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revision 335 by ph10, Sat Apr 12 14:36:14 2008 UTC revision 456 by ph10, Fri Oct 2 08:53:31 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 318  parenthesized subpatterns. Line 325  parenthesized subpatterns.
325  .SS "Absolute and relative subroutine calls"  .SS "Absolute and relative subroutine calls"
326  .rs  .rs
327  .sp  .sp
328  For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or  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  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  syntax for referencing a subpattern as a "subroutine". Details are discussed
331  .\" HTML <a href="#onigurumasubroutines">  .\" HTML <a href="#onigurumasubroutines">
332  .\" </a>  .\" </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.  reasons. Note that this also affects \eb, because it is defined in terms of \ew
380    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
383  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.
# Line 455  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 634  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 727  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 859  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 876  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 890  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 1031  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 1056  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 1117  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 1142  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    "defdef":
1169    .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  .P  .P
1179  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using the "branch reset" feature is to use
1180  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
1181  .  .
1182  .  .
# Line 1211  details of the interfaces for handling n Line 1242  details of the interfaces for handling n
1242  \fBpcreapi\fP  \fBpcreapi\fP
1243  .\"  .\"
1244  documentation.  documentation.
1245    .P
1246    \fBWarning:\fP You cannot use different names to distinguish between two
1247    subpatterns with the same number (see the previous section) because PCRE uses
1248    only the numbers when matching.
1249  .  .
1250  .  .
1251  .SH REPETITION  .SH REPETITION
# Line 1228  items: Line 1263  items:
1263    a character class    a character class
1264    a back reference (see next section)    a back reference (see next section)
1265    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1266      a recursive or "subroutine" call to a subpattern
1267  .sp  .sp
1268  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1269  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1259  support is available, \eX{3} matches thr Line 1295  support is available, \eX{3} matches thr
1295  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).
1296  .P  .P
1297  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
1298  previous item and the quantifier were not present. This may be useful for  previous item and the quantifier were not present. This may be useful for
1299  subpatterns that are referenced as  subpatterns that are referenced as
1300  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
1301  .\" </a>  .\" </a>
1302  subroutines  subroutines
1303  .\"  .\"
1304  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern. Items other than subpatterns that have a {0}
1305  quantifier are omitted from the compiled pattern.  quantifier are omitted from the compiled pattern.
1306  .P  .P
1307  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
# Line 1549  after the reference. Line 1585  after the reference.
1585  .P  .P
1586  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
1587  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1588  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1589  .sp  .sp
1590    (a|(bc))\e2    (a|(bc))\e2
1591  .sp  .sp
1592  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
1593  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
1594  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1595  with a digit character, some delimiter must be used to terminate the back  .P
1596  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1597  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1598    If the pattern continues with a digit character, some delimiter must be used to
1599    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1600    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1601  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1602  .\" </a>  .\" </a>
1603  "Comments"  "Comments"
# Line 1631  lookbehind assertion is needed to achiev Line 1670  lookbehind assertion is needed to achiev
1670  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
1671  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
1672  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.
1673    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1674    synonym for (?!).
1675  .  .
1676  .  .
1677  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1655  is permitted, but Line 1696  is permitted, but
1696  .sp  .sp
1697  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1698  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
1699  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
1700  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1701  .sp  .sp
1702    (?<=ab(c|de))    (?<=ab(c|de))
1703  .sp  .sp
1704  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
1705  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
1706    branches:
1707  .sp  .sp
1708    (?<=abc|abde)    (?<=abc|abde)
1709  .sp  .sp
# Line 1670  In some cases, the Perl 5.10 escape sequ Line 1712  In some cases, the Perl 5.10 escape sequ
1712  .\" </a>  .\" </a>
1713  (see above)  (see above)
1714  .\"  .\"
1715  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
1716  fixed-length.  restriction.
1717  .P  .P
1718  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1719  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 1683  to appear in lookbehind assertions, beca Line 1725  to appear in lookbehind assertions, beca
1725  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
1726  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1727  .P  .P
1728    .\" HTML <a href="#subpatternsassubroutines">
1729    .\" </a>
1730    "Subroutine"
1731    .\"
1732    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1733    as the subpattern matches a fixed-length string.
1734    .\" HTML <a href="#recursion">
1735    .\" </a>
1736    Recursion,
1737    .\"
1738    however, is not supported.
1739    .P
1740  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1741  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
1742  pattern such as  strings. Consider a simple pattern such as
1743  .sp  .sp
1744    abcd$    abcd$
1745  .sp  .sp
# Line 1749  characters that are not "999". Line 1803  characters that are not "999".
1803  .sp  .sp
1804  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1805  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1806  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1807  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1808  .sp  .sp
1809    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1810    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1766  recursion, a pseudo-condition called DEF Line 1820  recursion, a pseudo-condition called DEF
1820  .rs  .rs
1821  .sp  .sp
1822  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
1823  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1824  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
1825  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
1826  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
1827  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
1828  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
1829    section about duplicate subpattern numbers),
1830    .\"
1831    the condition is true if any of them have been set. An alternative notation is
1832    to precede the digits with a plus or minus sign. In this case, the subpattern
1833    number is relative rather than absolute. The most recently opened parentheses
1834    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1835    looping constructs it can also make sense to refer to subsequent groups with
1836    constructs such as (?(+2).
1837  .P  .P
1838  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1839  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 1823  letter R, for example: Line 1885  letter R, for example:
1885  .sp  .sp
1886    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
1887  .sp  .sp
1888  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
1889  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
1890  stack.  stack.
1891  .P  .P
1892  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
1893  patterns are described below.  .\" HTML <a href="#recursion">
1894    .\" </a>
1895    Recursive patterns
1896    .\"
1897    are described below.
1898  .  .
1899  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
1900  .rs  .rs
# Line 1837  If the condition is the string (DEFINE), Line 1903  If the condition is the string (DEFINE),
1903  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
1904  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
1905  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
1906  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
1907    .\" HTML <a href="#subpatternsassubroutines">
1908    .\" </a>
1909    "subroutines"
1910    .\"
1911  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
1912  written like this (ignore whitespace and line breaks):  written like this (ignore whitespace and line breaks):
1913  .sp  .sp
# Line 1847  written like this (ignore whitespace and Line 1917  written like this (ignore whitespace and
1917  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
1918  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
1919  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
1920  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
1921  .P  pattern uses references to the named group to match the four dot-separated
1922  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.  
1923  .  .
1924  .SS "Assertion conditions"  .SS "Assertion conditions"
1925  .rs  .rs
# Line 1908  recursively to the pattern in which it a Line 1976  recursively to the pattern in which it a
1976  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1977  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
1978  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
1979  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.
1980  .P  .P
1981  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
1982  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,
1983  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
1984    .\" HTML <a href="#subpatternsassubroutines">
1985    .\" </a>
1986    "subroutine"
1987    .\"
1988  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
1989  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
1990  .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  
1991  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1992  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
1993  .sp  .sp
1994    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
1995  .sp  .sp
1996  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1997  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
1998  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1999  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2000    to avoid backtracking into sequences of non-parentheses.
2001  .P  .P
2002  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
2003  pattern, so instead you could use this:  pattern, so instead you could use this:
2004  .sp  .sp
2005    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2006  .sp  .sp
2007  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
2008  them instead of the whole pattern.  them instead of the whole pattern.
2009  .P  .P
2010  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
2011  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).
2012  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
2013  most recently opened parentheses preceding the recursion. In other words, a  most recently opened parentheses preceding the recursion. In other words, a
2014  negative number counts capturing parentheses leftwards from the point at which  negative number counts capturing parentheses leftwards from the point at which
# Line 1949  it is encountered. Line 2017  it is encountered.
2017  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2018  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2019  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2020  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2021    .\" </a>
2022    "subroutine"
2023    .\"
2024    calls, as described in the next section.
2025  .P  .P
2026  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2027  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
2028  could rewrite the above example as follows:  could rewrite the above example as follows:
2029  .sp  .sp
2030    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2031  .sp  .sp
2032  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
2033  used.  used.
2034  .P  .P
2035  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2036  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
2037  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
2038  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2039  .sp  .sp
2040    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2041  .sp  .sp
2042  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,
2043  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
2044  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
2045  before failure can be reported.  before failure can be reported.
# Line 1986  documentation). If the pattern above is Line 2058  documentation). If the pattern above is
2058  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
2059  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
2060  .sp  .sp
2061    \e( ( ( (?>[^()]+) | (?R) )* ) \e)    \e( ( ( [^()]++ | (?R) )* ) \e)
2062       ^                        ^       ^                        ^
2063       ^                        ^       ^                        ^
2064  .sp  .sp
# Line 2008  different alternatives for the recursive Line 2080  different alternatives for the recursive
2080  is the actual recursive call.  is the actual recursive call.
2081  .  .
2082  .  .
2083    .\" HTML <a name="recursiondifference"></a>
2084    .SS "Recursion difference from Perl"
2085    .rs
2086    .sp
2087    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2088    treated as an atomic group. That is, once it has matched some of the subject
2089    string, it is never re-entered, even if it contains untried alternatives and
2090    there is a subsequent matching failure. This can be illustrated by the
2091    following pattern, which purports to match a palindromic string that contains
2092    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2093    .sp
2094      ^(.|(.)(?1)\e2)$
2095    .sp
2096    The idea is that it either matches a single character, or two identical
2097    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2098    it does not if the pattern is longer than three characters. Consider the
2099    subject string "abcba":
2100    .P
2101    At the top level, the first character is matched, but as it is not at the end
2102    of the string, the first alternative fails; the second alternative is taken
2103    and the recursion kicks in. The recursive call to subpattern 1 successfully
2104    matches the next character ("b"). (Note that the beginning and end of line
2105    tests are not part of the recursion).
2106    .P
2107    Back at the top level, the next character ("c") is compared with what
2108    subpattern 2 matched, which was "a". This fails. Because the recursion is
2109    treated as an atomic group, there are now no backtracking points, and so the
2110    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2111    try the second alternative.) However, if the pattern is written with the
2112    alternatives in the other order, things are different:
2113    .sp
2114      ^((.)(?1)\e2|.)$
2115    .sp
2116    This time, the recursing alternative is tried first, and continues to recurse
2117    until it runs out of characters, at which point the recursion fails. But this
2118    time we do have another alternative to try at the higher level. That is the big
2119    difference: in the previous case the remaining alternative is at a deeper
2120    recursion level, which PCRE cannot use.
2121    .P
2122    To change the pattern so that matches all palindromic strings, not just those
2123    with an odd number of characters, it is tempting to change the pattern to this:
2124    .sp
2125      ^((.)(?1)\e2|.?)$
2126    .sp
2127    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2128    deeper recursion has matched a single character, it cannot be entered again in
2129    order to match an empty string. The solution is to separate the two cases, and
2130    write out the odd and even cases as alternatives at the higher level:
2131    .sp
2132      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2133    .sp
2134    If you want to match typical palindromic phrases, the pattern has to ignore all
2135    non-word characters, which can be done like this:
2136    .sp
2137      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$
2138    .sp
2139    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2140    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2141    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2142    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2143    more) to match typical phrases, and Perl takes so long that you think it has
2144    gone into a loop.
2145    .P
2146    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2147    string does not start with a palindrome that is shorter than the entire string.
2148    For example, although "abcba" is correctly matched, if the subject is "ababa",
2149    PCRE finds the palindrome "aba" at the start, then fails at top level because
2150    the end of the string does not follow. Once again, it cannot jump back into the
2151    recursion to try other alternatives, so the entire match fails.
2152    .
2153    .
2154  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2155  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2156  .rs  .rs
# Line 2053  processing option does not affect the ca Line 2196  processing option does not affect the ca
2196  .SH "ONIGURUMA SUBROUTINE SYNTAX"  .SH "ONIGURUMA SUBROUTINE SYNTAX"
2197  .rs  .rs
2198  .sp  .sp
2199  For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or  For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2200  a number enclosed either in angle brackets or single quotes, is an alternative  a number enclosed either in angle brackets or single quotes, is an alternative
2201  syntax for referencing a subpattern as a subroutine, possibly recursively. Here  syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2202  are two of the examples used above, rewritten using this syntax:  are two of the examples used above, rewritten using this syntax:
2203  .sp  .sp
2204    (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )    (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2205    (sens|respons)e and \eg'1'ibility    (sens|respons)e and \eg'1'ibility
2206  .sp  .sp
2207  PCRE supports an extension to Oniguruma: if a number is preceded by a  PCRE supports an extension to Oniguruma: if a number is preceded by a
2208  plus or a minus sign it is taken as a relative reference. For example:  plus or a minus sign it is taken as a relative reference. For example:
2209  .sp  .sp
2210    (abc)(?i:\eg<-1>)    (abc)(?i:\eg<-1>)
2211  .sp  .sp
2212  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2213  synonymous. The former is a back reference; the latter is a subroutine call.  synonymous. The former is a back reference; the latter is a subroutine call.
2214  .  .
2215  .  .
# Line 2115  or removal in a future version of Perl". Line 2258  or removal in a future version of Perl".
2258  production code should be noted to avoid problems during upgrades." The same  production code should be noted to avoid problems during upgrades." The same
2259  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2260  .P  .P
2261  Since these verbs are specifically related to backtracking, they can be used  Since these verbs are specifically related to backtracking, most of them can be
2262  only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a  used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2263  backtracking algorithm. They cause an error if encountered by  a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2264    failing negative assertion, they cause an error if encountered by
2265  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2266  .P  .P
2267    If any of these verbs are used in an assertion subpattern, their effect is
2268    confined to that subpattern; it does not extend to the surrounding pattern.
2269    Note that assertion subpatterns are processed as anchored at the point where
2270    they are tested.
2271    .P
2272  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
2273  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
2274  (*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 2135  The following verbs act as soon as they Line 2284  The following verbs act as soon as they
2284  .sp  .sp
2285  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
2286  pattern. When inside a recursion, only the innermost pattern is ended  pattern. When inside a recursion, only the innermost pattern is ended
2287  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
2288  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:  
2289  .sp  .sp
2290    A(A|B(*ACCEPT)|C)D    A((?:A|B(*ACCEPT)|C)D)
2291  .sp  .sp
2292  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
2293  captured.  the outer parentheses.
2294  .sp  .sp
2295    (*FAIL) or (*F)    (*FAIL) or (*F)
2296  .sp  .sp
# Line 2168  The verbs differ in exactly what kind of Line 2316  The verbs differ in exactly what kind of
2316  .sp  .sp
2317  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
2318  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
2319  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
2320  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
2321  starting point, or not at all. For example:  starting point, or not at all. For example:
2322  .sp  .sp
# Line 2200  was matched leading up to it cannot be p Line 2348  was matched leading up to it cannot be p
2348  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
2349  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
2350  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
2351  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
2352  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2353  instead of skipping on to "c".  instead of skipping on to "c".
2354  .sp  .sp
# Line 2222  is used outside of any alternation, it a Line 2370  is used outside of any alternation, it a
2370  .SH "SEE ALSO"  .SH "SEE ALSO"
2371  .rs  .rs
2372  .sp  .sp
2373  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2374    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2375  .  .
2376  .  .
2377  .SH AUTHOR  .SH AUTHOR
# Line 2239  Cambridge CB2 3QH, England. Line 2388  Cambridge CB2 3QH, England.
2388  .rs  .rs
2389  .sp  .sp
2390  .nf  .nf
2391  Last updated: 12 April 2008  Last updated: 30 September 2009
2392  Copyright (c) 1997-2008 University of Cambridge.  Copyright (c) 1997-2009 University of Cambridge.
2393  .fi  .fi

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