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revision 394 by ph10, Wed Mar 18 16:38:23 2009 UTC revision 457 by ph10, Sat Oct 3 16:24:08 2009 UTC
# Line 21  published by O'Reilly, covers regular ex Line 21  published by O'Reilly, covers regular ex
21  description of PCRE's regular expressions is intended as reference material.  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,
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  PCRE must be built to include UTF-8 support, and you must call
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  places below. There is also a summary of UTF-8 features in the  is also a special sequence that can be given at 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 76  string with one of the following five se Line 83  string with one of the following five se
83    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
84    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
85  .sp  .sp
86  These override the default and the options given to \fBpcre_compile()\fP. For  These override the default and the options given to \fBpcre_compile()\fP or
87  example, on a Unix system where LF is the default newline sequence, the pattern  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
88    newline sequence, the pattern
89  .sp  .sp
90    (*CR)a.b    (*CR)a.b
91  .sp  .sp
# Line 199  The \eQ...\eE sequence is recognized bot Line 207  The \eQ...\eE sequence is recognized bot
207  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
208  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
209  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
210  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
211  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
212  .sp  .sp
213    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
214    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
# Line 326  syntax for referencing a subpattern as a Line 333  syntax for referencing a subpattern as a
333  later.  later.
334  .\"  .\"
335  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  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.  synonymous. The former is a back reference; the latter is a
337    .\" HTML <a href="#subpatternsassubroutines">
338    .\" </a>
339    subroutine
340    .\"
341    call.
342  .  .
343  .  .
344  .SS "Generic character types"  .SS "Generic character types"
# Line 364  In UTF-8 mode, characters with values gr Line 376  In UTF-8 mode, characters with values gr
376  \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
377  character property support is available. These sequences retain their original  character property support is available. These sequences retain their original
378  meanings from before UTF-8 support was available, mainly for efficiency  meanings from before UTF-8 support was available, mainly for efficiency
379  reasons. Note that this also affects \eb, because it is defined in terms of \ew  reasons. Note that this also affects \eb, because it is defined in terms of \ew
380  and \eW.  and \eW.
381  .P  .P
382  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
# Line 456  one of the following sequences: Line 468  one of the following sequences:
468    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
469    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
470  .sp  .sp
471  These override the default and the options given to \fBpcre_compile()\fP, but  These override the default and the options given to \fBpcre_compile()\fP or
472  they can be overridden by options given to \fBpcre_exec()\fP. Note that these  \fBpcre_compile2()\fP, but they can be overridden by options given to
473  special settings, which are not Perl-compatible, are recognized only at the  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
474  very start of a pattern, and that they must be in upper case. If more than one  which are not Perl-compatible, are recognized only at the very start of a
475  of them is present, the last one is used. They can be combined with a change of  pattern, and that they must be in upper case. If more than one of them is
476  newline convention, for example, a pattern can start with:  present, the last one is used. They can be combined with a change of newline
477    convention, for example, a pattern can start with:
478  .sp  .sp
479    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
480  .sp  .sp
# Line 635  cannot be tested by PCRE, unless UTF-8 v Line 648  cannot be tested by PCRE, unless UTF-8 v
648  .\" HREF  .\" HREF
649  \fBpcreapi\fP  \fBpcreapi\fP
650  .\"  .\"
651  page).  page). Perl does not support the Cs property.
652  .P  .P
653  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for property names that Perl supports (such as \ep{Letter})
654  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
655  properties with "Is".  properties with "Is".
656  .P  .P
# Line 728  different meaning, namely the backspace Line 741  different meaning, namely the backspace
741  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
742  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
743  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
744  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. Neither PCRE nor Perl has a
745    separte "start of word" or "end of word" metasequence. However, whatever
746    follows \eb normally determines which it is. For example, the fragment
747    \eba matches "a" at the start of a word.
748  .P  .P
749  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
750  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 860  the lookbehind. Line 876  the lookbehind.
876  .rs  .rs
877  .sp  .sp
878  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
879  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
880  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
881  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
882  escaped with a backslash.  a member of the class, it should be the first data character in the class
883    (after an initial circumflex, if present) or escaped with a backslash.
884  .P  .P
885  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
886  character may occupy more than one byte. A matched character must be in the set  character may be more than one byte long. A matched character must be in the
887  of characters defined by the class, unless the first character in the class  set of characters defined by the class, unless the first character in the class
888  definition is a circumflex, in which case the subject character must not be in  definition is a circumflex, in which case the subject character must not be in
889  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
890  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
# Line 877  For example, the character class [aeiou] Line 894  For example, the character class [aeiou]
894  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
895  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
896  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
897  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
898  string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
899  string.  string.
900  .P  .P
# Line 891  caseful version would. In UTF-8 mode, PC Line 908  caseful version would. In UTF-8 mode, PC
908  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
909  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
910  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
911  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in UTF8-mode for characters 128 and above,
912  ensure that PCRE is compiled with Unicode property support as well as with  you must ensure that PCRE is compiled with Unicode property support as well as
913  UTF-8 support.  with UTF-8 support.
914  .P  .P
915  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
916  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 1032  The PCRE-specific options PCRE_DUPNAMES, Line 1049  The PCRE-specific options PCRE_DUPNAMES,
1049  changed in the same way as the Perl-compatible options by using the characters  changed in the same way as the Perl-compatible options by using the characters
1050  J, U and X respectively.  J, U and X respectively.
1051  .P  .P
1052  When an option change occurs at top level (that is, not inside subpattern  When one of these option changes occurs at top level (that is, not inside
1053  parentheses), the change applies to the remainder of the pattern that follows.  subpattern parentheses), the change applies to the remainder of the pattern
1054  If the change is placed right at the start of a pattern, PCRE extracts it into  that follows. If the change is placed right at the start of a pattern, PCRE
1055  the global options (and it will therefore show up in data extracted by the  extracts it into the global options (and it will therefore show up in data
1056  \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1057  .P  .P
1058  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1059  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 1057  behaviour otherwise. Line 1074  behaviour otherwise.
1074  .P  .P
1075  \fBNote:\fP There are other PCRE-specific options that can be set by the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1076  application when the compile or match functions are called. In some cases the  application when the compile or match functions are called. In some cases the
1077  pattern can contain special leading sequences to override what the application  pattern can contain special leading sequences such as (*CRLF) to override what
1078  has set or what has been defaulted. Details are given in the section entitled  the application has set or what has been defaulted. Details are given in the
1079    section entitled
1080  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1081  .\" </a>  .\" </a>
1082  "Newline sequences"  "Newline sequences"
1083  .\"  .\"
1084  above.  above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1085    mode; this is equivalent to setting the PCRE_UTF8 option.
1086  .  .
1087  .  .
1088  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1118  is reached, an option setting in one bra Line 1137  is reached, an option setting in one bra
1137  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1138  .  .
1139  .  .
1140    .\" HTML <a name="dupsubpatternnumber"></a>
1141  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1142  .rs  .rs
1143  .sp  .sp
# Line 1143  stored. Line 1163  stored.
1163    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1164    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1165  .sp  .sp
1166  A backreference or a recursive call to a numbered subpattern always refers to  A backreference to a numbered subpattern uses the most recent value that is set
1167  the first one in the pattern with the given number.  for that number by any subpattern. The following pattern matches "abcabc" or
1168  .P  "defdef":
1169  An alternative approach to using this "branch reset" feature is to use  .sp
1170      /(?|(abc)|(def))\1/
1171    .sp
1172    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1173    refers to the first one in the pattern with the given number. The following
1174    pattern matches "abcabc" or "defabc":
1175    .sp
1176      /(?|(abc)|(def))(?1)/
1177    .sp
1178    An alternative approach to using the "branch reset" feature is to use
1179  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
1180  .  .
1181  .  .
# Line 1186  extracting the name-to-number translatio Line 1215  extracting the name-to-number translatio
1215  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1216  .P  .P
1217  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
1218  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1219  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1220  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1221  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1222  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1223    name, and in both cases you want to extract the abbreviation. This pattern
1224    (ignoring the line breaks) does the job:
1225  .sp  .sp
1226    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1227    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1214  details of the interfaces for handling n Line 1245  details of the interfaces for handling n
1245  documentation.  documentation.
1246  .P  .P
1247  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1248  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1249  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1250    are given to subpatterns with the same number. However, you can give the same
1251    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1252  .  .
1253  .  .
1254  .SH REPETITION  .SH REPETITION
# Line 1233  items: Line 1266  items:
1266    a character class    a character class
1267    a back reference (see next section)    a back reference (see next section)
1268    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1269      a recursive or "subroutine" call to a subpattern
1270  .sp  .sp
1271  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1272  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1554  after the reference. Line 1588  after the reference.
1588  .P  .P
1589  There may be more than one back reference to the same subpattern. If a  There may be more than one back reference to the same subpattern. If a
1590  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1591  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1592  .sp  .sp
1593    (a|(bc))\e2    (a|(bc))\e2
1594  .sp  .sp
1595  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". However, if the
1596  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1597  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1598  with a digit character, some delimiter must be used to terminate the back  .P
1599  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1600  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1601    If the pattern continues with a digit character, some delimiter must be used to
1602    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1603    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1604  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1605  .\" </a>  .\" </a>
1606  "Comments"  "Comments"
# Line 1636  lookbehind assertion is needed to achiev Line 1673  lookbehind assertion is needed to achiev
1673  If you want to force a matching failure at some point in a pattern, the most  If you want to force a matching failure at some point in a pattern, the most
1674  convenient way to do it is with (?!) because an empty string always matches, so  convenient way to do it is with (?!) because an empty string always matches, so
1675  an assertion that requires there not to be an empty string must always fail.  an assertion that requires there not to be an empty string must always fail.
1676    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1677    synonym for (?!).
1678  .  .
1679  .  .
1680  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1660  is permitted, but Line 1699  is permitted, but
1699  .sp  .sp
1700  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1701  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1702  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (5.8 and 5.10), which requires all branches to
1703  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1704  .sp  .sp
1705    (?<=ab(c|de))    (?<=ab(c|de))
1706  .sp  .sp
1707  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1708  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1709    branches:
1710  .sp  .sp
1711    (?<=abc|abde)    (?<=abc|abde)
1712  .sp  .sp
# Line 1675  In some cases, the Perl 5.10 escape sequ Line 1715  In some cases, the Perl 5.10 escape sequ
1715  .\" </a>  .\" </a>
1716  (see above)  (see above)
1717  .\"  .\"
1718  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
1719  fixed-length.  restriction.
1720  .P  .P
1721  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1722  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
# Line 1688  to appear in lookbehind assertions, beca Line 1728  to appear in lookbehind assertions, beca
1728  the length of the lookbehind. The \eX and \eR escapes, which can match  the length of the lookbehind. The \eX and \eR escapes, which can match
1729  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1730  .P  .P
1731    .\" HTML <a href="#subpatternsassubroutines">
1732    .\" </a>
1733    "Subroutine"
1734    .\"
1735    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1736    as the subpattern matches a fixed-length string.
1737    .\" HTML <a href="#recursion">
1738    .\" </a>
1739    Recursion,
1740    .\"
1741    however, is not supported.
1742    .P
1743  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1744  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching of fixed-length strings at the end of subject
1745  pattern such as  strings. Consider a simple pattern such as
1746  .sp  .sp
1747    abcd$    abcd$
1748  .sp  .sp
# Line 1754  characters that are not "999". Line 1806  characters that are not "999".
1806  .sp  .sp
1807  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1808  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1809  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1810  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1811  .sp  .sp
1812    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1813    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1771  recursion, a pseudo-condition called DEF Line 1823  recursion, a pseudo-condition called DEF
1823  .rs  .rs
1824  .sp  .sp
1825  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
1826  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1827  matched. An alternative notation is to precede the digits with a plus or minus  matched. If there is more than one capturing subpattern with the same number
1828  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
1829  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
1830  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
1831  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
1832    section about duplicate subpattern numbers),
1833    .\"
1834    the condition is true if any of them have been set. An alternative notation is
1835    to precede the digits with a plus or minus sign. In this case, the subpattern
1836    number is relative rather than absolute. The most recently opened parentheses
1837    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1838    looping constructs it can also make sense to refer to subsequent groups with
1839    constructs such as (?(+2).
1840  .P  .P
1841  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1842  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1828  letter R, for example: Line 1888  letter R, for example:
1888  .sp  .sp
1889    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
1890  .sp  .sp
1891  the condition is true if the most recent recursion is into the subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
1892  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
1893  stack.  stack.
1894  .P  .P
1895  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
1896  patterns are described below.  .\" HTML <a href="#recursion">
1897    .\" </a>
1898    Recursive patterns
1899    .\"
1900    are described below.
1901  .  .
1902  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
1903  .rs  .rs
# Line 1842  If the condition is the string (DEFINE), Line 1906  If the condition is the string (DEFINE),
1906  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
1907  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
1908  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
1909  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
1910    .\" HTML <a href="#subpatternsassubroutines">
1911    .\" </a>
1912    "subroutines"
1913    .\"
1914  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address could be
1915  written like this (ignore whitespace and line breaks):  written like this (ignore whitespace and line breaks):
1916  .sp  .sp
# Line 1852  written like this (ignore whitespace and Line 1920  written like this (ignore whitespace and
1920  The first part of the pattern is a DEFINE group inside which a another group  The first part of the pattern is a DEFINE group inside which a another group
1921  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
1922  address (a number less than 256). When matching takes place, this part of the  address (a number less than 256). When matching takes place, this part of the
1923  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
1924  .P  pattern uses references to the named group to match the four dot-separated
1925  The rest of the pattern uses references to the named group to match the four  components of an IPv4 address, insisting on a word boundary at each end.
 dot-separated components of an IPv4 address, insisting on a word boundary at  
 each end.  
1926  .  .
1927  .SS "Assertion conditions"  .SS "Assertion conditions"
1928  .rs  .rs
# Line 1913  recursively to the pattern in which it a Line 1979  recursively to the pattern in which it a
1979  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1980  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
1981  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
1982  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
1983  .P  .P
1984  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
1985  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive call of the subpattern of the given number,
1986  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  provided that it occurs inside that subpattern. (If not, it is a
1987    .\" HTML <a href="#subpatternsassubroutines">
1988    .\" </a>
1989    "subroutine"
1990    .\"
1991  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
1992  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
1993  .P  .P
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 .P  
1994  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1995  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
1996  .sp  .sp
1997    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
1998  .sp  .sp
1999  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2000  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
2001  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2002  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2003    to avoid backtracking into sequences of non-parentheses.
2004  .P  .P
2005  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
2006  pattern, so instead you could use this:  pattern, so instead you could use this:
2007  .sp  .sp
2008    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2009  .sp  .sp
2010  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
2011  them instead of the whole pattern.  them instead of the whole pattern.
2012  .P  .P
2013  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
2014  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).
2015  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2016  most recently opened parentheses preceding the recursion. In other words, a  most recently opened parentheses preceding the recursion. In other words, a
2017  negative number counts capturing parentheses leftwards from the point at which  negative number counts capturing parentheses leftwards from the point at which
# Line 1954  it is encountered. Line 2020  it is encountered.
2020  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2021  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2022  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2023  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2024    .\" </a>
2025    "subroutine"
2026    .\"
2027    calls, as described in the next section.
2028  .P  .P
2029  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2030  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2031  could rewrite the above example as follows:  could rewrite the above example as follows:
2032  .sp  .sp
2033    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2034  .sp  .sp
2035  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
2036  used.  used.
2037  .P  .P
2038  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2039  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of a possessive quantifier for matching
2040  non-parentheses is important when applying the pattern to strings that do not  strings of non-parentheses is important when applying the pattern to strings
2041  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2042  .sp  .sp
2043    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2044  .sp  .sp
2045  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2046  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
2047  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
2048  before failure can be reported.  before failure can be reported.
# Line 1991  documentation). If the pattern above is Line 2061  documentation). If the pattern above is
2061  the value for the capturing parentheses is "ef", which is the last value taken  the value for the capturing parentheses is "ef", which is the last value taken
2062  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
2063  .sp  .sp
2064    \e( ( ( (?>[^()]+) | (?R) )* ) \e)    \e( ( ( [^()]++ | (?R) )* ) \e)
2065       ^                        ^       ^                        ^
2066       ^                        ^       ^                        ^
2067  .sp  .sp
# Line 2013  different alternatives for the recursive Line 2083  different alternatives for the recursive
2083  is the actual recursive call.  is the actual recursive call.
2084  .  .
2085  .  .
2086    .\" HTML <a name="recursiondifference"></a>
2087    .SS "Recursion difference from Perl"
2088    .rs
2089    .sp
2090    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2091    treated as an atomic group. That is, once it has matched some of the subject
2092    string, it is never re-entered, even if it contains untried alternatives and
2093    there is a subsequent matching failure. This can be illustrated by the
2094    following pattern, which purports to match a palindromic string that contains
2095    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2096    .sp
2097      ^(.|(.)(?1)\e2)$
2098    .sp
2099    The idea is that it either matches a single character, or two identical
2100    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2101    it does not if the pattern is longer than three characters. Consider the
2102    subject string "abcba":
2103    .P
2104    At the top level, the first character is matched, but as it is not at the end
2105    of the string, the first alternative fails; the second alternative is taken
2106    and the recursion kicks in. The recursive call to subpattern 1 successfully
2107    matches the next character ("b"). (Note that the beginning and end of line
2108    tests are not part of the recursion).
2109    .P
2110    Back at the top level, the next character ("c") is compared with what
2111    subpattern 2 matched, which was "a". This fails. Because the recursion is
2112    treated as an atomic group, there are now no backtracking points, and so the
2113    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2114    try the second alternative.) However, if the pattern is written with the
2115    alternatives in the other order, things are different:
2116    .sp
2117      ^((.)(?1)\e2|.)$
2118    .sp
2119    This time, the recursing alternative is tried first, and continues to recurse
2120    until it runs out of characters, at which point the recursion fails. But this
2121    time we do have another alternative to try at the higher level. That is the big
2122    difference: in the previous case the remaining alternative is at a deeper
2123    recursion level, which PCRE cannot use.
2124    .P
2125    To change the pattern so that matches all palindromic strings, not just those
2126    with an odd number of characters, it is tempting to change the pattern to this:
2127    .sp
2128      ^((.)(?1)\e2|.?)$
2129    .sp
2130    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2131    deeper recursion has matched a single character, it cannot be entered again in
2132    order to match an empty string. The solution is to separate the two cases, and
2133    write out the odd and even cases as alternatives at the higher level:
2134    .sp
2135      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2136    .sp
2137    If you want to match typical palindromic phrases, the pattern has to ignore all
2138    non-word characters, which can be done like this:
2139    .sp
2140      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$
2141    .sp
2142    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2143    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2144    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2145    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2146    more) to match typical phrases, and Perl takes so long that you think it has
2147    gone into a loop.
2148    .P
2149    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2150    string does not start with a palindrome that is shorter than the entire string.
2151    For example, although "abcba" is correctly matched, if the subject is "ababa",
2152    PCRE finds the palindrome "aba" at the start, then fails at top level because
2153    the end of the string does not follow. Once again, it cannot jump back into the
2154    recursion to try other alternatives, so the entire match fails.
2155    .
2156    .
2157  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2158  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2159  .rs  .rs
# Line 2126  a backtracking algorithm. With the excep Line 2267  a backtracking algorithm. With the excep
2267  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2268  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2269  .P  .P
2270    If any of these verbs are used in an assertion subpattern, their effect is
2271    confined to that subpattern; it does not extend to the surrounding pattern.
2272    Note that assertion subpatterns are processed as anchored at the point where
2273    they are tested.
2274    .P
2275  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2276  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. In Perl, they are generally of the form
2277  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB:ARG) but PCRE does not support the use of arguments, so its general
# Line 2141  The following verbs act as soon as they Line 2287  The following verbs act as soon as they
2287  .sp  .sp
2288  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2289  pattern. When inside a recursion, only the innermost pattern is ended  pattern. When inside a recursion, only the innermost pattern is ended
2290  immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2291  capturing parentheses. In Perl, the data so far is captured: in PCRE no data is  captured. (This feature was added to PCRE at release 8.00.) For example:
 captured. For example:  
2292  .sp  .sp
2293    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2294  .sp  .sp
2295  This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2296  captured.  the outer parentheses.
2297  .sp  .sp
2298    (*FAIL) or (*F)    (*FAIL) or (*F)
2299  .sp  .sp
# Line 2174  The verbs differ in exactly what kind of Line 2319  The verbs differ in exactly what kind of
2319  .sp  .sp
2320  This verb causes the whole match to fail outright if the rest of the pattern  This verb causes the whole match to fail outright if the rest of the pattern
2321  does not match. Even if the pattern is unanchored, no further attempts to find  does not match. Even if the pattern is unanchored, no further attempts to find
2322  a match by advancing the start point take place. Once (*COMMIT) has been  a match by advancing the starting point take place. Once (*COMMIT) has been
2323  passed, \fBpcre_exec()\fP is committed to finding a match at the current  passed, \fBpcre_exec()\fP is committed to finding a match at the current
2324  starting point, or not at all. For example:  starting point, or not at all. For example:
2325  .sp  .sp
# Line 2206  was matched leading up to it cannot be p Line 2351  was matched leading up to it cannot be p
2351  If the subject is "aaaac...", after the first match attempt fails (starting at  If the subject is "aaaac...", after the first match attempt fails (starting at
2352  the first character in the string), the starting point skips on to start the  the first character in the string), the starting point skips on to start the
2353  next attempt at "c". Note that a possessive quantifer does not have the same  next attempt at "c". Note that a possessive quantifer does not have the same
2354  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
2355  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2356  instead of skipping on to "c".  instead of skipping on to "c".
2357  .sp  .sp
# Line 2228  is used outside of any alternation, it a Line 2373  is used outside of any alternation, it a
2373  .SH "SEE ALSO"  .SH "SEE ALSO"
2374  .rs  .rs
2375  .sp  .sp
2376  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2377    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2378  .  .
2379  .  .
2380  .SH AUTHOR  .SH AUTHOR
# Line 2245  Cambridge CB2 3QH, England. Line 2391  Cambridge CB2 3QH, England.
2391  .rs  .rs
2392  .sp  .sp
2393  .nf  .nf
2394  Last updated: 18 March 2009  Last updated: 03 October 2009
2395  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2009 University of Cambridge.
2396  .fi  .fi

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