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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. 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  .\"  .\"
# Line 162  Thus \ecz becomes hex 1A, but \ec{ becom Line 168  Thus \ecz becomes hex 1A, but \ec{ becom
168  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
169  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{
170  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
171  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
172  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
173  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
174  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
175  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
176    there is no terminating }, this form of escape is not recognized. Instead, the
177    initial \ex will be interpreted as a basic hexadecimal escape, with no
178    following digits, giving a character whose value is zero.
179  .P  .P
180  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
181  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 247  meanings
247  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
248  .rs  .rs
249  .sp  .sp
250  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
251  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
252  discussed  reference can be coded as \eg{name}. Back references are discussed
253  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
254  .\" </a>  .\" </a>
255  later,  later,
# Line 260  following are always recognized: Line 269  following are always recognized:
269  .sp  .sp
270    \ed     any decimal digit    \ed     any decimal digit
271    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
272      \eh     any horizontal whitespace character
273      \eH     any character that is not a horizontal whitespace character
274    \es     any whitespace character    \es     any whitespace character
275    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
276      \ev     any vertical whitespace character
277      \eV     any character that is not a vertical whitespace character
278    \ew     any "word" character    \ew     any "word" character
279    \eW     any "non-word" character    \eW     any "non-word" character
280  .sp  .sp
# Line 275  there is no character to match. Line 288  there is no character to match.
288  .P  .P
289  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).
290  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
291  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
292  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
293  does.)  does.
294    .P
295    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
296    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
297    character property support is available. These sequences retain their original
298    meanings from before UTF-8 support was available, mainly for efficiency
299    reasons.
300    .P
301    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
302    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
303    The horizontal space characters are:
304    .sp
305      U+0009     Horizontal tab
306      U+0020     Space
307      U+00A0     Non-break space
308      U+1680     Ogham space mark
309      U+180E     Mongolian vowel separator
310      U+2000     En quad
311      U+2001     Em quad
312      U+2002     En space
313      U+2003     Em space
314      U+2004     Three-per-em space
315      U+2005     Four-per-em space
316      U+2006     Six-per-em space
317      U+2007     Figure space
318      U+2008     Punctuation space
319      U+2009     Thin space
320      U+200A     Hair space
321      U+202F     Narrow no-break space
322      U+205F     Medium mathematical space
323      U+3000     Ideographic space
324    .sp
325    The vertical space characters are:
326    .sp
327      U+000A     Linefeed
328      U+000B     Vertical tab
329      U+000C     Formfeed
330      U+000D     Carriage return
331      U+0085     Next line
332      U+2028     Line separator
333      U+2029     Paragraph separator
334  .P  .P
335  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
336  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 346  in the
346  .\"  .\"
347  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,
348  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
349  accented letters, and these are matched by \ew.  accented letters, and these are matched by \ew. The use of locales with Unicode
350  .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.  
351  .  .
352  .  .
353  .SS "Newline sequences"  .SS "Newline sequences"
354  .rs  .rs
355  .sp  .sp
356  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, the escape sequence \eR matches any Unicode newline
357  sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to  sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
358  the following:  the following:
359  .sp  .sp
360    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
# Line 334  Inside a character class, \eR matches th Line 383  Inside a character class, \eR matches th
383  .rs  .rs
384  .sp  .sp
385  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
386  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
387  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
388    characters whose codepoints are less than 256, but they do work in this mode.
389    The extra escape sequences are:
390  .sp  .sp
391    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
392    \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 538  The special property L& is also supporte
538  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
539  a modifier or "other".  a modifier or "other".
540  .P  .P
541    The Cs (Surrogate) property applies only to characters in the range U+D800 to
542    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
543    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
544    (see the discussion of PCRE_NO_UTF8_CHECK in the
545    .\" HREF
546    \fBpcreapi\fP
547    .\"
548    page).
549    .P
550  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})
551  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
552  properties with "Is".  properties with "Is".
# Line 511  atomic group Line 571  atomic group
571  (see below).  (see below).
572  .\"  .\"
573  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
574  preceding character.  preceding character. None of them have codepoints less than 256, so in
575    non-UTF-8 mode \eX matches any one character.
576  .P  .P
577  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
578  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 580  why the traditional escape sequences suc
580  properties in PCRE.  properties in PCRE.
581  .  .
582  .  .
583    .\" HTML <a name="resetmatchstart"></a>
584    .SS "Resetting the match start"
585    .rs
586    .sp
587    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
588    matched characters not to be included in the final matched sequence. For
589    example, the pattern:
590    .sp
591      foo\eKbar
592    .sp
593    matches "foobar", but reports that it has matched "bar". This feature is
594    similar to a lookbehind assertion
595    .\" HTML <a href="#lookbehind">
596    .\" </a>
597    (described below).
598    .\"
599    However, in this case, the part of the subject before the real match does not
600    have to be of fixed length, as lookbehind assertions do. The use of \eK does
601    not interfere with the setting of
602    .\" HTML <a href="#subpattern">
603    .\" </a>
604    captured substrings.
605    .\"
606    For example, when the pattern
607    .sp
608      (foo)\eKbar
609    .sp
610    matches "foobar", the first substring is still set to "foo".
611    .
612    .
613  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
614  .SS "Simple assertions"  .SS "Simple assertions"
615  .rs  .rs
# Line 926  is reached, an option setting in one bra Line 1017  is reached, an option setting in one bra
1017  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1018  .  .
1019  .  .
1020    .SH "DUPLICATE SUBPATTERN NUMBERS"
1021    .rs
1022    .sp
1023    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1024    the same numbers for its capturing parentheses. Such a subpattern starts with
1025    (?| and is itself a non-capturing subpattern. For example, consider this
1026    pattern:
1027    .sp
1028      (?|(Sat)ur|(Sun))day
1029    .sp
1030    Because the two alternatives are inside a (?| group, both sets of capturing
1031    parentheses are numbered one. Thus, when the pattern matches, you can look
1032    at captured substring number one, whichever alternative matched. This construct
1033    is useful when you want to capture part, but not all, of one of a number of
1034    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1035    number is reset at the start of each branch. The numbers of any capturing
1036    buffers that follow the subpattern start after the highest number used in any
1037    branch. The following example is taken from the Perl documentation.
1038    The numbers underneath show in which buffer the captured content will be
1039    stored.
1040    .sp
1041      # before  ---------------branch-reset----------- after
1042      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1043      # 1            2         2  3        2     3     4
1044    .sp
1045    A backreference or a recursive call to a numbered subpattern always refers to
1046    the first one in the pattern with the given number.
1047    .P
1048    An alternative approach to using this "branch reset" feature is to use
1049    duplicate named subpatterns, as described in the next section.
1050    .
1051    .
1052  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1053  .rs  .rs
1054  .sp  .sp
# Line 975  abbreviation. This pattern (ignoring the Line 1098  abbreviation. This pattern (ignoring the
1098    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1099  .sp  .sp
1100  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.
1101    (An alternative way of solving this problem is to use a "branch reset"
1102    subpattern, as described in the previous section.)
1103    .P
1104  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1105  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
1106  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 1180  previous example can be rewritten as Line 1306  previous example can be rewritten as
1306  .sp  .sp
1307    \ed++foo    \ed++foo
1308  .sp  .sp
1309    Note that a possessive quantifier can be used with an entire group, for
1310    example:
1311    .sp
1312      (abc|xyz){2,3}+
1313    .sp
1314  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1315  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
1316  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 1385  subpattern is possible using named paren
1385  .P  .P
1386  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
1387  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
1388  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
1389  optionally enclosed in braces. These examples are all identical:  number, optionally enclosed in braces. These examples are all identical:
1390  .sp  .sp
1391    (ring), \e1    (ring), \e1
1392    (ring), \eg1    (ring), \eg1
1393    (ring), \eg{1}    (ring), \eg{1}
1394  .sp  .sp
1395  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1396  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
1397  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1398    example:
1399  .sp  .sp
1400    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1401  .sp  .sp
# Line 1293  back reference, the case of letters is r Line 1425  back reference, the case of letters is r
1425  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
1426  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1427  .P  .P
1428  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
1429  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
1430    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1431    back reference syntax, in which \eg can be used for both numeric and named
1432    references, is also supported. We could rewrite the above example in any of
1433  the following ways:  the following ways:
1434  .sp  .sp
1435    (?<p1>(?i)rah)\es+\ek<p1>    (?<p1>(?i)rah)\es+\ek<p1>
1436      (?'p1'(?i)rah)\es+\ek{p1}
1437    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1438      (?<p1>(?i)rah)\es+\eg{p1}
1439  .sp  .sp
1440  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
1441  after the reference.  after the reference.
# Line 1421  lengths, but it is acceptable if rewritt Line 1558  lengths, but it is acceptable if rewritt
1558  .sp  .sp
1559    (?<=abc|abde)    (?<=abc|abde)
1560  .sp  .sp
1561    In some cases, the Perl 5.10 escape sequence \eK
1562    .\" HTML <a href="#resetmatchstart">
1563    .\" </a>
1564    (see above)
1565    .\"
1566    can be used instead of a lookbehind assertion; this is not restricted to a
1567    fixed-length.
1568    .P
1569  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1570  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
1571  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
# Line 1515  recursion, a pseudo-condition called DEF Line 1660  recursion, a pseudo-condition called DEF
1660  .sp  .sp
1661  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
1662  condition is true if the capturing subpattern of that number has previously  condition is true if the capturing subpattern of that number has previously
1663  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
1664  sign. In this case, the subpattern number is relative rather than absolute.  sign. In this case, the subpattern number is relative rather than absolute.
1665  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
1666  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
1667  refer to subsequent groups with constructs such as (?(+2).  refer to subsequent groups with constructs such as (?(+2).
1668  .P  .P
# Line 1537  parenthesis is required. Otherwise, sinc Line 1682  parenthesis is required. Otherwise, sinc
1682  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1683  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1684  .P  .P
1685  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
1686  reference:  reference:
1687  .sp  .sp
1688    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
# Line 1685  pattern, so instead you could use this: Line 1830  pattern, so instead you could use this:
1830    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1831  .sp  .sp
1832  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
1833  them instead of the whole pattern.  them instead of the whole pattern.
1834  .P  .P
1835  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
1836  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 1706  could rewrite the above example as follo Line 1851  could rewrite the above example as follo
1851    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1852  .sp  .sp
1853  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
1854  used.  used.
1855  .P  .P
1856  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
1857  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 1768  relative, as in these examples: Line 1913  relative, as in these examples:
1913  .sp  .sp
1914    (...(absolute)...)...(?2)...    (...(absolute)...)...(?2)...
1915    (...(relative)...)...(?-1)...    (...(relative)...)...(?-1)...
1916    (...(?+1)...(relative)...    (...(?+1)...(relative)...
1917  .sp  .sp
1918  An earlier example pointed out that the pattern  An earlier example pointed out that the pattern
1919  .sp  .sp
# Line 1833  description of the interface to the call Line 1978  description of the interface to the call
1978  documentation.  documentation.
1979  .  .
1980  .  .
1981    .SH "BACTRACKING CONTROL"
1982    .rs
1983    .sp
1984    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
1985    are described in the Perl documentation as "experimental and subject to change
1986    or removal in a future version of Perl". It goes on to say: "Their usage in
1987    production code should be noted to avoid problems during upgrades." The same
1988    remarks apply to the PCRE features described in this section.
1989    .P
1990    Since these verbs are specifically related to backtracking, they can be used
1991    only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
1992    backtracking algorithm. They cause an error if encountered by
1993    \fBpcre_dfa_exec()\fP.
1994    .P
1995    The new verbs make use of what was previously invalid syntax: an opening
1996    parenthesis followed by an asterisk. In Perl, they are generally of the form
1997    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
1998    form is just (*VERB). Any number of these verbs may occur in a pattern. There
1999    are two kinds:
2000    .
2001    .SS "Verbs that act immediately"
2002    .rs
2003    .sp
2004    The following verbs act as soon as they are encountered:
2005    .sp
2006       (*ACCEPT)
2007    .sp
2008    This verb causes the match to end successfully, skipping the remainder of the
2009    pattern. When inside a recursion, only the innermost pattern is ended
2010    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2011    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2012    captured. For example:
2013    .sp
2014      A(A|B(*ACCEPT)|C)D
2015    .sp
2016    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2017    captured.
2018    .sp
2019      (*FAIL) or (*F)
2020    .sp
2021    This verb causes the match to fail, forcing backtracking to occur. It is
2022    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2023    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2024    Perl features that are not present in PCRE. The nearest equivalent is the
2025    callout feature, as for example in this pattern:
2026    .sp
2027      a+(?C)(*FAIL)
2028    .sp
2029    A match with the string "aaaa" always fails, but the callout is taken before
2030    each backtrack happens (in this example, 10 times).
2031    .
2032    .SS "Verbs that act after backtracking"
2033    .rs
2034    .sp
2035    The following verbs do nothing when they are encountered. Matching continues
2036    with what follows, but if there is no subsequent match, a failure is forced.
2037    The verbs differ in exactly what kind of failure occurs.
2038    .sp
2039      (*COMMIT)
2040    .sp
2041    This verb causes the whole match to fail outright if the rest of the pattern
2042    does not match. Even if the pattern is unanchored, no further attempts to find
2043    a match by advancing the start point take place. Once (*COMMIT) has been
2044    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2045    starting point, or not at all. For example:
2046    .sp
2047      a+(*COMMIT)b
2048    .sp
2049    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2050    dynamic anchor, or "I've started, so I must finish."
2051    .sp
2052      (*PRUNE)
2053    .sp
2054    This verb causes the match to fail at the current position if the rest of the
2055    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2056    advance to the next starting character then happens. Backtracking can occur as
2057    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2058    if there is no match to the right, backtracking cannot cross (*PRUNE).
2059    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2060    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2061    be expressed in any other way.
2062    .sp
2063      (*SKIP)
2064    .sp
2065    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2066    "bumpalong" advance is not to the next character, but to the position in the
2067    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2068    was matched leading up to it cannot be part of a successful match. Consider:
2069    .sp
2070      a+(*SKIP)b
2071    .sp
2072    If the subject is "aaaac...", after the first match attempt fails (starting at
2073    the first character in the string), the starting point skips on to start the
2074    next attempt at "c". Note that a possessive quantifer does not have the same
2075    effect in this example; although it would suppress backtracking during the
2076    first match attempt, the second attempt would start at the second character
2077    instead of skipping on to "c".
2078    .sp
2079      (*THEN)
2080    .sp
2081    This verb causes a skip to the next alternation if the rest of the pattern does
2082    not match. That is, it cancels pending backtracking, but only within the
2083    current alternation. Its name comes from the observation that it can be used
2084    for a pattern-based if-then-else block:
2085    .sp
2086      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2087    .sp
2088    If the COND1 pattern matches, FOO is tried (and possibly further items after
2089    the end of the group if FOO succeeds); on failure the matcher skips to the
2090    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2091    is used outside of any alternation, it acts exactly like (*PRUNE).
2092    .
2093    .
2094  .SH "SEE ALSO"  .SH "SEE ALSO"
2095  .rs  .rs
2096  .sp  .sp
# Line 1853  Cambridge CB2 3QH, England. Line 2111  Cambridge CB2 3QH, England.
2111  .rs  .rs
2112  .sp  .sp
2113  .nf  .nf
2114  Last updated: 09 May 2007  Last updated: 09 August 2007
2115  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2007 University of Cambridge.
2116  .fi  .fi

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