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# Line 4  PCRE - Perl-compatible regular expressio Line 4  PCRE - Perl-compatible regular expressio
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 character strings. To use this, you must
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  the PCRE_UTF8 option. There is also a special sequence that can be given at the
27  places below. There is also a summary of UTF-8 features in the  start of a pattern:
28    .sp
29      (*UTF8)
30    .sp
31    Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32    option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33    pattern matching is mentioned in several places below. There is also a summary
34    of UTF-8 features in the
35  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
36  .\" </a>  .\" </a>
37  section on UTF-8 support  section on UTF-8 support
# Line 30  The remainder of this document discusses Line 46  The remainder of this document discusses
46  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
47  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
48  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
49  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
50  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
51    alternative function, and how it differs from the normal function, are
52    discussed in the
53  .\" HREF  .\" HREF
54  \fBpcrematching\fP  \fBpcrematching\fP
55  .\"  .\"
56  page.  page.
57  .  .
58  .  .
59    .SH "NEWLINE CONVENTIONS"
60    .rs
61    .sp
62    PCRE supports five different conventions for indicating line breaks in
63    strings: a single CR (carriage return) character, a single LF (linefeed)
64    character, the two-character sequence CRLF, any of the three preceding, or any
65    Unicode newline sequence. The
66    .\" HREF
67    \fBpcreapi\fP
68    .\"
69    page has
70    .\" HTML <a href="pcreapi.html#newlines">
71    .\" </a>
72    further discussion
73    .\"
74    about newlines, and shows how to set the newline convention in the
75    \fIoptions\fP arguments for the compiling and matching functions.
76    .P
77    It is also possible to specify a newline convention by starting a pattern
78    string with one of the following five sequences:
79    .sp
80      (*CR)        carriage return
81      (*LF)        linefeed
82      (*CRLF)      carriage return, followed by linefeed
83      (*ANYCRLF)   any of the three above
84      (*ANY)       all Unicode newline sequences
85    .sp
86    These override the default and the options given to \fBpcre_compile()\fP. For
87    example, on a Unix system where LF is the default newline sequence, the pattern
88    .sp
89      (*CR)a.b
90    .sp
91    changes the convention to CR. That pattern matches "a\enb" because LF is no
92    longer a newline. Note that these special settings, which are not
93    Perl-compatible, are recognized only at the very start of a pattern, and that
94    they must be in upper case. If more than one of them is present, the last one
95    is used.
96    .P
97    The newline convention does not affect what the \eR escape sequence matches. By
98    default, this is any Unicode newline sequence, for Perl compatibility. However,
99    this can be changed; see the description of \eR in the section entitled
100    .\" HTML <a href="#newlineseq">
101    .\" </a>
102    "Newline sequences"
103    .\"
104    below. A change of \eR setting can be combined with a change of newline
105    convention.
106    .
107    .
108  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
109  .rs  .rs
110  .sp  .sp
# Line 147  represents: Line 214  represents:
214    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
215    \ee        escape (hex 1B)    \ee        escape (hex 1B)
216    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
217    \en        newline (hex 0A)    \en        linefeed (hex 0A)
218    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
219    \et        tab (hex 09)    \et        tab (hex 09)
220    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
# Line 162  Thus \ecz becomes hex 1A, but \ec{ becom Line 229  Thus \ecz becomes hex 1A, but \ec{ becom
229  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
230  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
231  and }, but the value of the character code must be less than 256 in non-UTF-8  and }, but the value of the character code must be less than 256 in non-UTF-8
232  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
233  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
234  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
235  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
236  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
237    there is no terminating }, this form of escape is not recognized. Instead, the
238    initial \ex will be interpreted as a basic hexadecimal escape, with no
239    following digits, giving a character whose value is zero.
240  .P  .P
241  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
242  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex. There is no difference in the way they are handled. For
# Line 238  meanings Line 308  meanings
308  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
309  .rs  .rs
310  .sp  .sp
311  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
312  in braces, is an absolute or relative back reference. Back references are  enclosed in braces, is an absolute or relative back reference. A named back
313  discussed  reference can be coded as \eg{name}. Back references are discussed
314  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
315  .\" </a>  .\" </a>
316  later,  later,
# Line 252  parenthesized subpatterns. Line 322  parenthesized subpatterns.
322  .\"  .\"
323  .  .
324  .  .
325    .SS "Absolute and relative subroutine calls"
326    .rs
327    .sp
328    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
329    a number enclosed either in angle brackets or single quotes, is an alternative
330    syntax for referencing a subpattern as a "subroutine". Details are discussed
331    .\" HTML <a href="#onigurumasubroutines">
332    .\" </a>
333    later.
334    .\"
335    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
336    synonymous. The former is a back reference; the latter is a
337    .\" HTML <a href="#subpatternsassubroutines">
338    .\" </a>
339    subroutine
340    .\"
341    call.
342    .
343    .
344  .SS "Generic character types"  .SS "Generic character types"
345  .rs  .rs
346  .sp  .sp
# Line 260  following are always recognized: Line 349  following are always recognized:
349  .sp  .sp
350    \ed     any decimal digit    \ed     any decimal digit
351    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
352      \eh     any horizontal whitespace character
353      \eH     any character that is not a horizontal whitespace character
354    \es     any whitespace character    \es     any whitespace character
355    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
356      \ev     any vertical whitespace character
357      \eV     any character that is not a vertical whitespace character
358    \ew     any "word" character    \ew     any "word" character
359    \eW     any "non-word" character    \eW     any "non-word" character
360  .sp  .sp
# Line 275  there is no character to match. Line 368  there is no character to match.
368  .P  .P
369  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).
370  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
371  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
372  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
373  does.)  does.
374    .P
375    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
376    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
377    character property support is available. These sequences retain their original
378    meanings from before UTF-8 support was available, mainly for efficiency
379    reasons. Note that this also affects \eb, because it is defined in terms of \ew
380    and \eW.
381    .P
382    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
383    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
384    The horizontal space characters are:
385    .sp
386      U+0009     Horizontal tab
387      U+0020     Space
388      U+00A0     Non-break space
389      U+1680     Ogham space mark
390      U+180E     Mongolian vowel separator
391      U+2000     En quad
392      U+2001     Em quad
393      U+2002     En space
394      U+2003     Em space
395      U+2004     Three-per-em space
396      U+2005     Four-per-em space
397      U+2006     Six-per-em space
398      U+2007     Figure space
399      U+2008     Punctuation space
400      U+2009     Thin space
401      U+200A     Hair space
402      U+202F     Narrow no-break space
403      U+205F     Medium mathematical space
404      U+3000     Ideographic space
405    .sp
406    The vertical space characters are:
407    .sp
408      U+000A     Linefeed
409      U+000B     Vertical tab
410      U+000C     Formfeed
411      U+000D     Carriage return
412      U+0085     Next line
413      U+2028     Line separator
414      U+2029     Paragraph separator
415  .P  .P
416  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
417  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 293  in the Line 427  in the
427  .\"  .\"
428  page). For example, in a French locale such as "fr_FR" in Unix-like systems,  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
429  or "french" in Windows, some character codes greater than 128 are used for  or "french" in Windows, some character codes greater than 128 are used for
430  accented letters, and these are matched by \ew.  accented letters, and these are matched by \ew. The use of locales with Unicode
431  .P  is discouraged.
 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  
 \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  
 character property support is available. The use of locales with Unicode is  
 discouraged.  
432  .  .
433  .  .
434    .\" HTML <a name="newlineseq"></a>
435  .SS "Newline sequences"  .SS "Newline sequences"
436  .rs  .rs
437  .sp  .sp
438  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
439  sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
440  the following:  equivalent to the following:
441  .sp  .sp
442    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
443  .sp  .sp
# Line 326  are added: LS (line separator, U+2028) a Line 457  are added: LS (line separator, U+2028) a
457  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
458  recognized.  recognized.
459  .P  .P
460    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
461    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
462    either at compile time or when the pattern is matched. (BSR is an abbrevation
463    for "backslash R".) This can be made the default when PCRE is built; if this is
464    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
465    It is also possible to specify these settings by starting a pattern string with
466    one of the following sequences:
467    .sp
468      (*BSR_ANYCRLF)   CR, LF, or CRLF only
469      (*BSR_UNICODE)   any Unicode newline sequence
470    .sp
471    These override the default and the options given to \fBpcre_compile()\fP, but
472    they can be overridden by options given to \fBpcre_exec()\fP. Note that these
473    special settings, which are not Perl-compatible, are recognized only at the
474    very start of a pattern, and that they must be in upper case. If more than one
475    of them is present, the last one is used. They can be combined with a change of
476    newline convention, for example, a pattern can start with:
477    .sp
478      (*ANY)(*BSR_ANYCRLF)
479    .sp
480  Inside a character class, \eR matches the letter "R".  Inside a character class, \eR matches the letter "R".
481  .  .
482  .  .
# Line 334  Inside a character class, \eR matches th Line 485  Inside a character class, \eR matches th
485  .rs  .rs
486  .sp  .sp
487  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
488  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
489  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
490    characters whose codepoints are less than 256, but they do work in this mode.
491    The extra escape sequences are:
492  .sp  .sp
493    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
494    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
# Line 487  The special property L& is also supporte Line 640  The special property L& is also supporte
640  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
641  a modifier or "other".  a modifier or "other".
642  .P  .P
643  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The Cs (Surrogate) property applies only to characters in the range U+D800 to
644    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
645    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
646    (see the discussion of PCRE_NO_UTF8_CHECK in the
647    .\" HREF
648    \fBpcreapi\fP
649    .\"
650    page). Perl does not support the Cs property.
651    .P
652    The long synonyms for property names that Perl supports (such as \ep{Letter})
653  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
654  properties with "Is".  properties with "Is".
655  .P  .P
# Line 511  atomic group Line 673  atomic group
673  (see below).  (see below).
674  .\"  .\"
675  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
676  preceding character.  preceding character. None of them have codepoints less than 256, so in
677    non-UTF-8 mode \eX matches any one character.
678  .P  .P
679  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
680  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 519  why the traditional escape sequences suc Line 682  why the traditional escape sequences suc
682  properties in PCRE.  properties in PCRE.
683  .  .
684  .  .
685    .\" HTML <a name="resetmatchstart"></a>
686    .SS "Resetting the match start"
687    .rs
688    .sp
689    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
690    matched characters not to be included in the final matched sequence. For
691    example, the pattern:
692    .sp
693      foo\eKbar
694    .sp
695    matches "foobar", but reports that it has matched "bar". This feature is
696    similar to a lookbehind assertion
697    .\" HTML <a href="#lookbehind">
698    .\" </a>
699    (described below).
700    .\"
701    However, in this case, the part of the subject before the real match does not
702    have to be of fixed length, as lookbehind assertions do. The use of \eK does
703    not interfere with the setting of
704    .\" HTML <a href="#subpattern">
705    .\" </a>
706    captured substrings.
707    .\"
708    For example, when the pattern
709    .sp
710      (foo)\eKbar
711    .sp
712    matches "foobar", the first substring is still set to "foo".
713    .
714    .
715  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
716  .SS "Simple assertions"  .SS "Simple assertions"
717  .rs  .rs
# Line 831  alternative in the subpattern. Line 1024  alternative in the subpattern.
1024  .rs  .rs
1025  .sp  .sp
1026  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1027  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
1028  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1029    The option letters are
1030  .sp  .sp
1031    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1032    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 846  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1040  PCRE_MULTILINE while unsetting PCRE_DOTA
1040  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
1041  unset.  unset.
1042  .P  .P
1043  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1044  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1045  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1046  the global options (and it will therefore show up in data extracted by the  .P
1047  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1048    subpattern parentheses), the change applies to the remainder of the pattern
1049    that follows. If the change is placed right at the start of a pattern, PCRE
1050    extracts it into the global options (and it will therefore show up in data
1051    extracted by the \fBpcre_fullinfo()\fP function).
1052  .P  .P
1053  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1054  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
# Line 869  branch is abandoned before the option se Line 1067  branch is abandoned before the option se
1067  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1068  behaviour otherwise.  behaviour otherwise.
1069  .P  .P
1070  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
1071  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
1072  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1073    the application has set or what has been defaulted. Details are given in the
1074    section entitled
1075    .\" HTML <a href="#newlineseq">
1076    .\" </a>
1077    "Newline sequences"
1078    .\"
1079    above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1080    mode; this is equivalent to setting the PCRE_UTF8 option.
1081  .  .
1082  .  .
1083  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 926  is reached, an option setting in one bra Line 1132  is reached, an option setting in one bra
1132  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1133  .  .
1134  .  .
1135    .SH "DUPLICATE SUBPATTERN NUMBERS"
1136    .rs
1137    .sp
1138    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1139    the same numbers for its capturing parentheses. Such a subpattern starts with
1140    (?| and is itself a non-capturing subpattern. For example, consider this
1141    pattern:
1142    .sp
1143      (?|(Sat)ur|(Sun))day
1144    .sp
1145    Because the two alternatives are inside a (?| group, both sets of capturing
1146    parentheses are numbered one. Thus, when the pattern matches, you can look
1147    at captured substring number one, whichever alternative matched. This construct
1148    is useful when you want to capture part, but not all, of one of a number of
1149    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1150    number is reset at the start of each branch. The numbers of any capturing
1151    buffers that follow the subpattern start after the highest number used in any
1152    branch. The following example is taken from the Perl documentation.
1153    The numbers underneath show in which buffer the captured content will be
1154    stored.
1155    .sp
1156      # before  ---------------branch-reset----------- after
1157      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1158      # 1            2         2  3        2     3     4
1159    .sp
1160    A backreference or a recursive call to a numbered subpattern always refers to
1161    the first one in the pattern with the given number.
1162    .P
1163    An alternative approach to using this "branch reset" feature is to use
1164    duplicate named subpatterns, as described in the next section.
1165    .
1166    .
1167  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1168  .rs  .rs
1169  .sp  .sp
# Line 975  abbreviation. This pattern (ignoring the Line 1213  abbreviation. This pattern (ignoring the
1213    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1214  .sp  .sp
1215  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.
1216    (An alternative way of solving this problem is to use a "branch reset"
1217    subpattern, as described in the previous section.)
1218    .P
1219  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1220  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
1221  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
# Line 985  details of the interfaces for handling n Line 1226  details of the interfaces for handling n
1226  \fBpcreapi\fP  \fBpcreapi\fP
1227  .\"  .\"
1228  documentation.  documentation.
1229    .P
1230    \fBWarning:\fP You cannot use different names to distinguish between two
1231    subpatterns with the same number (see the previous section) because PCRE uses
1232    only the numbers when matching.
1233  .  .
1234  .  .
1235  .SH REPETITION  .SH REPETITION
# Line 1033  support is available, \eX{3} matches thr Line 1278  support is available, \eX{3} matches thr
1278  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).
1279  .P  .P
1280  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
1281  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1282    subpatterns that are referenced as
1283    .\" HTML <a href="#subpatternsassubroutines">
1284    .\" </a>
1285    subroutines
1286    .\"
1287    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1288    quantifier are omitted from the compiled pattern.
1289  .P  .P
1290  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1291  abbreviations:  abbreviations:
# Line 1180  previous example can be rewritten as Line 1432  previous example can be rewritten as
1432  .sp  .sp
1433    \ed++foo    \ed++foo
1434  .sp  .sp
1435    Note that a possessive quantifier can be used with an entire group, for
1436    example:
1437    .sp
1438      (abc|xyz){2,3}+
1439    .sp
1440  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1441  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
1442  atomic group. However, there is no difference in the meaning of a possessive  atomic group. However, there is no difference in the meaning of a possessive
# Line 1254  subpattern is possible using named paren Line 1511  subpattern is possible using named paren
1511  .P  .P
1512  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1513  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence, which is a feature introduced in
1514  Perl 5.10. This escape must be followed by a positive or a negative number,  Perl 5.10. This escape must be followed by an unsigned number or a negative
1515  optionally enclosed in braces. These examples are all identical:  number, optionally enclosed in braces. These examples are all identical:
1516  .sp  .sp
1517    (ring), \e1    (ring), \e1
1518    (ring), \eg1    (ring), \eg1
1519    (ring), \eg{1}    (ring), \eg{1}
1520  .sp  .sp
1521  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1522  present in the older syntax. It is also useful when literal digits follow the  is present in the older syntax. It is also useful when literal digits follow
1523  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1524    example:
1525  .sp  .sp
1526    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1527  .sp  .sp
# Line 1293  back reference, the case of letters is r Line 1551  back reference, the case of letters is r
1551  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
1552  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1553  .P  .P
1554  Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'  There are several different ways of writing back references to named
1555  or the Python syntax (?P=name). We could rewrite the above example in either of  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1556    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1557    back reference syntax, in which \eg can be used for both numeric and named
1558    references, is also supported. We could rewrite the above example in any of
1559  the following ways:  the following ways:
1560  .sp  .sp
1561    (?<p1>(?i)rah)\es+\ek<p1>    (?<p1>(?i)rah)\es+\ek<p1>
1562      (?'p1'(?i)rah)\es+\ek{p1}
1563    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1564      (?<p1>(?i)rah)\es+\eg{p1}
1565  .sp  .sp
1566  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
1567  after the reference.  after the reference.
# Line 1411  is permitted, but Line 1674  is permitted, but
1674  .sp  .sp
1675  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1676  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1677  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (5.8 and 5.10), which requires all branches to
1678  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1679  .sp  .sp
1680    (?<=ab(c|de))    (?<=ab(c|de))
1681  .sp  .sp
1682  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1683  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1684    branches:
1685  .sp  .sp
1686    (?<=abc|abde)    (?<=abc|abde)
1687  .sp  .sp
1688    In some cases, the Perl 5.10 escape sequence \eK
1689    .\" HTML <a href="#resetmatchstart">
1690    .\" </a>
1691    (see above)
1692    .\"
1693    can be used instead of a lookbehind assertion to get round the fixed-length
1694    restriction.
1695    .P
1696  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1697  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
1698  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
# Line 1431  to appear in lookbehind assertions, beca Line 1703  to appear in lookbehind assertions, beca
1703  the length of the lookbehind. The \eX and \eR escapes, which can match  the length of the lookbehind. The \eX and \eR escapes, which can match
1704  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1705  .P  .P
1706    .\" HTML <a href="#subpatternsassubroutines">
1707    .\" </a>
1708    "Subroutine"
1709    .\"
1710    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1711    as the subpattern matches a fixed-length string.
1712    .\" HTML <a href="#recursion">
1713    .\" </a>
1714    Recursion,
1715    .\"
1716    however, is not supported.
1717    .P
1718  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1719  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching at the end of the subject string. Consider a simple
1720  pattern such as  pattern such as
# Line 1515  recursion, a pseudo-condition called DEF Line 1799  recursion, a pseudo-condition called DEF
1799  .sp  .sp
1800  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
1801  condition is true if the capturing subpattern of that number has previously  condition is true if the capturing subpattern of that number has previously
1802  matched. An alternative notation is to precede the digits with a plus or minus  matched. An alternative notation is to precede the digits with a plus or minus
1803  sign. In this case, the subpattern number is relative rather than absolute.  sign. In this case, the subpattern number is relative rather than absolute.
1804  The most recently opened parentheses can be referenced by (?(-1), the next most  The most recently opened parentheses can be referenced by (?(-1), the next most
1805  recent by (?(-2), and so on. In looping constructs it can also make sense to  recent by (?(-2), and so on. In looping constructs it can also make sense to
1806  refer to subsequent groups with constructs such as (?(+2).  refer to subsequent groups with constructs such as (?(+2).
1807  .P  .P
# Line 1537  parenthesis is required. Otherwise, sinc Line 1821  parenthesis is required. Otherwise, sinc
1821  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1822  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1823  .P  .P
1824  If you were embedding this pattern in a larger one, you could use a relative  If you were embedding this pattern in a larger one, you could use a relative
1825  reference:  reference:
1826  .sp  .sp
1827    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
# Line 1575  the condition is true if the most recent Line 1859  the condition is true if the most recent
1859  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
1860  stack.  stack.
1861  .P  .P
1862  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
1863  patterns are described below.  .\" HTML <a href="#recursion">
1864    .\" </a>
1865    Recursive patterns
1866    .\"
1867    are described below.
1868  .  .
1869  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
1870  .rs  .rs
# Line 1585  If the condition is the string (DEFINE), Line 1873  If the condition is the string (DEFINE),
1873  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
1874  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
1875  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
1876  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
1877    .\" HTML <a href="#subpatternsassubroutines">
1878    .\" </a>
1879    "subroutines"
1880    .\"
1881  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address could be
1882  written like this (ignore whitespace and line breaks):  written like this (ignore whitespace and line breaks):
1883  .sp  .sp
# Line 1656  recursively to the pattern in which it a Line 1948  recursively to the pattern in which it a
1948  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1949  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
1950  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
1951  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
1952  .P  .P
1953  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
1954  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive call of the subpattern of the given number,
1955  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  provided that it occurs inside that subpattern. (If not, it is a
1956    .\" HTML <a href="#subpatternsassubroutines">
1957    .\" </a>
1958    "subroutine"
1959    .\"
1960  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
1961  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
1962  .P  .P
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 .P  
1963  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1964  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
1965  .sp  .sp
# Line 1685  pattern, so instead you could use this: Line 1976  pattern, so instead you could use this:
1976    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1977  .sp  .sp
1978  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
1979  them instead of the whole pattern.  them instead of the whole pattern.
1980  .P  .P
1981  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1982  is made easier by the use of relative references. (A Perl 5.10 feature.)  is made easier by the use of relative references. (A Perl 5.10 feature.)
# Line 1697  it is encountered. Line 1988  it is encountered.
1988  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
1989  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
1990  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
1991  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
1992    .\" </a>
1993    "subroutine"
1994    .\"
1995    calls, as described in the next section.
1996  .P  .P
1997  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
1998  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
# Line 1706  could rewrite the above example as follo Line 2001  could rewrite the above example as follo
2001    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
2002  .sp  .sp
2003  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
2004  used.  used.
2005  .P  .P
2006  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2007  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of atomic grouping for matching strings of
# Line 1756  different alternatives for the recursive Line 2051  different alternatives for the recursive
2051  is the actual recursive call.  is the actual recursive call.
2052  .  .
2053  .  .
2054    .\" HTML <a name="recursiondifference"></a>
2055    .SS "Recursion difference from Perl"
2056    .rs
2057    .sp
2058    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2059    treated as an atomic group. That is, once it has matched some of the subject
2060    string, it is never re-entered, even if it contains untried alternatives and
2061    there is a subsequent matching failure. This can be illustrated by the
2062    following pattern, which purports to match a palindromic string that contains
2063    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2064    .sp
2065      ^(.|(.)(?1)\e2)$
2066    .sp
2067    The idea is that it either matches a single character, or two identical
2068    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2069    it does not if the pattern is longer than three characters. Consider the
2070    subject string "abcba":
2071    .P
2072    At the top level, the first character is matched, but as it is not at the end
2073    of the string, the first alternative fails; the second alternative is taken
2074    and the recursion kicks in. The recursive call to subpattern 1 successfully
2075    matches the next character ("b"). (Note that the beginning and end of line
2076    tests are not part of the recursion).
2077    .P
2078    Back at the top level, the next character ("c") is compared with what
2079    subpattern 2 matched, which was "a". This fails. Because the recursion is
2080    treated as an atomic group, there are now no backtracking points, and so the
2081    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2082    try the second alternative.) However, if the pattern is written with the
2083    alternatives in the other order, things are different:
2084    .sp
2085      ^((.)(?1)\e2|.)$
2086    .sp
2087    This time, the recursing alternative is tried first, and continues to recurse
2088    until it runs out of characters, at which point the recursion fails. But this
2089    time we do have another alternative to try at the higher level. That is the big
2090    difference: in the previous case the remaining alternative is at a deeper
2091    recursion level, which PCRE cannot use.
2092    .P
2093    To change the pattern so that matches all palindromic strings, not just those
2094    with an odd number of characters, it is tempting to change the pattern to this:
2095    .sp
2096      ^((.)(?1)\e2|.?)$
2097    .sp
2098    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2099    deeper recursion has matched a single character, it cannot be entered again in
2100    order to match an empty string. The solution is to separate the two cases, and
2101    write out the odd and even cases as alternatives at the higher level:
2102    .sp
2103      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2104    .sp
2105    If you want to match typical palindromic phrases, the pattern has to ignore all
2106    non-word characters, which can be done like this:
2107    .sp
2108      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$
2109    .sp
2110    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2111    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2112    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2113    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2114    more) to match typical phrases, and Perl takes so long that you think it has
2115    gone into a loop.
2116    .
2117    .
2118  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2119  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2120  .rs  .rs
# Line 1768  relative, as in these examples: Line 2127  relative, as in these examples:
2127  .sp  .sp
2128    (...(absolute)...)...(?2)...    (...(absolute)...)...(?2)...
2129    (...(relative)...)...(?-1)...    (...(relative)...)...(?-1)...
2130    (...(?+1)...(relative)...    (...(?+1)...(relative)...
2131  .sp  .sp
2132  An earlier example pointed out that the pattern  An earlier example pointed out that the pattern
2133  .sp  .sp
# Line 1797  It matches "abcabc". It does not match " Line 2156  It matches "abcabc". It does not match "
2156  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2157  .  .
2158  .  .
2159    .\" HTML <a name="onigurumasubroutines"></a>
2160    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2161    .rs
2162    .sp
2163    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2164    a number enclosed either in angle brackets or single quotes, is an alternative
2165    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2166    are two of the examples used above, rewritten using this syntax:
2167    .sp
2168      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2169      (sens|respons)e and \eg'1'ibility
2170    .sp
2171    PCRE supports an extension to Oniguruma: if a number is preceded by a
2172    plus or a minus sign it is taken as a relative reference. For example:
2173    .sp
2174      (abc)(?i:\eg<-1>)
2175    .sp
2176    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2177    synonymous. The former is a back reference; the latter is a subroutine call.
2178    .
2179    .
2180  .SH CALLOUTS  .SH CALLOUTS
2181  .rs  .rs
2182  .sp  .sp
# Line 1833  description of the interface to the call Line 2213  description of the interface to the call
2213  documentation.  documentation.
2214  .  .
2215  .  .
2216    .SH "BACKTRACKING CONTROL"
2217    .rs
2218    .sp
2219    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2220    are described in the Perl documentation as "experimental and subject to change
2221    or removal in a future version of Perl". It goes on to say: "Their usage in
2222    production code should be noted to avoid problems during upgrades." The same
2223    remarks apply to the PCRE features described in this section.
2224    .P
2225    Since these verbs are specifically related to backtracking, most of them can be
2226    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2227    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2228    failing negative assertion, they cause an error if encountered by
2229    \fBpcre_dfa_exec()\fP.
2230    .P
2231    If any of these verbs are used in an assertion subpattern, their effect is
2232    confined to that subpattern; it does not extend to the surrounding pattern.
2233    Note that assertion subpatterns are processed as anchored at the point where
2234    they are tested.
2235    .P
2236    The new verbs make use of what was previously invalid syntax: an opening
2237    parenthesis followed by an asterisk. In Perl, they are generally of the form
2238    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2239    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2240    are two kinds:
2241    .
2242    .SS "Verbs that act immediately"
2243    .rs
2244    .sp
2245    The following verbs act as soon as they are encountered:
2246    .sp
2247       (*ACCEPT)
2248    .sp
2249    This verb causes the match to end successfully, skipping the remainder of the
2250    pattern. When inside a recursion, only the innermost pattern is ended
2251    immediately. If the (*ACCEPT) is inside capturing parentheses, the data so far
2252    is captured. (This feature was added to PCRE at release 8.00.) For example:
2253    .sp
2254      A((?:A|B(*ACCEPT)|C)D)
2255    .sp
2256    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2257    the outer parentheses.
2258    .sp
2259      (*FAIL) or (*F)
2260    .sp
2261    This verb causes the match to fail, forcing backtracking to occur. It is
2262    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2263    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2264    Perl features that are not present in PCRE. The nearest equivalent is the
2265    callout feature, as for example in this pattern:
2266    .sp
2267      a+(?C)(*FAIL)
2268    .sp
2269    A match with the string "aaaa" always fails, but the callout is taken before
2270    each backtrack happens (in this example, 10 times).
2271    .
2272    .SS "Verbs that act after backtracking"
2273    .rs
2274    .sp
2275    The following verbs do nothing when they are encountered. Matching continues
2276    with what follows, but if there is no subsequent match, a failure is forced.
2277    The verbs differ in exactly what kind of failure occurs.
2278    .sp
2279      (*COMMIT)
2280    .sp
2281    This verb causes the whole match to fail outright if the rest of the pattern
2282    does not match. Even if the pattern is unanchored, no further attempts to find
2283    a match by advancing the start point take place. Once (*COMMIT) has been
2284    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2285    starting point, or not at all. For example:
2286    .sp
2287      a+(*COMMIT)b
2288    .sp
2289    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2290    dynamic anchor, or "I've started, so I must finish."
2291    .sp
2292      (*PRUNE)
2293    .sp
2294    This verb causes the match to fail at the current position if the rest of the
2295    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2296    advance to the next starting character then happens. Backtracking can occur as
2297    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2298    if there is no match to the right, backtracking cannot cross (*PRUNE).
2299    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2300    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2301    be expressed in any other way.
2302    .sp
2303      (*SKIP)
2304    .sp
2305    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2306    "bumpalong" advance is not to the next character, but to the position in the
2307    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2308    was matched leading up to it cannot be part of a successful match. Consider:
2309    .sp
2310      a+(*SKIP)b
2311    .sp
2312    If the subject is "aaaac...", after the first match attempt fails (starting at
2313    the first character in the string), the starting point skips on to start the
2314    next attempt at "c". Note that a possessive quantifer does not have the same
2315    effect in this example; although it would suppress backtracking during the
2316    first match attempt, the second attempt would start at the second character
2317    instead of skipping on to "c".
2318    .sp
2319      (*THEN)
2320    .sp
2321    This verb causes a skip to the next alternation if the rest of the pattern does
2322    not match. That is, it cancels pending backtracking, but only within the
2323    current alternation. Its name comes from the observation that it can be used
2324    for a pattern-based if-then-else block:
2325    .sp
2326      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2327    .sp
2328    If the COND1 pattern matches, FOO is tried (and possibly further items after
2329    the end of the group if FOO succeeds); on failure the matcher skips to the
2330    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2331    is used outside of any alternation, it acts exactly like (*PRUNE).
2332    .
2333    .
2334  .SH "SEE ALSO"  .SH "SEE ALSO"
2335  .rs  .rs
2336  .sp  .sp
# Line 1853  Cambridge CB2 3QH, England. Line 2351  Cambridge CB2 3QH, England.
2351  .rs  .rs
2352  .sp  .sp
2353  .nf  .nf
2354  Last updated: 09 May 2007  Last updated: 22 September 2009
2355  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2009 University of Cambridge.
2356  .fi  .fi

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