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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 259 by ph10, Wed Sep 19 09:52:44 2007 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. Perl's regular expressions are described in its own documentation, and
13    regular expressions in general are covered in a number of books, some of which
14    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
15    published by O'Reilly, covers regular expressions in great detail. This
16    description of PCRE's regular expressions is intended as reference material.
17  .P  .P
18  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,
19  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 34  The remainder of this document discusses
34  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
35  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
36  \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
37  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
38  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
39    alternative function, and how it differs from the normal function, are
40    discussed in the
41  .\" HREF  .\" HREF
42  \fBpcrematching\fP  \fBpcrematching\fP
43  .\"  .\"
44  page.  page.
45    .
46    .
47    .SH "NEWLINE CONVENTIONS"
48    .rs
49    .sp
50    PCRE supports five different conventions for indicating line breaks in
51    strings: a single CR (carriage return) character, a single LF (linefeed)
52    character, the two-character sequence CRLF, any of the three preceding, or any
53    Unicode newline sequence. The
54    .\" HREF
55    \fBpcreapi\fP
56    .\"
57    page has
58    .\" HTML <a href="pcreapi.html#newlines">
59    .\" </a>
60    further discussion
61    .\"
62    about newlines, and shows how to set the newline convention in the
63    \fIoptions\fP arguments for the compiling and matching functions.
64  .P  .P
65    It is also possible to specify a newline convention by starting a pattern
66    string with one of the following five sequences:
67    .sp
68      (*CR)        carriage return
69      (*LF)        linefeed
70      (*CRLF)      carriage return, followed by linefeed
71      (*ANYCRLF)   any of the three above
72      (*ANY)       all Unicode newline sequences
73    .sp
74    These override the default and the options given to \fBpcre_compile()\fP. For
75    example, on a Unix system where LF is the default newline sequence, the pattern
76    .sp
77      (*CR)a.b
78    .sp
79    changes the convention to CR. That pattern matches "a\enb" because LF is no
80    longer a newline. Note that these special settings, which are not
81    Perl-compatible, are recognized only at the very start of a pattern, and that
82    they must be in upper case. If more than one of them is present, the last one
83    is used.
84    .P
85    The newline convention does not affect what the \eR escape sequence matches. By
86    default, this is any Unicode newline sequence, for Perl compatibility. However,
87    this can be changed; see the description of \eR in the section entitled
88    .\" HTML <a href="#newlineseq">
89    .\" </a>
90    "Newline sequences"
91    .\"
92    below. A change of \eR setting can be combined with a change of newline
93    convention.
94    .
95    .
96    .SH "CHARACTERS AND METACHARACTERS"
97    .rs
98    .sp
99  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
100  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
101  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 119  interpreted in some special way.
119  .P  .P
120  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
121  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
122  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
123  as follows:  are as follows:
124  .sp  .sp
125    \e      general escape character with several uses    \e      general escape character with several uses
126    ^      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 151  a character class the only metacharacter
151  .sp  .sp
152  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
153  .  .
154    .
155  .SH BACKSLASH  .SH BACKSLASH
156  .rs  .rs
157  .sp  .sp
# Line 142  represents: Line 202  represents:
202    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
203    \ee        escape (hex 1B)    \ee        escape (hex 1B)
204    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
205    \en        newline (hex 0A)    \en        linefeed (hex 0A)
206    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
207    \et        tab (hex 09)    \et        tab (hex 09)
208    \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 217  Thus \ecz becomes hex 1A, but \ec{ becom
217  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
218  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{
219  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
220  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
221  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
222  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
223  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
224  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
225    there is no terminating }, this form of escape is not recognized. Instead, the
226    initial \ex will be interpreted as a basic hexadecimal escape, with no
227    following digits, giving a character whose value is zero.
228  .P  .P
229  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
230  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 253  parenthesized subpatterns.
253  .P  .P
254  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
255  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
256  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
257  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
258  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
259  to \e777 are permitted. For example:  to \e777 are permitted. For example:
# Line 221  zero, because no more than three octal d Line 284  zero, because no more than three octal d
284  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
285  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
286  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08), and the
287  sequence \eX is interpreted as the character "X". Outside a character class,  sequences \eR and \eX are interpreted as the characters "R" and "X",
288  these sequences have different meanings  respectively. Outside a character class, these sequences have different
289    meanings
290  .\" HTML <a href="#uniextseq">  .\" HTML <a href="#uniextseq">
291  .\" </a>  .\" </a>
292  (see below).  (see below).
293  .\"  .\"
294  .  .
295  .  .
296    .SS "Absolute and relative back references"
297    .rs
298    .sp
299    The sequence \eg followed by an unsigned or a negative number, optionally
300    enclosed in braces, is an absolute or relative back reference. A named back
301    reference can be coded as \eg{name}. Back references are discussed
302    .\" HTML <a href="#backreferences">
303    .\" </a>
304    later,
305    .\"
306    following the discussion of
307    .\" HTML <a href="#subpattern">
308    .\" </a>
309    parenthesized subpatterns.
310    .\"
311    .
312    .
313  .SS "Generic character types"  .SS "Generic character types"
314  .rs  .rs
315  .sp  .sp
316  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
317  following are always recognized:  following are always recognized:
318  .sp  .sp
319    \ed     any decimal digit    \ed     any decimal digit
320    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
321      \eh     any horizontal whitespace character
322      \eH     any character that is not a horizontal whitespace character
323    \es     any whitespace character    \es     any whitespace character
324    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
325      \ev     any vertical whitespace character
326      \eV     any character that is not a vertical whitespace character
327    \ew     any "word" character    \ew     any "word" character
328    \eW     any "non-word" character    \eW     any "non-word" character
329  .sp  .sp
# Line 252  there is no character to match. Line 337  there is no character to match.
337  .P  .P
338  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).
339  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
340  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
341  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
342  does.)  does.
343    .P
344    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
345    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
346    character property support is available. These sequences retain their original
347    meanings from before UTF-8 support was available, mainly for efficiency
348    reasons.
349    .P
350    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
351    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
352    The horizontal space characters are:
353    .sp
354      U+0009     Horizontal tab
355      U+0020     Space
356      U+00A0     Non-break space
357      U+1680     Ogham space mark
358      U+180E     Mongolian vowel separator
359      U+2000     En quad
360      U+2001     Em quad
361      U+2002     En space
362      U+2003     Em space
363      U+2004     Three-per-em space
364      U+2005     Four-per-em space
365      U+2006     Six-per-em space
366      U+2007     Figure space
367      U+2008     Punctuation space
368      U+2009     Thin space
369      U+200A     Hair space
370      U+202F     Narrow no-break space
371      U+205F     Medium mathematical space
372      U+3000     Ideographic space
373    .sp
374    The vertical space characters are:
375    .sp
376      U+000A     Linefeed
377      U+000B     Vertical tab
378      U+000C     Formfeed
379      U+000D     Carriage return
380      U+0085     Next line
381      U+2028     Line separator
382      U+2029     Paragraph separator
383  .P  .P
384  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
385  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 393  in the
393  .\" HREF  .\" HREF
394  \fBpcreapi\fP  \fBpcreapi\fP
395  .\"  .\"
396  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,
397  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
398  .P  accented letters, and these are matched by \ew. The use of locales with Unicode
399  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  is discouraged.
400  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .
401  character property support is available. The use of locales with Unicode is  .
402  discouraged.  .\" HTML <a name="newlineseq"></a>
403    .SS "Newline sequences"
404    .rs
405    .sp
406    Outside a character class, by default, the escape sequence \eR matches any
407    Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
408    equivalent to the following:
409    .sp
410      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
411    .sp
412    This is an example of an "atomic group", details of which are given
413    .\" HTML <a href="#atomicgroup">
414    .\" </a>
415    below.
416    .\"
417    This particular group matches either the two-character sequence CR followed by
418    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
419    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
420    line, U+0085). The two-character sequence is treated as a single unit that
421    cannot be split.
422    .P
423    In UTF-8 mode, two additional characters whose codepoints are greater than 255
424    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
425    Unicode character property support is not needed for these characters to be
426    recognized.
427    .P
428    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
429    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
430    either at compile time or when the pattern is matched. (BSR is an abbrevation
431    for "backslash R".) This can be made the default when PCRE is built; if this is
432    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
433    It is also possible to specify these settings by starting a pattern string with
434    one of the following sequences:
435    .sp
436      (*BSR_ANYCRLF)   CR, LF, or CRLF only
437      (*BSR_UNICODE)   any Unicode newline sequence
438    .sp
439    These override the default and the options given to \fBpcre_compile()\fP, but
440    they can be overridden by options given to \fBpcre_exec()\fP. Note that these
441    special settings, which are not Perl-compatible, are recognized only at the
442    very start of a pattern, and that they must be in upper case. If more than one
443    of them is present, the last one is used. They can be combined with a change of
444    newline convention, for example, a pattern can start with:
445    .sp
446      (*ANY)(*BSR_ANYCRLF)
447    .sp
448    Inside a character class, \eR matches the letter "R".
449  .  .
450  .  .
451  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 282  discouraged. Line 453  discouraged.
453  .rs  .rs
454  .sp  .sp
455  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
456  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
457  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
458    characters whose codepoints are less than 256, but they do work in this mode.
459    The extra escape sequences are:
460  .sp  .sp
461    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
462    \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 480  Those that are not part of an identified
480  .P  .P
481  Arabic,  Arabic,
482  Armenian,  Armenian,
483    Balinese,
484  Bengali,  Bengali,
485  Bopomofo,  Bopomofo,
486  Braille,  Braille,
# Line 316  Canadian_Aboriginal, Line 490  Canadian_Aboriginal,
490  Cherokee,  Cherokee,
491  Common,  Common,
492  Coptic,  Coptic,
493    Cuneiform,
494  Cypriot,  Cypriot,
495  Cyrillic,  Cyrillic,
496  Deseret,  Deseret,
# Line 345  Malayalam, Line 520  Malayalam,
520  Mongolian,  Mongolian,
521  Myanmar,  Myanmar,
522  New_Tai_Lue,  New_Tai_Lue,
523    Nko,
524  Ogham,  Ogham,
525  Old_Italic,  Old_Italic,
526  Old_Persian,  Old_Persian,
527  Oriya,  Oriya,
528  Osmanya,  Osmanya,
529    Phags_Pa,
530    Phoenician,
531  Runic,  Runic,
532  Shavian,  Shavian,
533  Sinhala,  Sinhala,
# Line 430  The special property L& is also supporte Line 608  The special property L& is also supporte
608  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
609  a modifier or "other".  a modifier or "other".
610  .P  .P
611    The Cs (Surrogate) property applies only to characters in the range U+D800 to
612    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
613    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
614    (see the discussion of PCRE_NO_UTF8_CHECK in the
615    .\" HREF
616    \fBpcreapi\fP
617    .\"
618    page).
619    .P
620  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})
621  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
622  properties with "Is".  properties with "Is".
# Line 454  atomic group Line 641  atomic group
641  (see below).  (see below).
642  .\"  .\"
643  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
644  preceding character.  preceding character. None of them have codepoints less than 256, so in
645    non-UTF-8 mode \eX matches any one character.
646  .P  .P
647  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
648  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 650  why the traditional escape sequences suc
650  properties in PCRE.  properties in PCRE.
651  .  .
652  .  .
653    .\" HTML <a name="resetmatchstart"></a>
654    .SS "Resetting the match start"
655    .rs
656    .sp
657    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
658    matched characters not to be included in the final matched sequence. For
659    example, the pattern:
660    .sp
661      foo\eKbar
662    .sp
663    matches "foobar", but reports that it has matched "bar". This feature is
664    similar to a lookbehind assertion
665    .\" HTML <a href="#lookbehind">
666    .\" </a>
667    (described below).
668    .\"
669    However, in this case, the part of the subject before the real match does not
670    have to be of fixed length, as lookbehind assertions do. The use of \eK does
671    not interfere with the setting of
672    .\" HTML <a href="#subpattern">
673    .\" </a>
674    captured substrings.
675    .\"
676    For example, when the pattern
677    .sp
678      (foo)\eKbar
679    .sp
680    matches "foobar", the first substring is still set to "foo".
681    .
682    .
683  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
684  .SS "Simple assertions"  .SS "Simple assertions"
685  .rs  .rs
686  .sp  .sp
687  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
688  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,
689  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
690  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 696  The backslashed assertions are:
696  .sp  .sp
697    \eb     matches at a word boundary    \eb     matches at a word boundary
698    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
699    \eA     matches at start of subject    \eA     matches at the start of the subject
700    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
701    \ez     matches at end of subject            also matches before a newline at the end of the subject
702    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
703      \eG     matches at the first matching position in the subject
704  .sp  .sp
705  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
706  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 797  end of the subject in both modes, and if
797  .sp  .sp
798  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
799  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
800  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.
801  a line ending is defined as a single character (CR or LF), dot never matches  .P
802  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
803  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
804  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
805    (including isolated CRs and LFs). When any Unicode line endings are being
806    recognized, dot does not match CR or LF or any of the other line ending
807    characters.
808  .P  .P
809  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
810  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
811  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
812    to match it.
813  .P  .P
814  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
815  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 820  special meaning in a character class.
820  .rs  .rs
821  .sp  .sp
822  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
823  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
824  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
825  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,
826  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,
827  sequence is best avoided.  the \eC escape sequence is best avoided.
828  .P  .P
829  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
830  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 652  If you want to use caseless matching for Line 875  If you want to use caseless matching for
875  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
876  UTF-8 support.  UTF-8 support.
877  .P  .P
878  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
879  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
880  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
881  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
882  .P  .P
883  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
884  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 902  example [\ex{100}-\ex{2ff}].
902  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
903  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
904  [][\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
905  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
906  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
907  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
908  property support.  property support.
# Line 769  alternative in the subpattern. Line 992  alternative in the subpattern.
992  .rs  .rs
993  .sp  .sp
994  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
995  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
996  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
997    The option letters are
998  .sp  .sp
999    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1000    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 784  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1008  PCRE_MULTILINE while unsetting PCRE_DOTA
1008  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
1009  unset.  unset.
1010  .P  .P
1011    The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1012    changed in the same way as the Perl-compatible options by using the characters
1013    J, U and X respectively.
1014    .P
1015  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
1016  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
1017  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
1018  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
1019  \fBpcre_fullinfo()\fP function).  \fBpcre_fullinfo()\fP function).
1020  .P  .P
1021  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
1022  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
1023  .sp  .sp
1024    (a(?i)b)c    (a(?i)b)c
1025  .sp  .sp
# Line 807  branch is abandoned before the option se Line 1035  branch is abandoned before the option se
1035  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1036  behaviour otherwise.  behaviour otherwise.
1037  .P  .P
1038  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
1039  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
1040  J, U and X respectively.  pattern can contain special leading sequences to override what the application
1041    has set or what has been defaulted. Details are given in the section entitled
1042    .\" HTML <a href="#newlineseq">
1043    .\" </a>
1044    "Newline sequences"
1045    .\"
1046    above.
1047  .  .
1048  .  .
1049  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 824  Turning part of a pattern into a subpatt Line 1058  Turning part of a pattern into a subpatt
1058    cat(aract|erpillar|)    cat(aract|erpillar|)
1059  .sp  .sp
1060  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1061  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1062  .sp  .sp
1063  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
1064  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 1083  the string "the white queen" is matched
1083    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1084  .sp  .sp
1085  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
1086  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.  
1087  .P  .P
1088  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
1089  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 1098  is reached, an option setting in one bra
1098  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1099  .  .
1100  .  .
1101    .SH "DUPLICATE SUBPATTERN NUMBERS"
1102    .rs
1103    .sp
1104    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1105    the same numbers for its capturing parentheses. Such a subpattern starts with
1106    (?| and is itself a non-capturing subpattern. For example, consider this
1107    pattern:
1108    .sp
1109      (?|(Sat)ur|(Sun))day
1110    .sp
1111    Because the two alternatives are inside a (?| group, both sets of capturing
1112    parentheses are numbered one. Thus, when the pattern matches, you can look
1113    at captured substring number one, whichever alternative matched. This construct
1114    is useful when you want to capture part, but not all, of one of a number of
1115    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1116    number is reset at the start of each branch. The numbers of any capturing
1117    buffers that follow the subpattern start after the highest number used in any
1118    branch. The following example is taken from the Perl documentation.
1119    The numbers underneath show in which buffer the captured content will be
1120    stored.
1121    .sp
1122      # before  ---------------branch-reset----------- after
1123      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1124      # 1            2         2  3        2     3     4
1125    .sp
1126    A backreference or a recursive call to a numbered subpattern always refers to
1127    the first one in the pattern with the given number.
1128    .P
1129    An alternative approach to using this "branch reset" feature is to use
1130    duplicate named subpatterns, as described in the next section.
1131    .
1132    .
1133  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1134  .rs  .rs
1135  .sp  .sp
1136  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
1137  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1138  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
1139  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1140  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
1141    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1142    the Perl and the Python syntax.
1143    .P
1144    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1145    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1146  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1147  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1148  .\" </a>  .\" </a>
# Line 890  conditions, Line 1160  conditions,
1160  can be made by name as well as by number.  can be made by name as well as by number.
1161  .P  .P
1162  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1163  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1164  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
1165  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
1166  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1167  .P  .P
1168  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
1169  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 1172  match. Suppose you want to match the nam
1172  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
1173  abbreviation. This pattern (ignoring the line breaks) does the job:  abbreviation. This pattern (ignoring the line breaks) does the job:
1174  .sp  .sp
1175    (?P<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1176    (?P<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
1177    (?P<DN>Wed)(?:nesday)?|    (?<DN>Wed)(?:nesday)?|
1178    (?P<DN>Thu)(?:rsday)?|    (?<DN>Thu)(?:rsday)?|
1179    (?P<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1180  .sp  .sp
1181  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.
1182    (An alternative way of solving this problem is to use a "branch reset"
1183    subpattern, as described in the previous section.)
1184    .P
1185  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1186  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
1187  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
1188  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
1189  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 1201  Repetition is specified by quantifiers,
1201  items:  items:
1202  .sp  .sp
1203    a literal data character    a literal data character
1204    the . metacharacter    the dot metacharacter
1205    the \eC escape sequence    the \eC escape sequence
1206    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1207      the \eR escape sequence
1208    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1209    a character class    a character class
1210    a back reference (see next section)    a back reference (see next section)
# Line 968  which may be several bytes long (and the Line 1242  which may be several bytes long (and the
1242  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
1243  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1244  .P  .P
1245  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1246  quantifiers have single-character abbreviations:  abbreviations:
1247  .sp  .sp
1248    *    is equivalent to {0,}    *    is equivalent to {0,}
1249    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1291  own right. Because it has two uses, it c
1291  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
1292  way the rest of the pattern matches.  way the rest of the pattern matches.
1293  .P  .P
1294  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),
1295  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
1296  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
1297  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1301  is greater than 1 or with a limited maxi
1301  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1302  .P  .P
1303  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
1304  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
1305  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1306  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
1307  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 1313  alternatively using ^ to indicate anchor
1313  .P  .P
1314  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1315  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1316  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
1317  succeed. Consider, for example:  succeeds. Consider, for example:
1318  .sp  .sp
1319    (.*)abc\e1    (.*)abc\e1
1320  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1340  matches "aba" the value of the second ca
1340  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1341  .rs  .rs
1342  .sp  .sp
1343  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1344  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
1345  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
1346  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
1347  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
1348  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1349  .P  .P
1350  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
1351  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1357  item, and then with 4, and so on, before
1357  (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
1358  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.
1359  .P  .P
1360  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
1361  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
1362  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1363  .sp  .sp
# Line 1113  previous example can be rewritten as Line 1387  previous example can be rewritten as
1387  .sp  .sp
1388    \ed++foo    \ed++foo
1389  .sp  .sp
1390    Note that a possessive quantifier can be used with an entire group, for
1391    example:
1392    .sp
1393      (abc|xyz){2,3}+
1394    .sp
1395  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1396  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
1397  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
1398  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1399  .P  difference; possessive quantifiers should be slightly faster.
1400  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1401  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.
1402  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
1403  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1404    package, and PCRE copied it from there. It ultimately found its way into Perl
1405    at release 5.10.
1406    .P
1407    PCRE has an optimization that automatically "possessifies" certain simple
1408    pattern constructs. For example, the sequence A+B is treated as A++B because
1409    there is no point in backtracking into a sequence of A's when B must follow.
1410  .P  .P
1411  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
1412  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 1452  numbers less than 10. A "forward back re
1452  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
1453  in an earlier iteration.  in an earlier iteration.
1454  .P  .P
1455  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
1456  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
1457  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1458  "Non-printing characters"  "Non-printing characters"
1459  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1460  .\" </a>  .\" </a>
1461  above  above
1462  .\"  .\"
1463  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1464    no such problem when named parentheses are used. A back reference to any
1465    subpattern is possible using named parentheses (see below).
1466    .P
1467    Another way of avoiding the ambiguity inherent in the use of digits following a
1468    backslash is to use the \eg escape sequence, which is a feature introduced in
1469    Perl 5.10. This escape must be followed by an unsigned number or a negative
1470    number, optionally enclosed in braces. These examples are all identical:
1471    .sp
1472      (ring), \e1
1473      (ring), \eg1
1474      (ring), \eg{1}
1475    .sp
1476    An unsigned number specifies an absolute reference without the ambiguity that
1477    is present in the older syntax. It is also useful when literal digits follow
1478    the reference. A negative number is a relative reference. Consider this
1479    example:
1480    .sp
1481      (abc(def)ghi)\eg{-1}
1482    .sp
1483    The sequence \eg{-1} is a reference to the most recently started capturing
1484    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1485    would be equivalent to \e1. The use of relative references can be helpful in
1486    long patterns, and also in patterns that are created by joining together
1487    fragments that contain references within themselves.
1488  .P  .P
1489  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1490  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 1506  back reference, the case of letters is r
1506  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
1507  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1508  .P  .P
1509  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1510  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1511    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1512    back reference syntax, in which \eg can be used for both numeric and named
1513    references, is also supported. We could rewrite the above example in any of
1514    the following ways:
1515  .sp  .sp
1516      (?<p1>(?i)rah)\es+\ek<p1>
1517      (?'p1'(?i)rah)\es+\ek{p1}
1518    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1519      (?<p1>(?i)rah)\es+\eg{p1}
1520  .sp  .sp
1521  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
1522  after the reference.  after the reference.
# Line 1323  lengths, but it is acceptable if rewritt Line 1639  lengths, but it is acceptable if rewritt
1639  .sp  .sp
1640    (?<=abc|abde)    (?<=abc|abde)
1641  .sp  .sp
1642    In some cases, the Perl 5.10 escape sequence \eK
1643    .\" HTML <a href="#resetmatchstart">
1644    .\" </a>
1645    (see above)
1646    .\"
1647    can be used instead of a lookbehind assertion; this is not restricted to a
1648    fixed-length.
1649    .P
1650  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1651  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
1652  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1653  match is deemed to fail.  assertion fails.
1654  .P  .P
1655  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)
1656  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1657  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
1658  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1659  .P  .P
1660  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1661  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
1662  such as  pattern such as
1663  .sp  .sp
1664    abcd$    abcd$
1665  .sp  .sp
# Line 1351  then all but the last two characters, an Line 1675  then all but the last two characters, an
1675  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,
1676  if the pattern is written as  if the pattern is written as
1677  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1678    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1679  .sp  .sp
1680  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
1681  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
1682  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1683  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 1733  If the condition is satisfied, the yes-p
1733  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
1734  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1735  .P  .P
1736  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1737  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1738  underscores, the condition is satisfied if the capturing subpattern of that  .
1739  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
1740  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
1741  named subpattern; if it cannot find one and the text consists entirely of  .sp
1742  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
1743  zero. Using subpattern names that consist entirely of digits is not  condition is true if the capturing subpattern of that number has previously
1744  recommended.  matched. An alternative notation is to precede the digits with a plus or minus
1745    sign. In this case, the subpattern number is relative rather than absolute.
1746    The most recently opened parentheses can be referenced by (?(-1), the next most
1747    recent by (?(-2), and so on. In looping constructs it can also make sense to
1748    refer to subsequent groups with constructs such as (?(+2).
1749  .P  .P
1750  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1751  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 1761  or not. If they did, that is, if subject
1761  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
1762  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1763  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1764  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
1765  named subpattern gives this:  .P
1766    If you were embedding this pattern in a larger one, you could use a relative
1767    reference:
1768    .sp
1769      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1770    .sp
1771    This makes the fragment independent of the parentheses in the larger pattern.
1772    .
1773    .SS "Checking for a used subpattern by name"
1774    .rs
1775    .sp
1776    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1777    subpattern by name. For compatibility with earlier versions of PCRE, which had
1778    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1779    there is a possible ambiguity with this syntax, because subpattern names may
1780    consist entirely of digits. PCRE looks first for a named subpattern; if it
1781    cannot find one and the name consists entirely of digits, PCRE looks for a
1782    subpattern of that number, which must be greater than zero. Using subpattern
1783    names that consist entirely of digits is not recommended.
1784    .P
1785    Rewriting the above example to use a named subpattern gives this:
1786    .sp
1787      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1788  .sp  .sp
1789    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )  .
1790    .SS "Checking for pattern recursion"
1791    .rs
1792  .sp  .sp
1793  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,
1794  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
1795  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
1796  Recursive patterns are described in the next section.  letter R, for example:
1797    .sp
1798      (?(R3)...) or (?(R&name)...)
1799    .sp
1800    the condition is true if the most recent recursion is into the subpattern whose
1801    number or name is given. This condition does not check the entire recursion
1802    stack.
1803  .P  .P
1804  If the condition is not a sequence of digits or (R), it must be an assertion.  At "top level", all these recursion test conditions are false. Recursive
1805    patterns are described below.
1806    .
1807    .SS "Defining subpatterns for use by reference only"
1808    .rs
1809    .sp
1810    If the condition is the string (DEFINE), and there is no subpattern with the
1811    name DEFINE, the condition is always false. In this case, there may be only one
1812    alternative in the subpattern. It is always skipped if control reaches this
1813    point in the pattern; the idea of DEFINE is that it can be used to define
1814    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1815    is described below.) For example, a pattern to match an IPv4 address could be
1816    written like this (ignore whitespace and line breaks):
1817    .sp
1818      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1819      \eb (?&byte) (\e.(?&byte)){3} \eb
1820    .sp
1821    The first part of the pattern is a DEFINE group inside which a another group
1822    named "byte" is defined. This matches an individual component of an IPv4
1823    address (a number less than 256). When matching takes place, this part of the
1824    pattern is skipped because DEFINE acts like a false condition.
1825    .P
1826    The rest of the pattern uses references to the named group to match the four
1827    dot-separated components of an IPv4 address, insisting on a word boundary at
1828    each end.
1829    .
1830    .SS "Assertion conditions"
1831    .rs
1832    .sp
1833    If the condition is not in any of the above formats, it must be an assertion.
1834  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1835  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1836  alternatives on the second line:  alternatives on the second line:
# Line 1483  next newline in the pattern. Line 1866  next newline in the pattern.
1866  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1867  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1868  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
1869  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1870  that allows regular expressions to recurse (amongst other things). It does this  .P
1871  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
1872  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
1873  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1874    pattern using code interpolation to solve the parentheses problem can be
1875    created like this:
1876  .sp  .sp
1877    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1878  .sp  .sp
1879  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
1880  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1881  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1882  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1883  .P  supports special syntax for recursion of the entire pattern, and also for
1884  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1885  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.
1886  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1887  "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
1888  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1889  .P  provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1890  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
1891  it has matched some of the subject string, it is never re-entered, even if  a recursive call of the entire regular expression.
1892  it contains untried alternatives and there is a subsequent matching failure.  .P
1893    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1894    treated as an atomic group. That is, once it has matched some of the subject
1895    string, it is never re-entered, even if it contains untried alternatives and
1896    there is a subsequent matching failure.
1897  .P  .P
1898  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1899  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 1911  pattern, so instead you could use this:
1911    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1912  .sp  .sp
1913  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
1914  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1915  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1916  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
1917  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.)
1918  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
1919  .sp  most recently opened parentheses preceding the recursion. In other words, a
1920    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
1921  .sp  it is encountered.
1922  This particular example pattern contains nested unlimited repeats, and so the  .P
1923  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1924  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
1925  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1926    "subroutine" calls, as described in the next section.
1927    .P
1928    An alternative approach is to use named parentheses instead. The Perl syntax
1929    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1930    could rewrite the above example as follows:
1931    .sp
1932      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1933    .sp
1934    If there is more than one subpattern with the same name, the earliest one is
1935    used.
1936    .P
1937    This particular example pattern that we have been looking at contains nested
1938    unlimited repeats, and so the use of atomic grouping for matching strings of
1939    non-parentheses is important when applying the pattern to strings that do not
1940    match. For example, when this pattern is applied to
1941  .sp  .sp
1942    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1943  .sp  .sp
# Line 1545  before failure can be reported. Line 1949  before failure can be reported.
1949  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
1950  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.
1951  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
1952  the next section and the  below and the
1953  .\" HREF  .\" HREF
1954  \fBpcrecallout\fP  \fBpcrecallout\fP
1955  .\"  .\"
# Line 1584  is the actual recursive call. Line 1988  is the actual recursive call.
1988  .sp  .sp
1989  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
1990  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
1991  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1992  pattern  before or after the reference. A numbered reference can be absolute or
1993    relative, as in these examples:
1994    .sp
1995      (...(absolute)...)...(?2)...
1996      (...(relative)...)...(?-1)...
1997      (...(?+1)...(relative)...
1998    .sp
1999    An earlier example pointed out that the pattern
2000  .sp  .sp
2001    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2002  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 2006  matches "sense and sensibility" and "res
2006    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2007  .sp  .sp
2008  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
2009  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.  
2010  .P  .P
2011  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2012  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
2013  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
2014  matching failure.  matching failure.
2015    .P
2016    When a subpattern is used as a subroutine, processing options such as
2017    case-independence are fixed when the subpattern is defined. They cannot be
2018    changed for different calls. For example, consider this pattern:
2019    .sp
2020      (abc)(?i:(?-1))
2021    .sp
2022    It matches "abcabc". It does not match "abcABC" because the change of
2023    processing option does not affect the called subpattern.
2024  .  .
2025  .  .
2026  .SH CALLOUTS  .SH CALLOUTS
# Line 1622  function is to be called. If you want to Line 2041  function is to be called. If you want to
2041  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.
2042  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2043  .sp  .sp
2044    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2045  .sp  .sp
2046  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
2047  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 2057  description of the interface to the call
2057  \fBpcrecallout\fP  \fBpcrecallout\fP
2058  .\"  .\"
2059  documentation.  documentation.
2060    .
2061    .
2062    .SH "BACKTRACKING CONTROL"
2063    .rs
2064    .sp
2065    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2066    are described in the Perl documentation as "experimental and subject to change
2067    or removal in a future version of Perl". It goes on to say: "Their usage in
2068    production code should be noted to avoid problems during upgrades." The same
2069    remarks apply to the PCRE features described in this section.
2070  .P  .P
2071  .in 0  Since these verbs are specifically related to backtracking, they can be used
2072  Last updated: 06 June 2006  only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
2073  .br  backtracking algorithm. They cause an error if encountered by
2074  Copyright (c) 1997-2006 University of Cambridge.  \fBpcre_dfa_exec()\fP.
2075    .P
2076    The new verbs make use of what was previously invalid syntax: an opening
2077    parenthesis followed by an asterisk. In Perl, they are generally of the form
2078    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2079    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2080    are two kinds:
2081    .
2082    .SS "Verbs that act immediately"
2083    .rs
2084    .sp
2085    The following verbs act as soon as they are encountered:
2086    .sp
2087       (*ACCEPT)
2088    .sp
2089    This verb causes the match to end successfully, skipping the remainder of the
2090    pattern. When inside a recursion, only the innermost pattern is ended
2091    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2092    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2093    captured. For example:
2094    .sp
2095      A(A|B(*ACCEPT)|C)D
2096    .sp
2097    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2098    captured.
2099    .sp
2100      (*FAIL) or (*F)
2101    .sp
2102    This verb causes the match to fail, forcing backtracking to occur. It is
2103    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2104    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2105    Perl features that are not present in PCRE. The nearest equivalent is the
2106    callout feature, as for example in this pattern:
2107    .sp
2108      a+(?C)(*FAIL)
2109    .sp
2110    A match with the string "aaaa" always fails, but the callout is taken before
2111    each backtrack happens (in this example, 10 times).
2112    .
2113    .SS "Verbs that act after backtracking"
2114    .rs
2115    .sp
2116    The following verbs do nothing when they are encountered. Matching continues
2117    with what follows, but if there is no subsequent match, a failure is forced.
2118    The verbs differ in exactly what kind of failure occurs.
2119    .sp
2120      (*COMMIT)
2121    .sp
2122    This verb causes the whole match to fail outright if the rest of the pattern
2123    does not match. Even if the pattern is unanchored, no further attempts to find
2124    a match by advancing the start point take place. Once (*COMMIT) has been
2125    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2126    starting point, or not at all. For example:
2127    .sp
2128      a+(*COMMIT)b
2129    .sp
2130    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2131    dynamic anchor, or "I've started, so I must finish."
2132    .sp
2133      (*PRUNE)
2134    .sp
2135    This verb causes the match to fail at the current position if the rest of the
2136    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2137    advance to the next starting character then happens. Backtracking can occur as
2138    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2139    if there is no match to the right, backtracking cannot cross (*PRUNE).
2140    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2141    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2142    be expressed in any other way.
2143    .sp
2144      (*SKIP)
2145    .sp
2146    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2147    "bumpalong" advance is not to the next character, but to the position in the
2148    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2149    was matched leading up to it cannot be part of a successful match. Consider:
2150    .sp
2151      a+(*SKIP)b
2152    .sp
2153    If the subject is "aaaac...", after the first match attempt fails (starting at
2154    the first character in the string), the starting point skips on to start the
2155    next attempt at "c". Note that a possessive quantifer does not have the same
2156    effect in this example; although it would suppress backtracking during the
2157    first match attempt, the second attempt would start at the second character
2158    instead of skipping on to "c".
2159    .sp
2160      (*THEN)
2161    .sp
2162    This verb causes a skip to the next alternation if the rest of the pattern does
2163    not match. That is, it cancels pending backtracking, but only within the
2164    current alternation. Its name comes from the observation that it can be used
2165    for a pattern-based if-then-else block:
2166    .sp
2167      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2168    .sp
2169    If the COND1 pattern matches, FOO is tried (and possibly further items after
2170    the end of the group if FOO succeeds); on failure the matcher skips to the
2171    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2172    is used outside of any alternation, it acts exactly like (*PRUNE).
2173    .
2174    .
2175    .SH "SEE ALSO"
2176    .rs
2177    .sp
2178    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
2179    .
2180    .
2181    .SH AUTHOR
2182    .rs
2183    .sp
2184    .nf
2185    Philip Hazel
2186    University Computing Service
2187    Cambridge CB2 3QH, England.
2188    .fi
2189    .
2190    .
2191    .SH REVISION
2192    .rs
2193    .sp
2194    .nf
2195    Last updated: 17 September 2007
2196    Copyright (c) 1997-2007 University of Cambridge.
2197    .fi

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