \ee        escape (hex 1B)
209    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
210    \en        newline (hex 0A)    \en        linefeed (hex 0A)
211    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
212    \et        tab (hex 09)    \et        tab (hex 09)
213    \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 222  Thus \ecz becomes hex 1A, but \ec{ becom
222  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
223  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{
224  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
225  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
226  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
227  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
228  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
229  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
230    there is no terminating }, this form of escape is not recognized. Instead, the
231    initial \ex will be interpreted as a basic hexadecimal escape, with no
232    following digits, giving a character whose value is zero.
233  .P  .P
234  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
235  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 258  parenthesized subpatterns.
258  .P  .P
259  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
260  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
261  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
262  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
263  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
264  to \e777 are permitted. For example:  to \e777 are permitted. For example:
# Line 221  zero, because no more than three octal d Line 289  zero, because no more than three octal d
289  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
290  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
291  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08), and the
292  sequence \eX is interpreted as the character "X". Outside a character class,  sequences \eR and \eX are interpreted as the characters "R" and "X",
293  these sequences have different meanings  respectively. Outside a character class, these sequences have different
294    meanings
295  .\" HTML <a href="#uniextseq">  .\" HTML <a href="#uniextseq">
296  .\" </a>  .\" </a>
297  (see below).  (see below).
298  .\"  .\"
299  .  .
300  .  .
301    .SS "Absolute and relative back references"
302    .rs
303    .sp
304    The sequence \eg followed by an unsigned or a negative number, optionally
305    enclosed in braces, is an absolute or relative back reference. A named back
306    reference can be coded as \eg{name}. Back references are discussed
307    .\" HTML <a href="#backreferences">
308    .\" </a>
309    later,
310    .\"
311    following the discussion of
312    .\" HTML <a href="#subpattern">
313    .\" </a>
314    parenthesized subpatterns.
315    .\"
316    .
317    .
318    .SS "Absolute and relative subroutine calls"
319    .rs
320    .sp
321    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
322    a number enclosed either in angle brackets or single quotes, is an alternative
323    syntax for referencing a subpattern as a "subroutine". Details are discussed
324    .\" HTML <a href="#onigurumasubroutines">
325    .\" </a>
326    later.
327    .\"
328    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
329    synonymous. The former is a back reference; the latter is a subroutine call.
330    .
331    .
332  .SS "Generic character types"  .SS "Generic character types"
333  .rs  .rs
334  .sp  .sp
335  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
336  following are always recognized:  following are always recognized:
337  .sp  .sp
338    \ed     any decimal digit    \ed     any decimal digit
339    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
340      \eh     any horizontal whitespace character
341      \eH     any character that is not a horizontal whitespace character
342    \es     any whitespace character    \es     any whitespace character
343    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
344      \ev     any vertical whitespace character
345      \eV     any character that is not a vertical whitespace character
346    \ew     any "word" character    \ew     any "word" character
347    \eW     any "non-word" character    \eW     any "non-word" character
348  .sp  .sp
# Line 252  there is no character to match. Line 356  there is no character to match.
356  .P  .P
357  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).
358  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
359  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
360  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
361  does.)  does.
362    .P
363    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
364    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
365    character property support is available. These sequences retain their original
366    meanings from before UTF-8 support was available, mainly for efficiency
367    reasons.
368    .P
369    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
370    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
371    The horizontal space characters are:
372    .sp
373      U+0009     Horizontal tab
374      U+0020     Space
375      U+00A0     Non-break space
376      U+1680     Ogham space mark
377      U+180E     Mongolian vowel separator
378      U+2000     En quad
379      U+2001     Em quad
380      U+2002     En space
381      U+2003     Em space
382      U+2004     Three-per-em space
383      U+2005     Four-per-em space
384      U+2006     Six-per-em space
385      U+2007     Figure space
386      U+2008     Punctuation space
387      U+2009     Thin space
388      U+200A     Hair space
389      U+202F     Narrow no-break space
390      U+205F     Medium mathematical space
391      U+3000     Ideographic space
392    .sp
393    The vertical space characters are:
394    .sp
395      U+000A     Linefeed
396      U+000B     Vertical tab
397      U+000C     Formfeed
398      U+000D     Carriage return
399      U+0085     Next line
400      U+2028     Line separator
401      U+2029     Paragraph separator
402  .P  .P
403  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
404  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 412  in the
412  .\" HREF  .\" HREF
413  \fBpcreapi\fP  \fBpcreapi\fP
414  .\"  .\"
415  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,
416  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
417  .P  accented letters, and these are matched by \ew. The use of locales with Unicode
418  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  is discouraged.
419  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .
420  character property support is available. The use of locales with Unicode is  .
421  discouraged.  .\" HTML <a name="newlineseq"></a>
422    .SS "Newline sequences"
423    .rs
424    .sp
425    Outside a character class, by default, the escape sequence \eR matches any
426    Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
427    equivalent to the following:
428    .sp
429      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
430    .sp
431    This is an example of an "atomic group", details of which are given
432    .\" HTML <a href="#atomicgroup">
433    .\" </a>
434    below.
435    .\"
436    This particular group matches either the two-character sequence CR followed by
437    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
438    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
439    line, U+0085). The two-character sequence is treated as a single unit that
440    cannot be split.
441    .P
442    In UTF-8 mode, two additional characters whose codepoints are greater than 255
443    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
444    Unicode character property support is not needed for these characters to be
445    recognized.
446    .P
447    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
448    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
449    either at compile time or when the pattern is matched. (BSR is an abbrevation
450    for "backslash R".) This can be made the default when PCRE is built; if this is
451    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
452    It is also possible to specify these settings by starting a pattern string with
453    one of the following sequences:
454    .sp
455      (*BSR_ANYCRLF)   CR, LF, or CRLF only
456      (*BSR_UNICODE)   any Unicode newline sequence
457    .sp
458    These override the default and the options given to \fBpcre_compile()\fP, but
459    they can be overridden by options given to \fBpcre_exec()\fP. Note that these
460    special settings, which are not Perl-compatible, are recognized only at the
461    very start of a pattern, and that they must be in upper case. If more than one
462    of them is present, the last one is used. They can be combined with a change of
463    newline convention, for example, a pattern can start with:
464    .sp
465      (*ANY)(*BSR_ANYCRLF)
466    .sp
467    Inside a character class, \eR matches the letter "R".
468  .  .
469  .  .
470  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 282  discouraged. Line 472  discouraged.
472  .rs  .rs
473  .sp  .sp
474  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
475  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
476  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
477    characters whose codepoints are less than 256, but they do work in this mode.
478    The extra escape sequences are:
479  .sp  .sp
480    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
481    \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 499  Those that are not part of an identified
499  .P  .P
500  Arabic,  Arabic,
501  Armenian,  Armenian,
502    Balinese,
503  Bengali,  Bengali,
504  Bopomofo,  Bopomofo,
505  Braille,  Braille,
# Line 316  Canadian_Aboriginal, Line 509  Canadian_Aboriginal,
509  Cherokee,  Cherokee,
510  Common,  Common,
511  Coptic,  Coptic,
512    Cuneiform,
513  Cypriot,  Cypriot,
514  Cyrillic,  Cyrillic,
515  Deseret,  Deseret,
# Line 345  Malayalam, Line 539  Malayalam,
539  Mongolian,  Mongolian,
540  Myanmar,  Myanmar,
541  New_Tai_Lue,  New_Tai_Lue,
542    Nko,
543  Ogham,  Ogham,
544  Old_Italic,  Old_Italic,
545  Old_Persian,  Old_Persian,
546  Oriya,  Oriya,
547  Osmanya,  Osmanya,
548    Phags_Pa,
549    Phoenician,
550  Runic,  Runic,
551  Shavian,  Shavian,
552  Sinhala,  Sinhala,
# Line 430  The special property L& is also supporte Line 627  The special property L& is also supporte
627  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
628  a modifier or "other".  a modifier or "other".
629  .P  .P
630    The Cs (Surrogate) property applies only to characters in the range U+D800 to
631    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
632    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
633    (see the discussion of PCRE_NO_UTF8_CHECK in the
634    .\" HREF
635    \fBpcreapi\fP
636    .\"
637    page).
638    .P
639  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})
640  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
641  properties with "Is".  properties with "Is".
# Line 454  atomic group Line 660  atomic group
660  (see below).  (see below).
661  .\"  .\"
662  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
663  preceding character.  preceding character. None of them have codepoints less than 256, so in
664    non-UTF-8 mode \eX matches any one character.
665  .P  .P
666  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
667  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 669  why the traditional escape sequences suc
669  properties in PCRE.  properties in PCRE.
670  .  .
671  .  .
672    .\" HTML <a name="resetmatchstart"></a>
673    .SS "Resetting the match start"
674    .rs
675    .sp
676    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
677    matched characters not to be included in the final matched sequence. For
678    example, the pattern:
679    .sp
680      foo\eKbar
681    .sp
682    matches "foobar", but reports that it has matched "bar". This feature is
683    similar to a lookbehind assertion
684    .\" HTML <a href="#lookbehind">
685    .\" </a>
686    (described below).
687    .\"
688    However, in this case, the part of the subject before the real match does not
689    have to be of fixed length, as lookbehind assertions do. The use of \eK does
690    not interfere with the setting of
691    .\" HTML <a href="#subpattern">
692    .\" </a>
693    captured substrings.
694    .\"
695    For example, when the pattern
696    .sp
697      (foo)\eKbar
698    .sp
699    matches "foobar", the first substring is still set to "foo".
700    .
701    .
702  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
703  .SS "Simple assertions"  .SS "Simple assertions"
704  .rs  .rs
705  .sp  .sp
706  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
707  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,
708  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
709  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 715  The backslashed assertions are:
715  .sp  .sp
716    \eb     matches at a word boundary    \eb     matches at a word boundary
717    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
718    \eA     matches at start of subject    \eA     matches at the start of the subject
719    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
720    \ez     matches at end of subject            also matches before a newline at the end of the subject
721    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
722      \eG     matches at the first matching position in the subject
723  .sp  .sp
724  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
725  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 816  end of the subject in both modes, and if
816  .sp  .sp
817  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
818  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
819  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.
820  a line ending is defined as a single character (CR or LF), dot never matches  .P
821  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
822  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
823  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
824    (including isolated CRs and LFs). When any Unicode line endings are being
825    recognized, dot does not match CR or LF or any of the other line ending
826    characters.
827  .P  .P
828  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
829  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
830  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
831    to match it.
832  .P  .P
833  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
834  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 839  special meaning in a character class.
839  .rs  .rs
840  .sp  .sp
841  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
842  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
843  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
844  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,
845  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,
846  sequence is best avoided.  the \eC escape sequence is best avoided.
847  .P  .P
848  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
849  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 652  If you want to use caseless matching for Line 894  If you want to use caseless matching for
894  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
895  UTF-8 support.  UTF-8 support.
896  .P  .P
897  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
898  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
899  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
900  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
901  .P  .P
902  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
903  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 921  example [\ex{100}-\ex{2ff}].
921  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
922  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
923  [][\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
924  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
925  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
926  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
927  property support.  property support.
# Line 769  alternative in the subpattern. Line 1011  alternative in the subpattern.
1011  .rs  .rs
1012  .sp  .sp
1013  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1014  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
1015  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1016    The option letters are
1017  .sp  .sp
1018    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1019    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 784  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1027  PCRE_MULTILINE while unsetting PCRE_DOTA
1027  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
1028  unset.  unset.
1029  .P  .P
1030    The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1031    changed in the same way as the Perl-compatible options by using the characters
1032    J, U and X respectively.
1033    .P
1034  When an option change occurs at top level (that is, not inside subpattern  When an option change occurs at top level (that is, not inside subpattern
1035  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
1036  If the change is placed right at the start of a pattern, PCRE extracts it into  If the change is placed right at the start of a pattern, PCRE extracts it into
1037  the global options (and it will therefore show up in data extracted by the  the global options (and it will therefore show up in data extracted by the
1038  \fBpcre_fullinfo()\fP function).  \fBpcre_fullinfo()\fP function).
1039  .P  .P
1040  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
1041  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
1042  .sp  .sp
1043    (a(?i)b)c    (a(?i)b)c
1044  .sp  .sp
# Line 807  branch is abandoned before the option se Line 1054  branch is abandoned before the option se
1054  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1055  behaviour otherwise.  behaviour otherwise.
1056  .P  .P
1057  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
1058  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
1059  J, U and X respectively.  pattern can contain special leading sequences to override what the application
1060    has set or what has been defaulted. Details are given in the section entitled
1061    .\" HTML <a href="#newlineseq">
1062    .\" </a>
1063    "Newline sequences"
1064    .\"
1065    above.
1066  .  .
1067  .  .
1068  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 824  Turning part of a pattern into a subpatt Line 1077  Turning part of a pattern into a subpatt
1077    cat(aract|erpillar|)    cat(aract|erpillar|)
1078  .sp  .sp
1079  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1080  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1081  .sp  .sp
1082  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
1083  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 1102  the string "the white queen" is matched
1102    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1103  .sp  .sp
1104  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
1105  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.  
1106  .P  .P
1107  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
1108  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 1117  is reached, an option setting in one bra
1117  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1118  .  .
1119  .  .
1120    .SH "DUPLICATE SUBPATTERN NUMBERS"
1121    .rs
1122    .sp
1123    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1124    the same numbers for its capturing parentheses. Such a subpattern starts with
1125    (?| and is itself a non-capturing subpattern. For example, consider this
1126    pattern:
1127    .sp
1128      (?|(Sat)ur|(Sun))day
1129    .sp
1130    Because the two alternatives are inside a (?| group, both sets of capturing
1131    parentheses are numbered one. Thus, when the pattern matches, you can look
1132    at captured substring number one, whichever alternative matched. This construct
1133    is useful when you want to capture part, but not all, of one of a number of
1134    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1135    number is reset at the start of each branch. The numbers of any capturing
1136    buffers that follow the subpattern start after the highest number used in any
1137    branch. The following example is taken from the Perl documentation.
1138    The numbers underneath show in which buffer the captured content will be
1139    stored.
1140    .sp
1141      # before  ---------------branch-reset----------- after
1142      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1143      # 1            2         2  3        2     3     4
1144    .sp
1145    A backreference or a recursive call to a numbered subpattern always refers to
1146    the first one in the pattern with the given number.
1147    .P
1148    An alternative approach to using this "branch reset" feature is to use
1149    duplicate named subpatterns, as described in the next section.
1150    .
1151    .
1152  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1153  .rs  .rs
1154  .sp  .sp
1155  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
1156  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1157  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
1158  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1159  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
1160    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1161    the Perl and the Python syntax.
1162    .P
1163    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1164    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1165  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1166  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1167  .\" </a>  .\" </a>
# Line 890  conditions, Line 1179  conditions,
1179  can be made by name as well as by number.  can be made by name as well as by number.
1180  .P  .P
1181  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1182  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1183  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
1184  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
1185  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1186  .P  .P
1187  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
1188  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 1191  match. Suppose you want to match the nam
1191  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
1192  abbreviation. This pattern (ignoring the line breaks) does the job:  abbreviation. This pattern (ignoring the line breaks) does the job:
1193  .sp  .sp
1194    (?P<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1195    (?P<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
1196    (?P<DN>Wed)(?:nesday)?|    (?<DN>Wed)(?:nesday)?|
1197    (?P<DN>Thu)(?:rsday)?|    (?<DN>Thu)(?:rsday)?|
1198    (?P<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1199  .sp  .sp
1200  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.
1201    (An alternative way of solving this problem is to use a "branch reset"
1202    subpattern, as described in the previous section.)
1203    .P
1204  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1205  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
1206  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
1207  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
1208  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 928  Repetition is specified by quantifiers, Line 1220  Repetition is specified by quantifiers,
1220  items:  items:
1221  .sp  .sp
1222    a literal data character    a literal data character
1223    the . metacharacter    the dot metacharacter
1224    the \eC escape sequence    the \eC escape sequence
1225    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1226      the \eR escape sequence
1227    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1228    a character class    a character class
1229    a back reference (see next section)    a back reference (see next section)
# Line 966  support is available, \eX{3} matches thr Line 1259  support is available, \eX{3} matches thr
1259  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).
1260  .P  .P
1261  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
1262  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1263    subpatterns that are referenced as
1264    .\" HTML <a href="#subpatternsassubroutines">
1265    .\" </a>
1266    subroutines
1267    .\"
1268    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1269    quantifier are omitted from the compiled pattern.
1270  .P  .P
1271  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1272  quantifiers have single-character abbreviations:  abbreviations:
1273  .sp  .sp
1274    *    is equivalent to {0,}    *    is equivalent to {0,}
1275    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1317  own right. Because it has two uses, it c
1317  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
1318  way the rest of the pattern matches.  way the rest of the pattern matches.
1319  .P  .P
1320  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),
1321  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
1322  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
1323  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1327  is greater than 1 or with a limited maxi
1327  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1328  .P  .P
1329  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
1330  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
1331  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1332  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
1333  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 1339  alternatively using ^ to indicate anchor
1339  .P  .P
1340  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1341  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1342  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
1343  succeed. Consider, for example:  succeeds. Consider, for example:
1344  .sp  .sp
1345    (.*)abc\e1    (.*)abc\e1
1346  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1366  matches "aba" the value of the second ca
1366  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1367  .rs  .rs
1368  .sp  .sp
1369  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1370  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
1371  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
1372  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
1373  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
1374  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1375  .P  .P
1376  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
1377  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1383  item, and then with 4, and so on, before
1383  (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
1384  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.
1385  .P  .P
1386  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
1387  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
1388  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1389  .sp  .sp
# Line 1113  previous example can be rewritten as Line 1413  previous example can be rewritten as
1413  .sp  .sp
1414    \ed++foo    \ed++foo
1415  .sp  .sp
1416    Note that a possessive quantifier can be used with an entire group, for
1417    example:
1418    .sp
1419      (abc|xyz){2,3}+
1420    .sp
1421  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1422  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
1423  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
1424  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1425  .P  difference; possessive quantifiers should be slightly faster.
1426  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1427  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.
1428  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
1429  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1430    package, and PCRE copied it from there. It ultimately found its way into Perl
1431    at release 5.10.
1432    .P
1433    PCRE has an optimization that automatically "possessifies" certain simple
1434    pattern constructs. For example, the sequence A+B is treated as A++B because
1435    there is no point in backtracking into a sequence of A's when B must follow.
1436  .P  .P
1437  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
1438  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 1478  numbers less than 10. A "forward back re
1478  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
1479  in an earlier iteration.  in an earlier iteration.
1480  .P  .P
1481  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
1482  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
1483  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1484  "Non-printing characters"  "Non-printing characters"
1485  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1486  .\" </a>  .\" </a>
1487  above  above
1488  .\"  .\"
1489  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1490    no such problem when named parentheses are used. A back reference to any
1491    subpattern is possible using named parentheses (see below).
1492    .P
1493    Another way of avoiding the ambiguity inherent in the use of digits following a
1494    backslash is to use the \eg escape sequence, which is a feature introduced in
1495    Perl 5.10. This escape must be followed by an unsigned number or a negative
1496    number, optionally enclosed in braces. These examples are all identical:
1497    .sp
1498      (ring), \e1
1499      (ring), \eg1
1500      (ring), \eg{1}
1501    .sp
1502    An unsigned number specifies an absolute reference without the ambiguity that
1503    is present in the older syntax. It is also useful when literal digits follow
1504    the reference. A negative number is a relative reference. Consider this
1505    example:
1506    .sp
1507      (abc(def)ghi)\eg{-1}
1508    .sp
1509    The sequence \eg{-1} is a reference to the most recently started capturing
1510    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1511    would be equivalent to \e1. The use of relative references can be helpful in
1512    long patterns, and also in patterns that are created by joining together
1513    fragments that contain references within themselves.
1514  .P  .P
1515  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1516  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 1532  back reference, the case of letters is r
1532  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
1533  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1534  .P  .P
1535  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1536  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1537    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1538    back reference syntax, in which \eg can be used for both numeric and named
1539    references, is also supported. We could rewrite the above example in any of
1540    the following ways:
1541  .sp  .sp
1542      (?<p1>(?i)rah)\es+\ek<p1>
1543      (?'p1'(?i)rah)\es+\ek{p1}
1544    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1545      (?<p1>(?i)rah)\es+\eg{p1}
1546  .sp  .sp
1547  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
1548  after the reference.  after the reference.
# Line 1323  lengths, but it is acceptable if rewritt Line 1665  lengths, but it is acceptable if rewritt
1665  .sp  .sp
1666    (?<=abc|abde)    (?<=abc|abde)
1667  .sp  .sp
1668    In some cases, the Perl 5.10 escape sequence \eK
1669    .\" HTML <a href="#resetmatchstart">
1670    .\" </a>
1671    (see above)
1672    .\"
1673    can be used instead of a lookbehind assertion; this is not restricted to a
1674    fixed-length.
1675    .P
1676  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1677  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
1678  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1679  match is deemed to fail.  assertion fails.
1680  .P  .P
1681  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)
1682  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1683  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
1684  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1685  .P  .P
1686  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1687  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
1688  such as  pattern such as
1689  .sp  .sp
1690    abcd$    abcd$
1691  .sp  .sp
# Line 1351  then all but the last two characters, an Line 1701  then all but the last two characters, an
1701  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,
1702  if the pattern is written as  if the pattern is written as
1703  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1704    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1705  .sp  .sp
1706  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
1707  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
1708  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1709  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 1759  If the condition is satisfied, the yes-p
1759  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
1760  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1761  .P  .P
1762  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1763  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1764  underscores, the condition is satisfied if the capturing subpattern of that  .
1765  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
1766  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
1767  named subpattern; if it cannot find one and the text consists entirely of  .sp
1768  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
1769  zero. Using subpattern names that consist entirely of digits is not  condition is true if the capturing subpattern of that number has previously
1770  recommended.  matched. An alternative notation is to precede the digits with a plus or minus
1771    sign. In this case, the subpattern number is relative rather than absolute.
1772    The most recently opened parentheses can be referenced by (?(-1), the next most
1773    recent by (?(-2), and so on. In looping constructs it can also make sense to
1774    refer to subsequent groups with constructs such as (?(+2).
1775  .P  .P
1776  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1777  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 1787  or not. If they did, that is, if subject
1787  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
1788  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1789  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1790  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
1791  named subpattern gives this:  .P
1792    If you were embedding this pattern in a larger one, you could use a relative
1793    reference:
1794    .sp
1795      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1796    .sp
1797    This makes the fragment independent of the parentheses in the larger pattern.
1798    .
1799    .SS "Checking for a used subpattern by name"
1800    .rs
1801    .sp
1802    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1803    subpattern by name. For compatibility with earlier versions of PCRE, which had
1804    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1805    there is a possible ambiguity with this syntax, because subpattern names may
1806    consist entirely of digits. PCRE looks first for a named subpattern; if it
1807    cannot find one and the name consists entirely of digits, PCRE looks for a
1808    subpattern of that number, which must be greater than zero. Using subpattern
1809    names that consist entirely of digits is not recommended.
1810    .P
1811    Rewriting the above example to use a named subpattern gives this:
1812  .sp  .sp
1813    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1814    .sp
1815    .
1816    .SS "Checking for pattern recursion"
1817    .rs
1818  .sp  .sp
1819  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,
1820  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
1821  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
1822  Recursive patterns are described in the next section.  letter R, for example:
1823    .sp
1824      (?(R3)...) or (?(R&name)...)
1825    .sp
1826    the condition is true if the most recent recursion is into the subpattern whose
1827    number or name is given. This condition does not check the entire recursion
1828    stack.
1829    .P
1830    At "top level", all these recursion test conditions are false. Recursive
1831    patterns are described below.
1832    .
1833    .SS "Defining subpatterns for use by reference only"
1834    .rs
1835    .sp
1836    If the condition is the string (DEFINE), and there is no subpattern with the
1837    name DEFINE, the condition is always false. In this case, there may be only one
1838    alternative in the subpattern. It is always skipped if control reaches this
1839    point in the pattern; the idea of DEFINE is that it can be used to define
1840    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1841    is described below.) For example, a pattern to match an IPv4 address could be
1842    written like this (ignore whitespace and line breaks):
1843    .sp
1844      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1845      \eb (?&byte) (\e.(?&byte)){3} \eb
1846    .sp
1847    The first part of the pattern is a DEFINE group inside which a another group
1848    named "byte" is defined. This matches an individual component of an IPv4
1849    address (a number less than 256). When matching takes place, this part of the
1850    pattern is skipped because DEFINE acts like a false condition.
1851  .P  .P
1852  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
1853    dot-separated components of an IPv4 address, insisting on a word boundary at
1854    each end.
1855    .
1856    .SS "Assertion conditions"
1857    .rs
1858    .sp
1859    If the condition is not in any of the above formats, it must be an assertion.
1860  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1861  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1862  alternatives on the second line:  alternatives on the second line:
# Line 1483  next newline in the pattern. Line 1892  next newline in the pattern.
1892  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1893  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1894  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
1895  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1896  that allows regular expressions to recurse (amongst other things). It does this  .P
1897  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
1898  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
1899  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1900    pattern using code interpolation to solve the parentheses problem can be
1901    created like this:
1902  .sp  .sp
1903    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1904  .sp  .sp
1905  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
1906  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1907  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1908  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1909  .P  supports special syntax for recursion of the entire pattern, and also for
1910  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1911  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.
1912  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1913  "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
1914  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1915  .P  provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1916  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
1917  it has matched some of the subject string, it is never re-entered, even if  a recursive call of the entire regular expression.
1918  it contains untried alternatives and there is a subsequent matching failure.  .P
1919    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1920    treated as an atomic group. That is, once it has matched some of the subject
1921    string, it is never re-entered, even if it contains untried alternatives and
1922    there is a subsequent matching failure.
1923  .P  .P
1924  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1925  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 1937  pattern, so instead you could use this:
1937    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1938  .sp  .sp
1939  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
1940  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1941  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1942  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
1943  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.)
1944  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
1945  .sp  most recently opened parentheses preceding the recursion. In other words, a
1946    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
1947  .sp  it is encountered.
1948  This particular example pattern contains nested unlimited repeats, and so the  .P
1949  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1950  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
1951  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1952    "subroutine" calls, as described in the next section.
1953    .P
1954    An alternative approach is to use named parentheses instead. The Perl syntax
1955    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1956    could rewrite the above example as follows:
1957    .sp
1958      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1959    .sp
1960    If there is more than one subpattern with the same name, the earliest one is
1961    used.
1962    .P
1963    This particular example pattern that we have been looking at contains nested
1964    unlimited repeats, and so the use of atomic grouping for matching strings of
1965    non-parentheses is important when applying the pattern to strings that do not
1966    match. For example, when this pattern is applied to
1967  .sp  .sp
1968    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1969  .sp  .sp
# Line 1545  before failure can be reported. Line 1975  before failure can be reported.
1975  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
1976  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.
1977  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
1978  the next section and the  below and the
1979  .\" HREF  .\" HREF
1980  \fBpcrecallout\fP  \fBpcrecallout\fP
1981  .\"  .\"
# Line 1584  is the actual recursive call. Line 2014  is the actual recursive call.
2014  .sp  .sp
2015  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
2016  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
2017  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2018  pattern  before or after the reference. A numbered reference can be absolute or
2019    relative, as in these examples:
2020    .sp
2021      (...(absolute)...)...(?2)...
2022      (...(relative)...)...(?-1)...
2023      (...(?+1)...(relative)...
2024    .sp
2025    An earlier example pointed out that the pattern
2026  .sp  .sp
2027    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2028  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 2032  matches "sense and sensibility" and "res
2032    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2033  .sp  .sp
2034  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
2035  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.  
2036  .P  .P
2037  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2038  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
2039  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
2040  matching failure.  matching failure.
2041    .P
2042    When a subpattern is used as a subroutine, processing options such as
2043    case-independence are fixed when the subpattern is defined. They cannot be
2044    changed for different calls. For example, consider this pattern:
2045    .sp
2046      (abc)(?i:(?-1))
2047    .sp
2048    It matches "abcabc". It does not match "abcABC" because the change of
2049    processing option does not affect the called subpattern.
2050    .
2051    .
2052    .\" HTML <a name="onigurumasubroutines"></a>
2053    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2054    .rs
2055    .sp
2056    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2057    a number enclosed either in angle brackets or single quotes, is an alternative
2058    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2059    are two of the examples used above, rewritten using this syntax:
2060    .sp
2061      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2062      (sens|respons)e and \eg'1'ibility
2063    .sp
2064    PCRE supports an extension to Oniguruma: if a number is preceded by a
2065    plus or a minus sign it is taken as a relative reference. For example:
2066    .sp
2067      (abc)(?i:\eg<-1>)
2068    .sp
2069    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2070    synonymous. The former is a back reference; the latter is a subroutine call.
2071  .  .
2072  .  .
2073  .SH CALLOUTS  .SH CALLOUTS
# Line 1622  function is to be called. If you want to Line 2088  function is to be called. If you want to
2088  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.
2089  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2090  .sp  .sp
2091    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2092  .sp  .sp
2093  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
2094  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 2104  description of the interface to the call
2104  \fBpcrecallout\fP  \fBpcrecallout\fP
2105  .\"  .\"
2106  documentation.  documentation.
2107    .
2108    .
2109    .SH "BACKTRACKING CONTROL"
2110    .rs
2111    .sp
2112    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2113    are described in the Perl documentation as "experimental and subject to change
2114    or removal in a future version of Perl". It goes on to say: "Their usage in
2115    production code should be noted to avoid problems during upgrades." The same
2116    remarks apply to the PCRE features described in this section.
2117    .P
2118    Since these verbs are specifically related to backtracking, most of them can be
2119    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2120    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2121    failing negative assertion, they cause an error if encountered by
2122    \fBpcre_dfa_exec()\fP.
2123  .P  .P
2124  .in 0  The new verbs make use of what was previously invalid syntax: an opening
2125  Last updated: 06 June 2006  parenthesis followed by an asterisk. In Perl, they are generally of the form
2126  .br  (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2127  Copyright (c) 1997-2006 University of Cambridge.  form is just (*VERB). Any number of these verbs may occur in a pattern. There
2128    are two kinds:
2129    .
2130    .SS "Verbs that act immediately"
2131    .rs
2132    .sp
2133    The following verbs act as soon as they are encountered:
2134    .sp
2135       (*ACCEPT)
2136    .sp
2137    This verb causes the match to end successfully, skipping the remainder of the
2138    pattern. When inside a recursion, only the innermost pattern is ended
2139    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2140    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2141    captured. For example:
2142    .sp
2143      A(A|B(*ACCEPT)|C)D
2144    .sp
2145    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2146    captured.
2147    .sp
2148      (*FAIL) or (*F)
2149    .sp
2150    This verb causes the match to fail, forcing backtracking to occur. It is
2151    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2152    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2153    Perl features that are not present in PCRE. The nearest equivalent is the
2154    callout feature, as for example in this pattern:
2155    .sp
2156      a+(?C)(*FAIL)
2157    .sp
2158    A match with the string "aaaa" always fails, but the callout is taken before
2159    each backtrack happens (in this example, 10 times).
2160    .
2161    .SS "Verbs that act after backtracking"
2162    .rs
2163    .sp
2164    The following verbs do nothing when they are encountered. Matching continues
2165    with what follows, but if there is no subsequent match, a failure is forced.
2166    The verbs differ in exactly what kind of failure occurs.
2167    .sp
2168      (*COMMIT)
2169    .sp
2170    This verb causes the whole match to fail outright if the rest of the pattern
2171    does not match. Even if the pattern is unanchored, no further attempts to find
2172    a match by advancing the start point take place. Once (*COMMIT) has been
2173    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2174    starting point, or not at all. For example:
2175    .sp
2176      a+(*COMMIT)b
2177    .sp
2178    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2179    dynamic anchor, or "I've started, so I must finish."
2180    .sp
2181      (*PRUNE)
2182    .sp
2183    This verb causes the match to fail at the current position if the rest of the
2184    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2185    advance to the next starting character then happens. Backtracking can occur as
2186    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2187    if there is no match to the right, backtracking cannot cross (*PRUNE).
2188    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2189    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2190    be expressed in any other way.
2191    .sp
2192      (*SKIP)
2193    .sp
2194    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2195    "bumpalong" advance is not to the next character, but to the position in the
2196    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2197    was matched leading up to it cannot be part of a successful match. Consider:
2198    .sp
2199      a+(*SKIP)b
2200    .sp
2201    If the subject is "aaaac...", after the first match attempt fails (starting at
2202    the first character in the string), the starting point skips on to start the
2203    next attempt at "c". Note that a possessive quantifer does not have the same
2204    effect in this example; although it would suppress backtracking during the
2205    first match attempt, the second attempt would start at the second character
2206    instead of skipping on to "c".
2207    .sp
2208      (*THEN)
2209    .sp
2210    This verb causes a skip to the next alternation if the rest of the pattern does
2211    not match. That is, it cancels pending backtracking, but only within the
2212    current alternation. Its name comes from the observation that it can be used
2213    for a pattern-based if-then-else block:
2214    .sp
2215      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2216    .sp
2217    If the COND1 pattern matches, FOO is tried (and possibly further items after
2218    the end of the group if FOO succeeds); on failure the matcher skips to the
2219    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2220    is used outside of any alternation, it acts exactly like (*PRUNE).
2221    .
2222    .
2223    .SH "SEE ALSO"
2224    .rs
2225    .sp
2226    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
2227    .
2228    .
2229    .SH AUTHOR
2230    .rs
2231    .sp
2232    .nf
2233    Philip Hazel
2234    University Computing Service
2235    Cambridge CB2 3QH, England.
2236    .fi
2237    .
2238    .
2239    .SH REVISION
2240    .rs
2241    .sp
2242    .nf
2243    Last updated: 19 April 2008
2244    Copyright (c) 1997-2008 University of Cambridge.
2245    .fi

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