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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 40  discussed in the Line 56  discussed in the
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 149  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 164  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 240  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. A named back reference  enclosed in braces, is an absolute or relative back reference. A named back
313  can be coded as \eg{name}. Back references are 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 254  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 subroutine call.
337    .
338    .
339  .SS "Generic character types"  .SS "Generic character types"
340  .rs  .rs
341  .sp  .sp
342  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
343  following are always recognized:  following are always recognized:
344  .sp  .sp
345    \ed     any decimal digit    \ed     any decimal digit
346    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
347    \eh     any horizontal whitespace character    \eh     any horizontal whitespace character
348    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal whitespace character
349    \es     any whitespace character    \es     any whitespace character
350    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
351    \ev     any vertical whitespace character    \ev     any vertical whitespace character
352    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical whitespace character
353    \ew     any "word" character    \ew     any "word" character
354    \eW     any "non-word" character    \eW     any "non-word" character
355  .sp  .sp
# Line 287  does. Line 369  does.
369  .P  .P
370  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
371  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
372  character property support is available. These sequences retain their original  character property support is available. These sequences retain their original
373  meanings from before UTF-8 support was available, mainly for efficiency  meanings from before UTF-8 support was available, mainly for efficiency
374  reasons.  reasons. Note that this also affects \eb, because it is defined in terms of \ew
375    and \eW.
376  .P  .P
377  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
378  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  other sequences, these do match certain high-valued codepoints in UTF-8 mode.
379  The horizontal space characters are:  The horizontal space characters are:
380  .sp  .sp
# Line 343  accented letters, and these are matched Line 426  accented letters, and these are matched
426  is discouraged.  is discouraged.
427  .  .
428  .  .
429    .\" HTML <a name="newlineseq"></a>
430  .SS "Newline sequences"  .SS "Newline sequences"
431  .rs  .rs
432  .sp  .sp
433  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
434  sequence. This is a Perl 5.10 feature. 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
435  the following:  equivalent to the following:
436  .sp  .sp
437    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
438  .sp  .sp
# Line 368  are added: LS (line separator, U+2028) a Line 452  are added: LS (line separator, U+2028) a
452  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
453  recognized.  recognized.
454  .P  .P
455    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
456    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
457    either at compile time or when the pattern is matched. (BSR is an abbrevation
458    for "backslash R".) This can be made the default when PCRE is built; if this is
459    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
460    It is also possible to specify these settings by starting a pattern string with
461    one of the following sequences:
462    .sp
463      (*BSR_ANYCRLF)   CR, LF, or CRLF only
464      (*BSR_UNICODE)   any Unicode newline sequence
465    .sp
466    These override the default and the options given to \fBpcre_compile()\fP, but
467    they can be overridden by options given to \fBpcre_exec()\fP. Note that these
468    special settings, which are not Perl-compatible, are recognized only at the
469    very start of a pattern, and that they must be in upper case. If more than one
470    of them is present, the last one is used. They can be combined with a change of
471    newline convention, for example, a pattern can start with:
472    .sp
473      (*ANY)(*BSR_ANYCRLF)
474    .sp
475  Inside a character class, \eR matches the letter "R".  Inside a character class, \eR matches the letter "R".
476  .  .
477  .  .
# Line 376  Inside a character class, \eR matches th Line 480  Inside a character class, \eR matches th
480  .rs  .rs
481  .sp  .sp
482  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
483  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
484  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
485    characters whose codepoints are less than 256, but they do work in this mode.
486    The extra escape sequences are:
487  .sp  .sp
488    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
489    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
# Line 529  The special property L& is also supporte Line 635  The special property L& is also supporte
635  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
636  a modifier or "other".  a modifier or "other".
637  .P  .P
638  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
639    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
640    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
641    (see the discussion of PCRE_NO_UTF8_CHECK in the
642    .\" HREF
643    \fBpcreapi\fP
644    .\"
645    page). Perl does not support the Cs property.
646    .P
647    The long synonyms for property names that Perl supports (such as \ep{Letter})
648  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
649  properties with "Is".  properties with "Is".
650  .P  .P
# Line 553  atomic group Line 668  atomic group
668  (see below).  (see below).
669  .\"  .\"
670  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
671  preceding character.  preceding character. None of them have codepoints less than 256, so in
672    non-UTF-8 mode \eX matches any one character.
673  .P  .P
674  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
675  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 903  alternative in the subpattern. Line 1019  alternative in the subpattern.
1019  .rs  .rs
1020  .sp  .sp
1021  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1022  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
1023  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1024    The option letters are
1025  .sp  .sp
1026    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1027    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 918  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1035  PCRE_MULTILINE while unsetting PCRE_DOTA
1035  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
1036  unset.  unset.
1037  .P  .P
1038  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
1039  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
1040  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1041  the global options (and it will therefore show up in data extracted by the  .P
1042  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1043    subpattern parentheses), the change applies to the remainder of the pattern
1044    that follows. If the change is placed right at the start of a pattern, PCRE
1045    extracts it into the global options (and it will therefore show up in data
1046    extracted by the \fBpcre_fullinfo()\fP function).
1047  .P  .P
1048  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1049  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 941  branch is abandoned before the option se Line 1062  branch is abandoned before the option se
1062  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1063  behaviour otherwise.  behaviour otherwise.
1064  .P  .P
1065  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
1066  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
1067  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1068    the application has set or what has been defaulted. Details are given in the
1069    section entitled
1070    .\" HTML <a href="#newlineseq">
1071    .\" </a>
1072    "Newline sequences"
1073    .\"
1074    above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1075    mode; this is equivalent to setting the PCRE_UTF8 option.
1076  .  .
1077  .  .
1078  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1001  the above patterns match "SUNDAY" as wel Line 1130  the above patterns match "SUNDAY" as wel
1130  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1131  .rs  .rs
1132  .sp  .sp
1133  Perl 5.10 introduced a feature whereby each alternative in a subpattern uses  Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1134  the same numbers for its capturing parentheses. Such a subpattern starts with  the same numbers for its capturing parentheses. Such a subpattern starts with
1135  (?| and is itself a non-capturing subpattern. For example, consider this  (?| and is itself a non-capturing subpattern. For example, consider this
1136  pattern:  pattern:
1137  .sp  .sp
1138    (?|(Sat)ur|(Sun))day    (?|(Sat)ur|(Sun))day
1139  .sp  .sp
1140  Because the two alternatives are inside a (?| group, both sets of capturing  Because the two alternatives are inside a (?| group, both sets of capturing
1141  parentheses are numbered one. Thus, when the pattern matches, you can look  parentheses are numbered one. Thus, when the pattern matches, you can look
1142  at captured substring number one, whichever alternative matched. This construct  at captured substring number one, whichever alternative matched. This construct
1143  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1144  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1145  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1146  buffers that follow the subpattern start after the highest number used in any  buffers that follow the subpattern start after the highest number used in any
1147  branch. The following example is taken from the Perl documentation.  branch. The following example is taken from the Perl documentation.
1148  The numbers underneath show in which buffer the captured content will be  The numbers underneath show in which buffer the captured content will be
1149  stored.  stored.
1150  .sp  .sp
1151    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1152    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1153    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1154  .sp  .sp
1155  A backreference or a recursive call to a numbered subpattern always refers to  A backreference or a recursive call to a numbered subpattern always refers to
1156  the first one in the pattern with the given number.  the first one in the pattern with the given number.
1157  .P  .P
# Line 1079  abbreviation. This pattern (ignoring the Line 1208  abbreviation. This pattern (ignoring the
1208    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1209  .sp  .sp
1210  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.
1211  (An alternative way of solving this problem is to use a "branch reset"  (An alternative way of solving this problem is to use a "branch reset"
1212  subpattern, as described in the previous section.)  subpattern, as described in the previous section.)
1213  .P  .P
1214  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
# Line 1092  details of the interfaces for handling n Line 1221  details of the interfaces for handling n
1221  \fBpcreapi\fP  \fBpcreapi\fP
1222  .\"  .\"
1223  documentation.  documentation.
1224    .P
1225    \fBWarning:\fP You cannot use different names to distinguish between two
1226    subpatterns with the same number (see the previous section) because PCRE uses
1227    only the numbers when matching.
1228  .  .
1229  .  .
1230  .SH REPETITION  .SH REPETITION
# Line 1140  support is available, \eX{3} matches thr Line 1273  support is available, \eX{3} matches thr
1273  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).
1274  .P  .P
1275  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
1276  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1277    subpatterns that are referenced as
1278    .\" HTML <a href="#subpatternsassubroutines">
1279    .\" </a>
1280    subroutines
1281    .\"
1282    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1283    quantifier are omitted from the compiled pattern.
1284  .P  .P
1285  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1286  abbreviations:  abbreviations:
# Line 1287  previous example can be rewritten as Line 1427  previous example can be rewritten as
1427  .sp  .sp
1428    \ed++foo    \ed++foo
1429  .sp  .sp
1430    Note that a possessive quantifier can be used with an entire group, for
1431    example:
1432    .sp
1433      (abc|xyz){2,3}+
1434    .sp
1435  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1436  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
1437  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 1361  subpattern is possible using named paren Line 1506  subpattern is possible using named paren
1506  .P  .P
1507  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
1508  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
1509  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
1510  optionally enclosed in braces. These examples are all identical:  number, optionally enclosed in braces. These examples are all identical:
1511  .sp  .sp
1512    (ring), \e1    (ring), \e1
1513    (ring), \eg1    (ring), \eg1
1514    (ring), \eg{1}    (ring), \eg{1}
1515  .sp  .sp
1516  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1517  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
1518  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1519    example:
1520  .sp  .sp
1521    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1522  .sp  .sp
# Line 1917  It matches "abcabc". It does not match " Line 2063  It matches "abcabc". It does not match "
2063  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2064  .  .
2065  .  .
2066    .\" HTML <a name="onigurumasubroutines"></a>
2067    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2068    .rs
2069    .sp
2070    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2071    a number enclosed either in angle brackets or single quotes, is an alternative
2072    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2073    are two of the examples used above, rewritten using this syntax:
2074    .sp
2075      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2076      (sens|respons)e and \eg'1'ibility
2077    .sp
2078    PCRE supports an extension to Oniguruma: if a number is preceded by a
2079    plus or a minus sign it is taken as a relative reference. For example:
2080    .sp
2081      (abc)(?i:\eg<-1>)
2082    .sp
2083    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2084    synonymous. The former is a back reference; the latter is a subroutine call.
2085    .
2086    .
2087  .SH CALLOUTS  .SH CALLOUTS
2088  .rs  .rs
2089  .sp  .sp
# Line 1953  description of the interface to the call Line 2120  description of the interface to the call
2120  documentation.  documentation.
2121  .  .
2122  .  .
2123    .SH "BACKTRACKING CONTROL"
2124    .rs
2125    .sp
2126    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2127    are described in the Perl documentation as "experimental and subject to change
2128    or removal in a future version of Perl". It goes on to say: "Their usage in
2129    production code should be noted to avoid problems during upgrades." The same
2130    remarks apply to the PCRE features described in this section.
2131    .P
2132    Since these verbs are specifically related to backtracking, most of them can be
2133    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2134    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2135    failing negative assertion, they cause an error if encountered by
2136    \fBpcre_dfa_exec()\fP.
2137    .P
2138    If any of these verbs are used in an assertion subpattern, their effect is
2139    confined to that subpattern; it does not extend to the surrounding pattern.
2140    Note that assertion subpatterns are processed as anchored at the point where
2141    they are tested.
2142    .P
2143    The new verbs make use of what was previously invalid syntax: an opening
2144    parenthesis followed by an asterisk. In Perl, they are generally of the form
2145    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2146    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2147    are two kinds:
2148    .
2149    .SS "Verbs that act immediately"
2150    .rs
2151    .sp
2152    The following verbs act as soon as they are encountered:
2153    .sp
2154       (*ACCEPT)
2155    .sp
2156    This verb causes the match to end successfully, skipping the remainder of the
2157    pattern. When inside a recursion, only the innermost pattern is ended
2158    immediately. If the (*ACCEPT) is inside capturing parentheses, the data so far
2159    is captured. (This feature was added to PCRE at release 8.00.) For example:
2160    .sp
2161      A((?:A|B(*ACCEPT)|C)D)
2162    .sp
2163    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2164    the outer parentheses.
2165    .sp
2166      (*FAIL) or (*F)
2167    .sp
2168    This verb causes the match to fail, forcing backtracking to occur. It is
2169    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2170    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2171    Perl features that are not present in PCRE. The nearest equivalent is the
2172    callout feature, as for example in this pattern:
2173    .sp
2174      a+(?C)(*FAIL)
2175    .sp
2176    A match with the string "aaaa" always fails, but the callout is taken before
2177    each backtrack happens (in this example, 10 times).
2178    .
2179    .SS "Verbs that act after backtracking"
2180    .rs
2181    .sp
2182    The following verbs do nothing when they are encountered. Matching continues
2183    with what follows, but if there is no subsequent match, a failure is forced.
2184    The verbs differ in exactly what kind of failure occurs.
2185    .sp
2186      (*COMMIT)
2187    .sp
2188    This verb causes the whole match to fail outright if the rest of the pattern
2189    does not match. Even if the pattern is unanchored, no further attempts to find
2190    a match by advancing the start point take place. Once (*COMMIT) has been
2191    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2192    starting point, or not at all. For example:
2193    .sp
2194      a+(*COMMIT)b
2195    .sp
2196    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2197    dynamic anchor, or "I've started, so I must finish."
2198    .sp
2199      (*PRUNE)
2200    .sp
2201    This verb causes the match to fail at the current position if the rest of the
2202    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2203    advance to the next starting character then happens. Backtracking can occur as
2204    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2205    if there is no match to the right, backtracking cannot cross (*PRUNE).
2206    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2207    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2208    be expressed in any other way.
2209    .sp
2210      (*SKIP)
2211    .sp
2212    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2213    "bumpalong" advance is not to the next character, but to the position in the
2214    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2215    was matched leading up to it cannot be part of a successful match. Consider:
2216    .sp
2217      a+(*SKIP)b
2218    .sp
2219    If the subject is "aaaac...", after the first match attempt fails (starting at
2220    the first character in the string), the starting point skips on to start the
2221    next attempt at "c". Note that a possessive quantifer does not have the same
2222    effect in this example; although it would suppress backtracking during the
2223    first match attempt, the second attempt would start at the second character
2224    instead of skipping on to "c".
2225    .sp
2226      (*THEN)
2227    .sp
2228    This verb causes a skip to the next alternation if the rest of the pattern does
2229    not match. That is, it cancels pending backtracking, but only within the
2230    current alternation. Its name comes from the observation that it can be used
2231    for a pattern-based if-then-else block:
2232    .sp
2233      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2234    .sp
2235    If the COND1 pattern matches, FOO is tried (and possibly further items after
2236    the end of the group if FOO succeeds); on failure the matcher skips to the
2237    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2238    is used outside of any alternation, it acts exactly like (*PRUNE).
2239    .
2240    .
2241  .SH "SEE ALSO"  .SH "SEE ALSO"
2242  .rs  .rs
2243  .sp  .sp
# Line 1973  Cambridge CB2 3QH, England. Line 2258  Cambridge CB2 3QH, England.
2258  .rs  .rs
2259  .sp  .sp
2260  .nf  .nf
2261  Last updated: 13 June 2007  Last updated: 16 September 2009
2262  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2009 University of Cambridge.
2263  .fi  .fi

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