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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
# Line 30  The remainder of this document discusses Line 39  The remainder of this document discusses
39  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
40  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
41  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
42  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
43  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
44    alternative function, and how it differs from the normal function, are
45    discussed in the
46  .\" HREF  .\" HREF
47  \fBpcrematching\fP  \fBpcrematching\fP
48  .\"  .\"
49  page.  page.
50  .  .
51  .  .
52    .SH "NEWLINE CONVENTIONS"
53    .rs
54    .sp
55    PCRE supports five different conventions for indicating line breaks in
56    strings: a single CR (carriage return) character, a single LF (linefeed)
57    character, the two-character sequence CRLF, any of the three preceding, or any
58    Unicode newline sequence. The
59    .\" HREF
60    \fBpcreapi\fP
61    .\"
62    page has
63    .\" HTML <a href="pcreapi.html#newlines">
64    .\" </a>
65    further discussion
66    .\"
67    about newlines, and shows how to set the newline convention in the
68    \fIoptions\fP arguments for the compiling and matching functions.
69    .P
70    It is also possible to specify a newline convention by starting a pattern
71    string with one of the following five sequences:
72    .sp
73      (*CR)        carriage return
74      (*LF)        linefeed
75      (*CRLF)      carriage return, followed by linefeed
76      (*ANYCRLF)   any of the three above
77      (*ANY)       all Unicode newline sequences
78    .sp
79    These override the default and the options given to \fBpcre_compile()\fP. For
80    example, on a Unix system where LF is the default newline sequence, the pattern
81    .sp
82      (*CR)a.b
83    .sp
84    changes the convention to CR. That pattern matches "a\enb" because LF is no
85    longer a newline. Note that these special settings, which are not
86    Perl-compatible, are recognized only at the very start of a pattern, and that
87    they must be in upper case. If more than one of them is present, the last one
88    is used.
89    .P
90    The newline convention does not affect what the \eR escape sequence matches. By
91    default, this is any Unicode newline sequence, for Perl compatibility. However,
92    this can be changed; see the description of \eR in the section entitled
93    .\" HTML <a href="#newlineseq">
94    .\" </a>
95    "Newline sequences"
96    .\"
97    below. A change of \eR setting can be combined with a change of newline
98    convention.
99    .
100    .
101  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
102  .rs  .rs
103  .sp  .sp
# Line 147  represents: Line 207  represents:
207    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
208    \ee        escape (hex 1B)    \ee        escape (hex 1B)
209    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
210    \en        newline (hex 0A)    \en        linefeed (hex 0A)
211    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
212    \et        tab (hex 09)    \et        tab (hex 09)
213    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
# Line 162  Thus \ecz becomes hex 1A, but \ec{ becom Line 222  Thus \ecz becomes hex 1A, but \ec{ becom
222  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
223  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
224  and }, but the value of the character code must be less than 256 in non-UTF-8  and }, but the value of the character code must be less than 256 in non-UTF-8
225  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
226  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
227  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
228  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
229  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
230    there is no terminating }, this form of escape is not recognized. Instead, the
231    initial \ex will be interpreted as a basic hexadecimal escape, with no
232    following digits, giving a character whose value is zero.
233  .P  .P
234  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
235  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex. There is no difference in the way they are handled. For
# Line 238  meanings Line 301  meanings
301  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
302  .rs  .rs
303  .sp  .sp
304  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
305  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
306  discussed  reference can be coded as \eg{name}. Back references are discussed
307  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
308  .\" </a>  .\" </a>
309  later,  later,
# Line 252  parenthesized subpatterns. Line 315  parenthesized subpatterns.
315  .\"  .\"
316  .  .
317  .  .
318    .SS "Absolute and relative subroutine calls"
319    .rs
320    .sp
321    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
322    a number enclosed either in angle brackets or single quotes, is an alternative
323    syntax for referencing a subpattern as a "subroutine". Details are discussed
324    .\" HTML <a href="#onigurumasubroutines">
325    .\" </a>
326    later.
327    .\"
328    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
329    synonymous. The former is a back reference; the latter is a subroutine call.
330    .
331    .
332  .SS "Generic character types"  .SS "Generic character types"
333  .rs  .rs
334  .sp  .sp
# Line 260  following are always recognized: Line 337  following are always recognized:
337  .sp  .sp
338    \ed     any decimal digit    \ed     any decimal digit
339    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
340      \eh     any horizontal whitespace character
341      \eH     any character that is not a horizontal whitespace character
342    \es     any whitespace character    \es     any whitespace character
343    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
344      \ev     any vertical whitespace character
345      \eV     any character that is not a vertical whitespace character
346    \ew     any "word" character    \ew     any "word" character
347    \eW     any "non-word" character    \eW     any "non-word" character
348  .sp  .sp
# Line 275  there is no character to match. Line 356  there is no character to match.
356  .P  .P
357  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
358  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
359  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
360  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
361  does.)  does.
362    .P
363    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
364    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
365    character property support is available. These sequences retain their original
366    meanings from before UTF-8 support was available, mainly for efficiency
367    reasons.
368    .P
369    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
370    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
371    The horizontal space characters are:
372    .sp
373      U+0009     Horizontal tab
374      U+0020     Space
375      U+00A0     Non-break space
376      U+1680     Ogham space mark
377      U+180E     Mongolian vowel separator
378      U+2000     En quad
379      U+2001     Em quad
380      U+2002     En space
381      U+2003     Em space
382      U+2004     Three-per-em space
383      U+2005     Four-per-em space
384      U+2006     Six-per-em space
385      U+2007     Figure space
386      U+2008     Punctuation space
387      U+2009     Thin space
388      U+200A     Hair space
389      U+202F     Narrow no-break space
390      U+205F     Medium mathematical space
391      U+3000     Ideographic space
392    .sp
393    The vertical space characters are:
394    .sp
395      U+000A     Linefeed
396      U+000B     Vertical tab
397      U+000C     Formfeed
398      U+000D     Carriage return
399      U+0085     Next line
400      U+2028     Line separator
401      U+2029     Paragraph separator
402  .P  .P
403  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character less than 256 that is a
404  letter or digit. The definition of letters and digits is controlled by PCRE's  letter or digit. The definition of letters and digits is controlled by PCRE's
# Line 293  in the Line 414  in the
414  .\"  .\"
415  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,
416  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
417  accented letters, and these are matched by \ew.  accented letters, and these are matched by \ew. The use of locales with Unicode
418  .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.  
419  .  .
420  .  .
421    .\" HTML <a name="newlineseq"></a>
422  .SS "Newline sequences"  .SS "Newline sequences"
423  .rs  .rs
424  .sp  .sp
425  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
426  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
427  the following:  equivalent to the following:
428  .sp  .sp
429    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
430  .sp  .sp
# Line 326  are added: LS (line separator, U+2028) a Line 444  are added: LS (line separator, U+2028) a
444  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
445  recognized.  recognized.
446  .P  .P
447    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
448    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
449    either at compile time or when the pattern is matched. (BSR is an abbrevation
450    for "backslash R".) This can be made the default when PCRE is built; if this is
451    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
452    It is also possible to specify these settings by starting a pattern string with
453    one of the following sequences:
454    .sp
455      (*BSR_ANYCRLF)   CR, LF, or CRLF only
456      (*BSR_UNICODE)   any Unicode newline sequence
457    .sp
458    These override the default and the options given to \fBpcre_compile()\fP, but
459    they can be overridden by options given to \fBpcre_exec()\fP. Note that these
460    special settings, which are not Perl-compatible, are recognized only at the
461    very start of a pattern, and that they must be in upper case. If more than one
462    of them is present, the last one is used. They can be combined with a change of
463    newline convention, for example, a pattern can start with:
464    .sp
465      (*ANY)(*BSR_ANYCRLF)
466    .sp
467  Inside a character class, \eR matches the letter "R".  Inside a character class, \eR matches the letter "R".
468  .  .
469  .  .
# Line 334  Inside a character class, \eR matches th Line 472  Inside a character class, \eR matches th
472  .rs  .rs
473  .sp  .sp
474  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
475  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
476  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
477    characters whose codepoints are less than 256, but they do work in this mode.
478    The extra escape sequences are:
479  .sp  .sp
480    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
481    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
# Line 487  The special property L& is also supporte Line 627  The special property L& is also supporte
627  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
628  a modifier or "other".  a modifier or "other".
629  .P  .P
630    The Cs (Surrogate) property applies only to characters in the range U+D800 to
631    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
632    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
633    (see the discussion of PCRE_NO_UTF8_CHECK in the
634    .\" HREF
635    \fBpcreapi\fP
636    .\"
637    page).
638    .P
639  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for these properties that Perl supports (such as \ep{Letter})
640  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
641  properties with "Is".  properties with "Is".
# Line 511  atomic group Line 660  atomic group
660  (see below).  (see below).
661  .\"  .\"
662  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
663  preceding character.  preceding character. None of them have codepoints less than 256, so in
664    non-UTF-8 mode \eX matches any one character.
665  .P  .P
666  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
667  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
# Line 519  why the traditional escape sequences suc Line 669  why the traditional escape sequences suc
669  properties in PCRE.  properties in PCRE.
670  .  .
671  .  .
672    .\" HTML <a name="resetmatchstart"></a>
673    .SS "Resetting the match start"
674    .rs
675    .sp
676    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
677    matched characters not to be included in the final matched sequence. For
678    example, the pattern:
679    .sp
680      foo\eKbar
681    .sp
682    matches "foobar", but reports that it has matched "bar". This feature is
683    similar to a lookbehind assertion
684    .\" HTML <a href="#lookbehind">
685    .\" </a>
686    (described below).
687    .\"
688    However, in this case, the part of the subject before the real match does not
689    have to be of fixed length, as lookbehind assertions do. The use of \eK does
690    not interfere with the setting of
691    .\" HTML <a href="#subpattern">
692    .\" </a>
693    captured substrings.
694    .\"
695    For example, when the pattern
696    .sp
697      (foo)\eKbar
698    .sp
699    matches "foobar", the first substring is still set to "foo".
700    .
701    .
702  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
703  .SS "Simple assertions"  .SS "Simple assertions"
704  .rs  .rs
# Line 831  alternative in the subpattern. Line 1011  alternative in the subpattern.
1011  .rs  .rs
1012  .sp  .sp
1013  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1014  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1015  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1016    The option letters are
1017  .sp  .sp
1018    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1019    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 846  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1027  PCRE_MULTILINE while unsetting PCRE_DOTA
1027  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
1028  unset.  unset.
1029  .P  .P
1030    The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1031    changed in the same way as the Perl-compatible options by using the characters
1032    J, U and X respectively.
1033    .P
1034  When an option change occurs at top level (that is, not inside subpattern  When an option change occurs at top level (that is, not inside subpattern
1035  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
1036  If the change is placed right at the start of a pattern, PCRE extracts it into  If the change is placed right at the start of a pattern, PCRE extracts it into
# Line 869  branch is abandoned before the option se Line 1054  branch is abandoned before the option se
1054  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1055  behaviour otherwise.  behaviour otherwise.
1056  .P  .P
1057  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  \fBNote:\fP There are other PCRE-specific options that can be set by the
1058  changed in the same way as the Perl-compatible options by using the characters  application when the compile or match functions are called. In some cases the
1059  J, U and X respectively.  pattern can contain special leading sequences to override what the application
1060    has set or what has been defaulted. Details are given in the section entitled
1061    .\" HTML <a href="#newlineseq">
1062    .\" </a>
1063    "Newline sequences"
1064    .\"
1065    above.
1066  .  .
1067  .  .
1068  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 926  is reached, an option setting in one bra Line 1117  is reached, an option setting in one bra
1117  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1118  .  .
1119  .  .
1120    .SH "DUPLICATE SUBPATTERN NUMBERS"
1121    .rs
1122    .sp
1123    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1124    the same numbers for its capturing parentheses. Such a subpattern starts with
1125    (?| and is itself a non-capturing subpattern. For example, consider this
1126    pattern:
1127    .sp
1128      (?|(Sat)ur|(Sun))day
1129    .sp
1130    Because the two alternatives are inside a (?| group, both sets of capturing
1131    parentheses are numbered one. Thus, when the pattern matches, you can look
1132    at captured substring number one, whichever alternative matched. This construct
1133    is useful when you want to capture part, but not all, of one of a number of
1134    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1135    number is reset at the start of each branch. The numbers of any capturing
1136    buffers that follow the subpattern start after the highest number used in any
1137    branch. The following example is taken from the Perl documentation.
1138    The numbers underneath show in which buffer the captured content will be
1139    stored.
1140    .sp
1141      # before  ---------------branch-reset----------- after
1142      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1143      # 1            2         2  3        2     3     4
1144    .sp
1145    A backreference or a recursive call to a numbered subpattern always refers to
1146    the first one in the pattern with the given number.
1147    .P
1148    An alternative approach to using this "branch reset" feature is to use
1149    duplicate named subpatterns, as described in the next section.
1150    .
1151    .
1152  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1153  .rs  .rs
1154  .sp  .sp
# Line 975  abbreviation. This pattern (ignoring the Line 1198  abbreviation. This pattern (ignoring the
1198    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1199  .sp  .sp
1200  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1201    (An alternative way of solving this problem is to use a "branch reset"
1202    subpattern, as described in the previous section.)
1203    .P
1204  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1205  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1206  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was. If you
# Line 1033  support is available, \eX{3} matches thr Line 1259  support is available, \eX{3} matches thr
1259  which may be several bytes long (and they may be of different lengths).  which may be several bytes long (and they may be of different lengths).
1260  .P  .P
1261  The quantifier {0} is permitted, causing the expression to behave as if the  The quantifier {0} is permitted, causing the expression to behave as if the
1262  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1263    subpatterns that are referenced as
1264    .\" HTML <a href="#subpatternsassubroutines">
1265    .\" </a>
1266    subroutines
1267    .\"
1268    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1269    quantifier are omitted from the compiled pattern.
1270  .P  .P
1271  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1272  abbreviations:  abbreviations:
# Line 1180  previous example can be rewritten as Line 1413  previous example can be rewritten as
1413  .sp  .sp
1414    \ed++foo    \ed++foo
1415  .sp  .sp
1416    Note that a possessive quantifier can be used with an entire group, for
1417    example:
1418    .sp
1419      (abc|xyz){2,3}+
1420    .sp
1421  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1422  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1423  atomic group. However, there is no difference in the meaning 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 1492  subpattern is possible using named paren
1492  .P  .P
1493  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
1494  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
1495  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
1496  optionally enclosed in braces. These examples are all identical:  number, optionally enclosed in braces. These examples are all identical:
1497  .sp  .sp
1498    (ring), \e1    (ring), \e1
1499    (ring), \eg1    (ring), \eg1
1500    (ring), \eg{1}    (ring), \eg{1}
1501  .sp  .sp
1502  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1503  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
1504  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1505    example:
1506  .sp  .sp
1507    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1508  .sp  .sp
# Line 1293  back reference, the case of letters is r Line 1532  back reference, the case of letters is r
1532  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1533  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1534  .P  .P
1535  Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'  There are several different ways of writing back references to named
1536  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
1537    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1538    back reference syntax, in which \eg can be used for both numeric and named
1539    references, is also supported. We could rewrite the above example in any of
1540  the following ways:  the following ways:
1541  .sp  .sp
1542    (?<p1>(?i)rah)\es+\ek<p1>    (?<p1>(?i)rah)\es+\ek<p1>
1543      (?'p1'(?i)rah)\es+\ek{p1}
1544    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1545      (?<p1>(?i)rah)\es+\eg{p1}
1546  .sp  .sp
1547  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
1548  after the reference.  after the reference.
# Line 1421  lengths, but it is acceptable if rewritt Line 1665  lengths, but it is acceptable if rewritt
1665  .sp  .sp
1666    (?<=abc|abde)    (?<=abc|abde)
1667  .sp  .sp
1668    In some cases, the Perl 5.10 escape sequence \eK
1669    .\" HTML <a href="#resetmatchstart">
1670    .\" </a>
1671    (see above)
1672    .\"
1673    can be used instead of a lookbehind assertion; this is not restricted to a
1674    fixed-length.
1675    .P
1676  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1677  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
1678  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
# Line 1515  recursion, a pseudo-condition called DEF Line 1767  recursion, a pseudo-condition called DEF
1767  .sp  .sp
1768  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
1769  condition is true if the capturing subpattern of that number has previously  condition is true if the capturing subpattern of that number has previously
1770  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
1771  sign. In this case, the subpattern number is relative rather than absolute.  sign. In this case, the subpattern number is relative rather than absolute.
1772  The most recently opened parentheses can be referenced by (?(-1), the next most  The most recently opened parentheses can be referenced by (?(-1), the next most
1773  recent by (?(-2), and so on. In looping constructs it can also make sense to  recent by (?(-2), and so on. In looping constructs it can also make sense to
1774  refer to subsequent groups with constructs such as (?(+2).  refer to subsequent groups with constructs such as (?(+2).
1775  .P  .P
# Line 1537  parenthesis is required. Otherwise, sinc Line 1789  parenthesis is required. Otherwise, sinc
1789  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1790  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1791  .P  .P
1792  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
1793  reference:  reference:
1794  .sp  .sp
1795    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
# Line 1685  pattern, so instead you could use this: Line 1937  pattern, so instead you could use this:
1937    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1938  .sp  .sp
1939  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
1940  them instead of the whole pattern.  them instead of the whole pattern.
1941  .P  .P
1942  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
1943  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 1958  could rewrite the above example as follo
1958    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1959  .sp  .sp
1960  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
1961  used.  used.
1962  .P  .P
1963  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
1964  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of atomic grouping for matching strings of
# Line 1768  relative, as in these examples: Line 2020  relative, as in these examples:
2020  .sp  .sp
2021    (...(absolute)...)...(?2)...    (...(absolute)...)...(?2)...
2022    (...(relative)...)...(?-1)...    (...(relative)...)...(?-1)...
2023    (...(?+1)...(relative)...    (...(?+1)...(relative)...
2024  .sp  .sp
2025  An earlier example pointed out that the pattern  An earlier example pointed out that the pattern
2026  .sp  .sp
# Line 1797  It matches "abcabc". It does not match " Line 2049  It matches "abcabc". It does not match "
2049  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2050  .  .
2051  .  .
2052    .\" HTML <a name="onigurumasubroutines"></a>
2053    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2054    .rs
2055    .sp
2056    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2057    a number enclosed either in angle brackets or single quotes, is an alternative
2058    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2059    are two of the examples used above, rewritten using this syntax:
2060    .sp
2061      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2062      (sens|respons)e and \eg'1'ibility
2063    .sp
2064    PCRE supports an extension to Oniguruma: if a number is preceded by a
2065    plus or a minus sign it is taken as a relative reference. For example:
2066    .sp
2067      (abc)(?i:\eg<-1>)
2068    .sp
2069    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2070    synonymous. The former is a back reference; the latter is a subroutine call.
2071    .
2072    .
2073  .SH CALLOUTS  .SH CALLOUTS
2074  .rs  .rs
2075  .sp  .sp
# Line 1833  description of the interface to the call Line 2106  description of the interface to the call
2106  documentation.  documentation.
2107  .  .
2108  .  .
2109    .SH "BACKTRACKING CONTROL"
2110    .rs
2111    .sp
2112    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2113    are described in the Perl documentation as "experimental and subject to change
2114    or removal in a future version of Perl". It goes on to say: "Their usage in
2115    production code should be noted to avoid problems during upgrades." The same
2116    remarks apply to the PCRE features described in this section.
2117    .P
2118    Since these verbs are specifically related to backtracking, most of them can be
2119    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2120    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2121    failing negative assertion, they cause an error if encountered by
2122    \fBpcre_dfa_exec()\fP.
2123    .P
2124    The new verbs make use of what was previously invalid syntax: an opening
2125    parenthesis followed by an asterisk. In Perl, they are generally of the form
2126    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2127    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2128    are two kinds:
2129    .
2130    .SS "Verbs that act immediately"
2131    .rs
2132    .sp
2133    The following verbs act as soon as they are encountered:
2134    .sp
2135       (*ACCEPT)
2136    .sp
2137    This verb causes the match to end successfully, skipping the remainder of the
2138    pattern. When inside a recursion, only the innermost pattern is ended
2139    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2140    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2141    captured. For example:
2142    .sp
2143      A(A|B(*ACCEPT)|C)D
2144    .sp
2145    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2146    captured.
2147    .sp
2148      (*FAIL) or (*F)
2149    .sp
2150    This verb causes the match to fail, forcing backtracking to occur. It is
2151    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2152    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2153    Perl features that are not present in PCRE. The nearest equivalent is the
2154    callout feature, as for example in this pattern:
2155    .sp
2156      a+(?C)(*FAIL)
2157    .sp
2158    A match with the string "aaaa" always fails, but the callout is taken before
2159    each backtrack happens (in this example, 10 times).
2160    .
2161    .SS "Verbs that act after backtracking"
2162    .rs
2163    .sp
2164    The following verbs do nothing when they are encountered. Matching continues
2165    with what follows, but if there is no subsequent match, a failure is forced.
2166    The verbs differ in exactly what kind of failure occurs.
2167    .sp
2168      (*COMMIT)
2169    .sp
2170    This verb causes the whole match to fail outright if the rest of the pattern
2171    does not match. Even if the pattern is unanchored, no further attempts to find
2172    a match by advancing the start point take place. Once (*COMMIT) has been
2173    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2174    starting point, or not at all. For example:
2175    .sp
2176      a+(*COMMIT)b
2177    .sp
2178    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2179    dynamic anchor, or "I've started, so I must finish."
2180    .sp
2181      (*PRUNE)
2182    .sp
2183    This verb causes the match to fail at the current position if the rest of the
2184    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2185    advance to the next starting character then happens. Backtracking can occur as
2186    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2187    if there is no match to the right, backtracking cannot cross (*PRUNE).
2188    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2189    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2190    be expressed in any other way.
2191    .sp
2192      (*SKIP)
2193    .sp
2194    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2195    "bumpalong" advance is not to the next character, but to the position in the
2196    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2197    was matched leading up to it cannot be part of a successful match. Consider:
2198    .sp
2199      a+(*SKIP)b
2200    .sp
2201    If the subject is "aaaac...", after the first match attempt fails (starting at
2202    the first character in the string), the starting point skips on to start the
2203    next attempt at "c". Note that a possessive quantifer does not have the same
2204    effect in this example; although it would suppress backtracking during the
2205    first match attempt, the second attempt would start at the second character
2206    instead of skipping on to "c".
2207    .sp
2208      (*THEN)
2209    .sp
2210    This verb causes a skip to the next alternation if the rest of the pattern does
2211    not match. That is, it cancels pending backtracking, but only within the
2212    current alternation. Its name comes from the observation that it can be used
2213    for a pattern-based if-then-else block:
2214    .sp
2215      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2216    .sp
2217    If the COND1 pattern matches, FOO is tried (and possibly further items after
2218    the end of the group if FOO succeeds); on failure the matcher skips to the
2219    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2220    is used outside of any alternation, it acts exactly like (*PRUNE).
2221    .
2222    .
2223  .SH "SEE ALSO"  .SH "SEE ALSO"
2224  .rs  .rs
2225  .sp  .sp
# Line 1853  Cambridge CB2 3QH, England. Line 2240  Cambridge CB2 3QH, England.
2240  .rs  .rs
2241  .sp  .sp
2242  .nf  .nf
2243  Last updated: 09 May 2007  Last updated: 19 April 2008
2244  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2008 University of Cambridge.
2245  .fi  .fi

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