Version reporting & module ABI change.

[users/jgh/exim.git] / doc / doc-txt / pcrepattern.txt

diff --git a/doc/doc-txt/pcrepattern.txt b/doc/doc-txt/pcrepattern.txt
index 1dc800af4e3fd35882b0ba8d9c69d98a714900f6..bfc1cab4cd9c6c7dfa60e6ab6b340ebfd6dc7c33 100644 (file)
--- a/doc/doc-txt/pcrepattern.txt
+++ b/doc/doc-txt/pcrepattern.txt
@@ -1,18 +1,18 @@
-This file contains the PCRE man page that describes the regular expressions 
-supported by PCRE version 5.0. Note that not all of the features are relevant 
+This file contains the PCRE man page that describes the regular expressions
+supported by PCRE version 7.2. Note that not all of the features are relevant

 in the context of Exim. In particular, the version of PCRE that is compiled
 with Exim does not include UTF-8 support, there is no mechanism for changing
 the options with which the PCRE functions are called, and features such as
 callout are not accessible.
 -----------------------------------------------------------------------------
 
 in the context of Exim. In particular, the version of PCRE that is compiled
 with Exim does not include UTF-8 support, there is no mechanism for changing
 the options with which the PCRE functions are called, and features such as
 callout are not accessible.
 -----------------------------------------------------------------------------
 

-PCRE(3)                                                                PCRE(3)
-
+PCREPATTERN(3)                                                  PCREPATTERN(3)

 
 
 NAME
        PCRE - Perl-compatible regular expressions
 
 
 
 NAME
        PCRE - Perl-compatible regular expressions
 

+

 PCRE REGULAR EXPRESSION DETAILS
 
        The  syntax  and semantics of the regular expressions supported by PCRE
 PCRE REGULAR EXPRESSION DETAILS
 
        The  syntax  and semantics of the regular expressions supported by PCRE


@@ -30,23 +30,44 @@ PCRE REGULAR EXPRESSION DETAILS
        of  UTF-8  features  in  the  section on UTF-8 support in the main pcre
        page.
 
        of  UTF-8  features  in  the  section on UTF-8 support in the main pcre
        page.
 

-       A regular expression is a pattern that is  matched  against  a  subject
-       string  from  left  to right. Most characters stand for themselves in a
-       pattern, and match the corresponding characters in the  subject.  As  a
+       The remainder of this document discusses the  patterns  that  are  sup-
+       ported  by  PCRE when its main matching function, pcre_exec(), is used.
+       From  release  6.0,   PCRE   offers   a   second   matching   function,
+       pcre_dfa_exec(),  which matches using a different algorithm that is not
+       Perl-compatible. Some of the features discussed below are not available
+       when  pcre_dfa_exec()  is used. The advantages and disadvantages of the
+       alternative function, and how it differs from the normal function,  are
+       discussed in the pcrematching page.
+
+
+CHARACTERS AND METACHARACTERS
+
+       A  regular  expression  is  a pattern that is matched against a subject
+       string from left to right. Most characters stand for  themselves  in  a
+       pattern,  and  match  the corresponding characters in the subject. As a

        trivial example, the pattern
 
          The quick brown fox
 
        trivial example, the pattern
 
          The quick brown fox
 

-       matches  a portion of a subject string that is identical to itself. The
-       power of regular expressions comes from the ability to include alterna-
-       tives  and repetitions in the pattern. These are encoded in the pattern
-       by the use of metacharacters, which do not  stand  for  themselves  but
-       instead are interpreted in some special way.
+       matches a portion of a subject string that is identical to itself. When
+       caseless  matching is specified (the PCRE_CASELESS option), letters are
+       matched independently of case. In UTF-8 mode, PCRE  always  understands
+       the  concept  of case for characters whose values are less than 128, so
+       caseless matching is always possible. For characters with  higher  val-
+       ues,  the concept of case is supported if PCRE is compiled with Unicode
+       property support, but not otherwise.   If  you  want  to  use  caseless
+       matching  for  characters  128  and above, you must ensure that PCRE is
+       compiled with Unicode property support as well as with UTF-8 support.
+
+       The power of regular expressions comes  from  the  ability  to  include
+       alternatives  and  repetitions in the pattern. These are encoded in the
+       pattern by the use of metacharacters, which do not stand for themselves
+       but instead are interpreted in some special way.

 
        There  are  two different sets of metacharacters: those that are recog-
        nized anywhere in the pattern except within square brackets, and  those
 
        There  are  two different sets of metacharacters: those that are recog-
        nized anywhere in the pattern except within square brackets, and  those

-       that  are  recognized  in square brackets. Outside square brackets, the
-       metacharacters are as follows:
+       that  are  recognized  within square brackets. Outside square brackets,
+       the metacharacters are as follows:

 
          \      general escape character with several uses
          ^      assert start of string (or line, in multiline mode)
 
          \      general escape character with several uses
          ^      assert start of string (or line, in multiline mode)


@@ -93,9 +114,9 @@ BACKSLASH
 
        If a pattern is compiled with the PCRE_EXTENDED option,  whitespace  in
        the  pattern (other than in a character class) and characters between a
 
        If a pattern is compiled with the PCRE_EXTENDED option,  whitespace  in
        the  pattern (other than in a character class) and characters between a

-       # outside a character class and the next newline character are ignored.
-       An  escaping backslash can be used to include a whitespace or # charac-
-       ter as part of the pattern.
+       # outside a character class and the next newline are ignored. An escap-
+       ing  backslash  can  be  used to include a whitespace or # character as
+       part of the pattern.

 
        If you want to remove the special meaning from a  sequence  of  charac-
        ters,  you can do so by putting them between \Q and \E. This is differ-
 
        If you want to remove the special meaning from a  sequence  of  charac-
        ters,  you can do so by putting them between \Q and \E. This is differ-


@@ -131,7 +152,7 @@ BACKSLASH
          \t        tab (hex 09)
          \ddd      character with octal code ddd, or backreference
          \xhh      character with hex code hh
          \t        tab (hex 09)
          \ddd      character with octal code ddd, or backreference
          \xhh      character with hex code hh

-         \x{hhh..} character with hex code hhh... (UTF-8 mode only)
+         \x{hhh..} character with hex code hhh..

 
        The precise effect of \cx is as follows: if x is a lower  case  letter,
        it  is converted to upper case. Then bit 6 of the character (hex 40) is
 
        The precise effect of \cx is as follows: if x is a lower  case  letter,
        it  is converted to upper case. Then bit 6 of the character (hex 40) is


@@ -139,26 +160,24 @@ BACKSLASH
        becomes hex 7B.
 
        After  \x, from zero to two hexadecimal digits are read (letters can be
        becomes hex 7B.
 
        After  \x, from zero to two hexadecimal digits are read (letters can be

-       in upper or lower case). In UTF-8 mode, any number of hexadecimal  dig-
-       its  may  appear between \x{ and }, but the value of the character code
-       must be less than 2**31 (that is,  the  maximum  hexadecimal  value  is
-       7FFFFFFF).  If  characters other than hexadecimal digits appear between
-       \x{ and }, or if there is no terminating }, this form of escape is  not
-       recognized. Instead, the initial \x will be interpreted as a basic hex-
-       adecimal escape, with no following digits,  giving  a  character  whose
-       value is zero.
+       in upper or lower case). Any number of hexadecimal  digits  may  appear
+       between  \x{  and  },  but the value of the character code must be less
+       than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode (that is,
+       the  maximum  hexadecimal  value is 7FFFFFFF). If characters other than
+       hexadecimal digits appear between \x{ and }, or if there is  no  termi-
+       nating  }, this form of escape is not recognized.  Instead, the initial
+       \x will be interpreted as a basic hexadecimal escape, with no following
+       digits, giving a character whose value is zero.

 
        Characters whose value is less than 256 can be defined by either of the
 
        Characters whose value is less than 256 can be defined by either of the

-       two syntaxes for \x when PCRE is in UTF-8 mode. There is no  difference
-       in  the  way they are handled. For example, \xdc is exactly the same as
-       \x{dc}.
+       two syntaxes for \x. There is no difference in the way  they  are  han-
+       dled. For example, \xdc is exactly the same as \x{dc}.

 
 

-       After \0 up to two further octal digits are read.  In  both  cases,  if
-       there  are fewer than two digits, just those that are present are used.
-       Thus the sequence \0\x\07 specifies two binary zeros followed by a  BEL
-       character  (code  value  7).  Make sure you supply two digits after the
-       initial zero if the pattern character that follows is itself  an  octal
-       digit.
+       After  \0  up  to two further octal digits are read. If there are fewer
+       than two digits, just  those  that  are  present  are  used.  Thus  the
+       sequence \0\x\07 specifies two binary zeros followed by a BEL character
+       (code value 7). Make sure you supply two digits after the initial  zero
+       if the pattern character that follows is itself an octal digit.

 
        The handling of a backslash followed by a digit other than 0 is compli-
        cated.  Outside a character class, PCRE reads it and any following dig-
 
        The handling of a backslash followed by a digit other than 0 is compli-
        cated.  Outside a character class, PCRE reads it and any following dig-


@@ -170,9 +189,11 @@ BACKSLASH
 
        Inside  a  character  class, or if the decimal number is greater than 9
        and there have not been that many capturing subpatterns, PCRE  re-reads
 
        Inside  a  character  class, or if the decimal number is greater than 9
        and there have not been that many capturing subpatterns, PCRE  re-reads

-       up  to three octal digits following the backslash, and generates a sin-
-       gle byte from the least significant 8 bits of the value. Any subsequent
-       digits stand for themselves.  For example:
+       up to three octal digits following the backslash, and uses them to gen-
+       erate a data character. Any subsequent digits stand for themselves.  In
+       non-UTF-8  mode,  the  value  of a character specified in octal must be
+       less than \400. In UTF-8 mode, values up to  \777  are  permitted.  For
+       example:

 
          \040   is another way of writing a space
          \40    is the same, provided there are fewer than 40
 
          \040   is another way of writing a space
          \40    is the same, provided there are fewer than 40


@@ -192,76 +213,174 @@ BACKSLASH
        Note  that  octal  values of 100 or greater must not be introduced by a
        leading zero, because no more than three octal digits are ever read.
 
        Note  that  octal  values of 100 or greater must not be introduced by a
        leading zero, because no more than three octal digits are ever read.
 

-       All the sequences that define a single byte value  or  a  single  UTF-8
-       character (in UTF-8 mode) can be used both inside and outside character
-       classes. In addition, inside a character  class,  the  sequence  \b  is
-       interpreted as the backspace character (hex 08), and the sequence \X is
-       interpreted as the character "X".  Outside  a  character  class,  these
-       sequences have different meanings (see below).
+       All the sequences that define a single character value can be used both
+       inside  and  outside character classes. In addition, inside a character
+       class, the sequence \b is interpreted as the backspace  character  (hex
+       08),  and the sequences \R and \X are interpreted as the characters "R"
+       and "X", respectively. Outside a character class, these sequences  have
+       different meanings (see below).
+
+   Absolute and relative back references
+
+       The  sequence  \g followed by a positive or negative number, optionally
+       enclosed in braces, is an absolute or relative back reference. A  named
+       back  reference can be coded as \g{name}. Back references are discussed
+       later, following the discussion of parenthesized subpatterns.

 
    Generic character types
 
 
    Generic character types
 

-       The  third  use of backslash is for specifying generic character types.
-       The following are always recognized:
+       Another use of backslash is for specifying generic character types. The
+       following are always recognized:

 
          \d     any decimal digit
          \D     any character that is not a decimal digit
 
          \d     any decimal digit
          \D     any character that is not a decimal digit

+         \h     any horizontal whitespace character
+         \H     any character that is not a horizontal whitespace character

          \s     any whitespace character
          \S     any character that is not a whitespace character
          \s     any whitespace character
          \S     any character that is not a whitespace character

+         \v     any vertical whitespace character
+         \V     any character that is not a vertical whitespace character

          \w     any "word" character
          \W     any "non-word" character
 
        Each pair of escape sequences partitions the complete set of characters
          \w     any "word" character
          \W     any "non-word" character
 
        Each pair of escape sequences partitions the complete set of characters

-       into  two disjoint sets. Any given character matches one, and only one,
+       into two disjoint sets. Any given character matches one, and only  one,

        of each pair.
 
        These character type sequences can appear both inside and outside char-
        of each pair.
 
        These character type sequences can appear both inside and outside char-

-       acter  classes.  They each match one character of the appropriate type.
-       If the current matching point is at the end of the subject string,  all
+       acter classes. They each match one character of the  appropriate  type.
+       If  the current matching point is at the end of the subject string, all

        of them fail, since there is no character to match.
 
        of them fail, since there is no character to match.
 

-       For  compatibility  with Perl, \s does not match the VT character (code
-       11).  This makes it different from the the POSIX "space" class. The  \s
-       characters are HT (9), LF (10), FF (12), CR (13), and space (32).
+       For compatibility with Perl, \s does not match the VT  character  (code
+       11).   This makes it different from the the POSIX "space" class. The \s
+       characters are HT (9), LF (10), FF (12), CR (13), and  space  (32).  If
+       "use locale;" is included in a Perl script, \s may match the VT charac-
+       ter. In PCRE, it never does.

 
 

-       A "word" character is an underscore or any character less than 256 that
-       is a letter or digit. The definition of  letters  and  digits  is  con-
-       trolled  by PCRE's low-valued character tables, and may vary if locale-
-       specific matching is taking place (see "Locale support" in the  pcreapi
-       page).  For  example,  in  the  "fr_FR" (French) locale, some character
-       codes greater than 128 are used for accented  letters,  and  these  are
-       matched by \w.
-
-       In  UTF-8 mode, characters with values greater than 128 never match \d,
+       In UTF-8 mode, characters with values greater than 128 never match  \d,

        \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
        \s, or \w, and always match \D, \S, and \W. This is true even when Uni-

-       code character property support is available.
+       code character property support is available.  These  sequences  retain
+       their original meanings from before UTF-8 support was available, mainly
+       for efficiency reasons.
+
+       The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
+       the  other  sequences, these do match certain high-valued codepoints in
+       UTF-8 mode.  The horizontal space characters are:
+
+         U+0009     Horizontal tab
+         U+0020     Space
+         U+00A0     Non-break space
+         U+1680     Ogham space mark
+         U+180E     Mongolian vowel separator
+         U+2000     En quad
+         U+2001     Em quad
+         U+2002     En space
+         U+2003     Em space
+         U+2004     Three-per-em space
+         U+2005     Four-per-em space
+         U+2006     Six-per-em space
+         U+2007     Figure space
+         U+2008     Punctuation space
+         U+2009     Thin space
+         U+200A     Hair space
+         U+202F     Narrow no-break space
+         U+205F     Medium mathematical space
+         U+3000     Ideographic space
+
+       The vertical space characters are:
+
+         U+000A     Linefeed
+         U+000B     Vertical tab
+         U+000C     Formfeed
+         U+000D     Carriage return
+         U+0085     Next line
+         U+2028     Line separator
+         U+2029     Paragraph separator
+
+       A "word" character is an underscore or any character less than 256 that
+       is  a  letter  or  digit.  The definition of letters and digits is con-
+       trolled by PCRE's low-valued character tables, and may vary if  locale-
+       specific  matching is taking place (see "Locale support" in the pcreapi
+       page). For example, in a French locale such  as  "fr_FR"  in  Unix-like
+       systems,  or "french" in Windows, some character codes greater than 128
+       are used for accented letters, and these are matched by \w. The use  of
+       locales with Unicode is discouraged.
+
+   Newline sequences
+
+       Outside  a  character class, the escape sequence \R matches any Unicode
+       newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R  is
+       equivalent to the following:
+
+         (?>\r\n|\n|\x0b|\f|\r|\x85)
+
+       This  is  an  example  of an "atomic group", details of which are given
+       below.  This particular group matches either the two-character sequence
+       CR  followed  by  LF,  or  one  of  the single characters LF (linefeed,
+       U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
+       return, U+000D), or NEL (next line, U+0085). The two-character sequence
+       is treated as a single unit that cannot be split.
+
+       In UTF-8 mode, two additional characters whose codepoints  are  greater
+       than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
+       rator, U+2029).  Unicode character property support is not  needed  for
+       these characters to be recognized.
+
+       Inside a character class, \R matches the letter "R".

 
    Unicode character properties
 
        When PCRE is built with Unicode character property support, three addi-
 
    Unicode character properties
 
        When PCRE is built with Unicode character property support, three addi-

-       tional escape sequences to match generic character types are  available
-       when UTF-8 mode is selected. They are:
+       tional escape sequences that match characters with specific  properties
+       are  available.   When not in UTF-8 mode, these sequences are of course
+       limited to testing characters whose codepoints are less than  256,  but
+       they do work in this mode.  The extra escape sequences are:

 
 

-        \p{xx}   a character with the xx property
-        \P{xx}   a character without the xx property
-        \X       an extended Unicode sequence
+         \p{xx}   a character with the xx property
+         \P{xx}   a character without the xx property
+         \X       an extended Unicode sequence

 
        The  property  names represented by xx above are limited to the Unicode
 
        The  property  names represented by xx above are limited to the Unicode

-       general category properties. Each character has exactly one such  prop-
-       erty,  specified  by  a two-letter abbreviation. For compatibility with
-       Perl, negation can be specified by including a circumflex  between  the
-       opening  brace  and the property name. For example, \p{^Lu} is the same
-       as \P{Lu}.
-
-       If only one letter is specified with \p or  \P,  it  includes  all  the
-       properties that start with that letter. In this case, in the absence of
-       negation, the curly brackets in the escape sequence are optional; these
-       two examples have the same effect:
+       script names, the general category properties, and "Any", which matches
+       any character (including newline). Other properties such as "InMusical-
+       Symbols" are not currently supported by PCRE. Note  that  \P{Any}  does
+       not match any characters, so always causes a match failure.
+
+       Sets of Unicode characters are defined as belonging to certain scripts.
+       A character from one of these sets can be matched using a script  name.
+       For example:
+
+         \p{Greek}
+         \P{Han}
+
+       Those  that are not part of an identified script are lumped together as
+       "Common". The current list of scripts is:
+
+       Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,
+       Buhid,   Canadian_Aboriginal,   Cherokee,  Common,  Coptic,  Cuneiform,
+       Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
+       Gothic,  Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
+       gana, Inherited, Kannada,  Katakana,  Kharoshthi,  Khmer,  Lao,  Latin,
+       Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,
+       Ogham, Old_Italic, Old_Persian, Oriya, Osmanya,  Phags_Pa,  Phoenician,
+       Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,
+       Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
+
+       Each character has exactly one general category property, specified  by
+       a two-letter abbreviation. For compatibility with Perl, negation can be
+       specified by including a circumflex between the opening brace  and  the
+       property name. For example, \p{^Lu} is the same as \P{Lu}.
+
+       If only one letter is specified with \p or \P, it includes all the gen-
+       eral category properties that start with that letter. In this case,  in
+       the  absence of negation, the curly brackets in the escape sequence are
+       optional; these two examples have the same effect:

 
          \p{L}
          \pL
 
 
          \p{L}
          \pL
 

-       The following property codes are supported:
+       The following general category property codes are supported:

 
          C     Other
          Cc    Control
 
          C     Other
          Cc    Control


@@ -307,8 +426,17 @@ BACKSLASH
          Zp    Paragraph separator
          Zs    Space separator
 
          Zp    Paragraph separator
          Zs    Space separator
 

-       Extended  properties such as "Greek" or "InMusicalSymbols" are not sup-
-       ported by PCRE.
+       The special property L& is also supported: it matches a character  that
+       has  the  Lu,  Ll, or Lt property, in other words, a letter that is not
+       classified as a modifier or "other".
+
+       The long synonyms for these properties  that  Perl  supports  (such  as
+       \p{Letter})  are  not  supported by PCRE, nor is it permitted to prefix
+       any of these properties with "Is".
+
+       No character that is in the Unicode table has the Cn (unassigned) prop-
+       erty.  Instead, this property is assumed for any code point that is not
+       in the Unicode table.

 
        Specifying caseless matching does not affect  these  escape  sequences.
        For example, \p{Lu} always matches only upper case letters.
 
        Specifying caseless matching does not affect  these  escape  sequences.
        For example, \p{Lu} always matches only upper case letters.


@@ -321,49 +449,70 @@ BACKSLASH
        That is, it matches a character without the "mark"  property,  followed
        by  zero  or  more  characters with the "mark" property, and treats the
        sequence as an atomic group (see below).  Characters  with  the  "mark"
        That is, it matches a character without the "mark"  property,  followed
        by  zero  or  more  characters with the "mark" property, and treats the
        sequence as an atomic group (see below).  Characters  with  the  "mark"

-       property are typically accents that affect the preceding character.
+       property  are  typically  accents  that affect the preceding character.
+       None of them have codepoints less than 256, so  in  non-UTF-8  mode  \X
+       matches any one character.

 
        Matching  characters  by Unicode property is not fast, because PCRE has
        to search a structure that contains  data  for  over  fifteen  thousand
        characters. That is why the traditional escape sequences such as \d and
        \w do not use Unicode properties in PCRE.
 
 
        Matching  characters  by Unicode property is not fast, because PCRE has
        to search a structure that contains  data  for  over  fifteen  thousand
        characters. That is why the traditional escape sequences such as \d and
        \w do not use Unicode properties in PCRE.
 

+   Resetting the match start
+
+       The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
+       ously  matched  characters  not  to  be  included  in the final matched
+       sequence. For example, the pattern:
+
+         foo\Kbar
+
+       matches "foobar", but reports that it has matched "bar".  This  feature
+       is  similar  to  a lookbehind assertion (described below).  However, in
+       this case, the part of the subject before the real match does not  have
+       to  be of fixed length, as lookbehind assertions do. The use of \K does
+       not interfere with the setting of captured  substrings.   For  example,
+       when the pattern
+
+         (foo)\Kbar
+
+       matches "foobar", the first substring is still set to "foo".
+

    Simple assertions
 
    Simple assertions
 

-       The fourth use of backslash is for certain simple assertions. An asser-
-       tion  specifies a condition that has to be met at a particular point in
-       a match, without consuming any characters from the subject string.  The
-       use  of subpatterns for more complicated assertions is described below.
+       The  final use of backslash is for certain simple assertions. An asser-
+       tion specifies a condition that has to be met at a particular point  in
+       a  match, without consuming any characters from the subject string. The
+       use of subpatterns for more complicated assertions is described  below.

        The backslashed assertions are:
 
          \b     matches at a word boundary
          \B     matches when not at a word boundary
        The backslashed assertions are:
 
          \b     matches at a word boundary
          \B     matches when not at a word boundary

-         \A     matches at start of subject
-         \Z     matches at end of subject or before newline at end
-         \z     matches at end of subject
-         \G     matches at first matching position in subject
+         \A     matches at the start of the subject
+         \Z     matches at the end of the subject
+                 also matches before a newline at the end of the subject
+         \z     matches only at the end of the subject
+         \G     matches at the first matching position in the subject

 
 

-       These assertions may not appear in character classes (but note that  \b
+       These  assertions may not appear in character classes (but note that \b

        has a different meaning, namely the backspace character, inside a char-
        acter class).
 
        has a different meaning, namely the backspace character, inside a char-
        acter class).
 

-       A word boundary is a position in the subject string where  the  current
-       character  and  the previous character do not both match \w or \W (i.e.
-       one matches \w and the other matches \W), or the start or  end  of  the
+       A  word  boundary is a position in the subject string where the current
+       character and the previous character do not both match \w or  \W  (i.e.
+       one  matches  \w  and the other matches \W), or the start or end of the

        string if the first or last character matches \w, respectively.
 
        string if the first or last character matches \w, respectively.
 

-       The  \A,  \Z,  and \z assertions differ from the traditional circumflex
+       The \A, \Z, and \z assertions differ from  the  traditional  circumflex

        and dollar (described in the next section) in that they only ever match
        and dollar (described in the next section) in that they only ever match

-       at  the  very start and end of the subject string, whatever options are
-       set. Thus, they are independent of multiline mode. These  three  asser-
+       at the very start and end of the subject string, whatever  options  are
+       set.  Thus,  they are independent of multiline mode. These three asser-

        tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
        tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which

-       affect only the behaviour of the circumflex and dollar  metacharacters.
-       However,  if the startoffset argument of pcre_exec() is non-zero, indi-
+       affect  only the behaviour of the circumflex and dollar metacharacters.
+       However, if the startoffset argument of pcre_exec() is non-zero,  indi-

        cating that matching is to start at a point other than the beginning of
        cating that matching is to start at a point other than the beginning of

-       the  subject,  \A  can never match. The difference between \Z and \z is
-       that \Z matches before a newline that is  the  last  character  of  the
-       string  as well as at the end of the string, whereas \z matches only at
-       the end.
+       the subject, \A can never match. The difference between \Z  and  \z  is
+       that \Z matches before a newline at the end of the string as well as at
+       the very end, whereas \z matches only at the end.

 
        The \G assertion is true only when the current matching position is  at
        the  start point of the match, as specified by the startoffset argument
 
        The \G assertion is true only when the current matching position is  at
        the  start point of the match, as specified by the startoffset argument


@@ -402,57 +551,74 @@ CIRCUMFLEX AND DOLLAR
 
        A dollar character is an assertion that is true  only  if  the  current
        matching  point  is  at  the  end of the subject string, or immediately
 
        A dollar character is an assertion that is true  only  if  the  current
        matching  point  is  at  the  end of the subject string, or immediately

-       before a newline character that is the last character in the string (by
-       default).  Dollar  need  not  be the last character of the pattern if a
-       number of alternatives are involved, but it should be the last item  in
-       any  branch  in  which  it appears.  Dollar has no special meaning in a
-       character class.
-
-       The meaning of dollar can be changed so that it  matches  only  at  the
-       very  end  of  the string, by setting the PCRE_DOLLAR_ENDONLY option at
+       before a newline at the end of the string (by default). Dollar need not
+       be  the  last  character of the pattern if a number of alternatives are
+       involved, but it should be the last item in  any  branch  in  which  it
+       appears. Dollar has no special meaning in a character class.
+
+       The  meaning  of  dollar  can be changed so that it matches only at the
+       very end of the string, by setting the  PCRE_DOLLAR_ENDONLY  option  at

        compile time. This does not affect the \Z assertion.
 
        The meanings of the circumflex and dollar characters are changed if the
        compile time. This does not affect the \Z assertion.
 
        The meanings of the circumflex and dollar characters are changed if the

-       PCRE_MULTILINE option is set. When this is the case, they match immedi-
-       ately after and  immediately  before  an  internal  newline  character,
-       respectively,  in addition to matching at the start and end of the sub-
-       ject string. For example,  the  pattern  /^abc$/  matches  the  subject
-       string  "def\nabc"  (where \n represents a newline character) in multi-
-       line mode, but not otherwise.  Consequently, patterns that are anchored
-       in  single line mode because all branches start with ^ are not anchored
-       in multiline mode, and a match for  circumflex  is  possible  when  the
-       startoffset   argument   of  pcre_exec()  is  non-zero.  The  PCRE_DOL-
-       LAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
+       PCRE_MULTILINE option is set. When  this  is  the  case,  a  circumflex
+       matches  immediately after internal newlines as well as at the start of
+       the subject string. It does not match after a  newline  that  ends  the
+       string.  A dollar matches before any newlines in the string, as well as
+       at the very end, when PCRE_MULTILINE is set. When newline is  specified
+       as  the  two-character  sequence CRLF, isolated CR and LF characters do
+       not indicate newlines.
+
+       For example, the pattern /^abc$/ matches the subject string  "def\nabc"
+       (where  \n  represents a newline) in multiline mode, but not otherwise.
+       Consequently, patterns that are anchored in single  line  mode  because
+       all  branches  start  with  ^ are not anchored in multiline mode, and a
+       match for circumflex is  possible  when  the  startoffset  argument  of
+       pcre_exec()  is  non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
+       PCRE_MULTILINE is set.

 
        Note that the sequences \A, \Z, and \z can be used to match  the  start
        and  end of the subject in both modes, and if all branches of a pattern
 
        Note that the sequences \A, \Z, and \z can be used to match  the  start
        and  end of the subject in both modes, and if all branches of a pattern

-       start with \A it is always anchored, whether PCRE_MULTILINE is  set  or
-       not.
+       start with \A it is always anchored, whether or not  PCRE_MULTILINE  is
+       set.

 
 
 FULL STOP (PERIOD, DOT)
 
        Outside a character class, a dot in the pattern matches any one charac-
 
 
 FULL STOP (PERIOD, DOT)
 
        Outside a character class, a dot in the pattern matches any one charac-

-       ter in the subject, including a non-printing  character,  but  not  (by
-       default)  newline.   In  UTF-8 mode, a dot matches any UTF-8 character,
-       which might be more than one byte long, except (by default) newline. If
-       the  PCRE_DOTALL  option  is set, dots match newlines as well. The han-
-       dling of dot is entirely independent of the handling of circumflex  and
-       dollar,  the  only  relationship  being  that they both involve newline
-       characters. Dot has no special meaning in a character class.
+       ter in the subject string except (by default) a character  that  signi-
+       fies  the  end  of  a line. In UTF-8 mode, the matched character may be
+       more than one byte long.
+
+       When a line ending is defined as a single character, dot never  matches
+       that  character; when the two-character sequence CRLF is used, dot does
+       not match CR if it is immediately followed  by  LF,  but  otherwise  it
+       matches  all characters (including isolated CRs and LFs). When any Uni-
+       code line endings are being recognized, dot does not match CR or LF  or
+       any of the other line ending characters.
+
+       The  behaviour  of  dot  with regard to newlines can be changed. If the
+       PCRE_DOTALL option is set, a dot matches  any  one  character,  without
+       exception. If the two-character sequence CRLF is present in the subject
+       string, it takes two dots to match it.
+
+       The handling of dot is entirely independent of the handling of  circum-
+       flex  and  dollar,  the  only relationship being that they both involve
+       newlines. Dot has no special meaning in a character class.

 
 
 MATCHING A SINGLE BYTE
 
        Outside a character class, the escape sequence \C matches any one byte,
 
 
 MATCHING A SINGLE BYTE
 
        Outside a character class, the escape sequence \C matches any one byte,

-       both  in  and  out of UTF-8 mode. Unlike a dot, it can match a newline.
-       The feature is provided in Perl in order to match individual  bytes  in
-       UTF-8  mode.  Because  it  breaks  up  UTF-8 characters into individual
-       bytes, what remains in the string may be a malformed UTF-8 string.  For
-       this reason, the \C escape sequence is best avoided.
-
-       PCRE  does  not  allow \C to appear in lookbehind assertions (described
-       below), because in UTF-8 mode this would make it impossible  to  calcu-
+       both  in  and  out  of  UTF-8 mode. Unlike a dot, it always matches any
+       line-ending characters. The feature is provided in  Perl  in  order  to
+       match  individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
+       acters into individual bytes, what remains in the string may be a  mal-
+       formed  UTF-8  string.  For this reason, the \C escape sequence is best
+       avoided.
+
+       PCRE does not allow \C to appear in  lookbehind  assertions  (described
+       below),  because  in UTF-8 mode this would make it impossible to calcu-

        late the length of the lookbehind.
 
 
        late the length of the lookbehind.
 
 


@@ -461,39 +627,46 @@ SQUARE BRACKETS AND CHARACTER CLASSES
        An opening square bracket introduces a character class, terminated by a
        closing square bracket. A closing square bracket on its own is not spe-
        cial. If a closing square bracket is required as a member of the class,
        An opening square bracket introduces a character class, terminated by a
        closing square bracket. A closing square bracket on its own is not spe-
        cial. If a closing square bracket is required as a member of the class,

-       it should be the first data character in the class  (after  an  initial
+       it  should  be  the first data character in the class (after an initial

        circumflex, if present) or escaped with a backslash.
 
        circumflex, if present) or escaped with a backslash.
 

-       A  character  class matches a single character in the subject. In UTF-8
-       mode, the character may occupy more than one byte. A matched  character
+       A character class matches a single character in the subject.  In  UTF-8
+       mode,  the character may occupy more than one byte. A matched character

        must be in the set of characters defined by the class, unless the first
        must be in the set of characters defined by the class, unless the first

-       character in the class definition is a circumflex, in  which  case  the
-       subject  character  must  not  be in the set defined by the class. If a
-       circumflex is actually required as a member of the class, ensure it  is
+       character  in  the  class definition is a circumflex, in which case the
+       subject character must not be in the set defined by  the  class.  If  a
+       circumflex  is actually required as a member of the class, ensure it is

        not the first character, or escape it with a backslash.
 
        not the first character, or escape it with a backslash.
 

-       For  example, the character class [aeiou] matches any lower case vowel,
-       while [^aeiou] matches any character that is not a  lower  case  vowel.
+       For example, the character class [aeiou] matches any lower case  vowel,
+       while  [^aeiou]  matches  any character that is not a lower case vowel.

        Note that a circumflex is just a convenient notation for specifying the
        Note that a circumflex is just a convenient notation for specifying the

-       characters that are in the class by enumerating those that are  not.  A
-       class  that starts with a circumflex is not an assertion: it still con-
-       sumes a character from the subject string, and therefore  it  fails  if
+       characters  that  are in the class by enumerating those that are not. A
+       class that starts with a circumflex is not an assertion: it still  con-
+       sumes  a  character  from the subject string, and therefore it fails if

        the current pointer is at the end of the string.
 
        the current pointer is at the end of the string.
 

-       In  UTF-8 mode, characters with values greater than 255 can be included
-       in a class as a literal string of bytes, or by using the  \x{  escaping
+       In UTF-8 mode, characters with values greater than 255 can be  included
+       in  a  class as a literal string of bytes, or by using the \x{ escaping

        mechanism.
 
        mechanism.
 

-       When  caseless  matching  is set, any letters in a class represent both
-       their upper case and lower case versions, so for  example,  a  caseless
-       [aeiou]  matches  "A"  as well as "a", and a caseless [^aeiou] does not
-       match "A", whereas a caseful version would. When running in UTF-8 mode,
-       PCRE  supports  the  concept of case for characters with values greater
-       than 128 only when it is compiled with Unicode property support.
-
-       The newline character is never treated in any special way in  character
-       classes,  whatever  the  setting  of  the PCRE_DOTALL or PCRE_MULTILINE
-       options is. A class such as [^a] will always match a newline.
+       When caseless matching is set, any letters in a  class  represent  both
+       their  upper  case  and lower case versions, so for example, a caseless
+       [aeiou] matches "A" as well as "a", and a caseless  [^aeiou]  does  not
+       match  "A", whereas a caseful version would. In UTF-8 mode, PCRE always
+       understands the concept of case for characters whose  values  are  less
+       than  128, so caseless matching is always possible. For characters with
+       higher values, the concept of case is supported  if  PCRE  is  compiled
+       with  Unicode  property support, but not otherwise.  If you want to use
+       caseless matching for characters 128 and above, you  must  ensure  that
+       PCRE  is  compiled  with Unicode property support as well as with UTF-8
+       support.
+
+       Characters that might indicate line breaks are  never  treated  in  any
+       special  way  when  matching  character  classes,  whatever line-ending
+       sequence is in  use,  and  whatever  setting  of  the  PCRE_DOTALL  and
+       PCRE_MULTILINE options is used. A class such as [^a] always matches one
+       of these characters.

 
        The minus (hyphen) character can be used to specify a range of  charac-
        ters  in  a  character  class.  For  example,  [d-m] matches any letter
 
        The minus (hyphen) character can be used to specify a range of  charac-
        ters  in  a  character  class.  For  example,  [d-m] matches any letter


@@ -519,7 +692,7 @@ SQUARE BRACKETS AND CHARACTER CLASSES
        If a range that includes letters is used when caseless matching is set,
        it matches the letters in either case. For example, [W-c] is equivalent
        to  [][\\^_`wxyzabc],  matched  caselessly,  and  in non-UTF-8 mode, if
        If a range that includes letters is used when caseless matching is set,
        it matches the letters in either case. For example, [W-c] is equivalent
        to  [][\\^_`wxyzabc],  matched  caselessly,  and  in non-UTF-8 mode, if

-       character tables for the "fr_FR" locale are in use, [\xc8-\xcb] matches
+       character tables for a French locale are in  use,  [\xc8-\xcb]  matches

        accented  E  characters in both cases. In UTF-8 mode, PCRE supports the
        concept of case for characters with values greater than 128  only  when
        it is compiled with Unicode property support.
        accented  E  characters in both cases. In UTF-8 mode, PCRE supports the
        concept of case for characters with values greater than 128  only  when
        it is compiled with Unicode property support.


@@ -594,11 +767,10 @@ VERTICAL BAR
 
        matches  either "gilbert" or "sullivan". Any number of alternatives may
        appear, and an empty  alternative  is  permitted  (matching  the  empty
 
        matches  either "gilbert" or "sullivan". Any number of alternatives may
        appear, and an empty  alternative  is  permitted  (matching  the  empty

-       string).   The  matching  process  tries each alternative in turn, from
-       left to right, and the first one that succeeds is used. If the alterna-
-       tives  are within a subpattern (defined below), "succeeds" means match-
-       ing the rest of the main pattern as well as the alternative in the sub-
-       pattern.
+       string). The matching process tries each alternative in turn, from left
+       to right, and the first one that succeeds is used. If the  alternatives
+       are  within a subpattern (defined below), "succeeds" means matching the
+       rest of the main pattern as well as the alternative in the  subpattern.

 
 
 INTERNAL OPTION SETTING
 
 
 INTERNAL OPTION SETTING


@@ -626,30 +798,28 @@ INTERNAL OPTION SETTING
        PCRE extracts it into the global options (and it will therefore show up
        in data extracted by the pcre_fullinfo() function).
 
        PCRE extracts it into the global options (and it will therefore show up
        in data extracted by the pcre_fullinfo() function).
 

-       An option change within a subpattern affects only that part of the cur-
-       rent pattern that follows it, so
+       An  option  change  within a subpattern (see below for a description of
+       subpatterns) affects only that part of the current pattern that follows
+       it, so

 
          (a(?i)b)c
 
        matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
 
          (a(?i)b)c
 
        matches abc and aBc and no other strings (assuming PCRE_CASELESS is not

-       used).   By  this means, options can be made to have different settings
-       in different parts of the pattern. Any changes made in one  alternative
-       do  carry  on  into subsequent branches within the same subpattern. For
+       used).  By this means, options can be made to have  different  settings
+       in  different parts of the pattern. Any changes made in one alternative
+       do carry on into subsequent branches within the  same  subpattern.  For

        example,
 
          (a(?i)b|c)
 
        example,
 
          (a(?i)b|c)
 

-       matches "ab", "aB", "c", and "C", even though  when  matching  "C"  the
-       first  branch  is  abandoned before the option setting. This is because
-       the effects of option settings happen at compile time. There  would  be
+       matches  "ab",  "aB",  "c",  and "C", even though when matching "C" the
+       first branch is abandoned before the option setting.  This  is  because
+       the  effects  of option settings happen at compile time. There would be

        some very weird behaviour otherwise.
 
        some very weird behaviour otherwise.
 

-       The  PCRE-specific  options PCRE_UNGREEDY and PCRE_EXTRA can be changed
-       in the same way as the Perl-compatible options by using the  characters
-       U  and X respectively. The (?X) flag setting is special in that it must
-       always occur earlier in the pattern than any of the additional features
-       it  turns on, even when it is at top level. It is best to put it at the
-       start.
+       The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and  PCRE_EXTRA
+       can  be changed in the same way as the Perl-compatible options by using
+       the characters J, U and X respectively.

 
 
 SUBPATTERNS
 
 
 SUBPATTERNS


@@ -662,7 +832,7 @@ SUBPATTERNS
          cat(aract|erpillar|)
 
        matches  one  of the words "cat", "cataract", or "caterpillar". Without
          cat(aract|erpillar|)
 
        matches  one  of the words "cat", "cataract", or "caterpillar". Without

-       the parentheses, it would match "cataract",  "erpillar"  or  the  empty
+       the parentheses, it would match  "cataract",  "erpillar"  or  an  empty

        string.
 
        2.  It  sets  up  the  subpattern as a capturing subpattern. This means
        string.
 
        2.  It  sets  up  the  subpattern as a capturing subpattern. This means


@@ -691,9 +861,7 @@ SUBPATTERNS
          the ((?:red|white) (king|queen))
 
        the captured substrings are "white queen" and "queen", and are numbered
          the ((?:red|white) (king|queen))
 
        the captured substrings are "white queen" and "queen", and are numbered

-       1  and 2. The maximum number of capturing subpatterns is 65535, and the
-       maximum depth of nesting of all subpatterns, both  capturing  and  non-
-       capturing, is 200.
+       1 and 2. The maximum number of capturing subpatterns is 65535.

 
        As  a  convenient shorthand, if any option settings are required at the
        start of a non-capturing subpattern,  the  option  letters  may  appear
 
        As  a  convenient shorthand, if any option settings are required at the
        start of a non-capturing subpattern,  the  option  letters  may  appear


@@ -709,21 +877,86 @@ SUBPATTERNS
        "Saturday".
 
 
        "Saturday".
 
 

+DUPLICATE SUBPATTERN NUMBERS
+
+       Perl 5.10 introduced a feature whereby each alternative in a subpattern
+       uses the same numbers for its capturing parentheses. Such a  subpattern
+       starts  with (?| and is itself a non-capturing subpattern. For example,
+       consider this pattern:
+
+         (?|(Sat)ur|(Sun))day
+
+       Because the two alternatives are inside a (?| group, both sets of  cap-
+       turing  parentheses  are  numbered one. Thus, when the pattern matches,
+       you can look at captured substring number  one,  whichever  alternative
+       matched.  This  construct  is useful when you want to capture part, but
+       not all, of one of a number of alternatives. Inside a (?| group, paren-
+       theses  are  numbered as usual, but the number is reset at the start of
+       each branch. The numbers of any capturing buffers that follow the  sub-
+       pattern  start after the highest number used in any branch. The follow-
+       ing example is taken from the Perl documentation.  The  numbers  under-
+       neath show in which buffer the captured content will be stored.
+
+         # before  ---------------branch-reset----------- after
+         / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
+         # 1            2         2  3        2     3     4
+
+       A  backreference  or  a  recursive call to a numbered subpattern always
+       refers to the first one in the pattern with the given number.
+
+       An alternative approach to using this "branch reset" feature is to  use
+       duplicate named subpatterns, as described in the next section.
+
+

 NAMED SUBPATTERNS
 
        Identifying  capturing  parentheses  by number is simple, but it can be
        very hard to keep track of the numbers in complicated  regular  expres-
        sions.  Furthermore,  if  an  expression  is  modified, the numbers may
        change. To help with this difficulty, PCRE supports the naming of  sub-
 NAMED SUBPATTERNS
 
        Identifying  capturing  parentheses  by number is simple, but it can be
        very hard to keep track of the numbers in complicated  regular  expres-
        sions.  Furthermore,  if  an  expression  is  modified, the numbers may
        change. To help with this difficulty, PCRE supports the naming of  sub-

-       patterns,  something  that  Perl  does  not  provide. The Python syntax
-       (?P<name>...) is used. Names consist  of  alphanumeric  characters  and
-       underscores, and must be unique within a pattern.
-
+       patterns. This feature was not added to Perl until release 5.10. Python
+       had the feature earlier, and PCRE introduced it at release  4.0,  using
+       the  Python syntax. PCRE now supports both the Perl and the Python syn-
+       tax.
+
+       In PCRE, a subpattern can be named in one of three  ways:  (?<name>...)
+       or  (?'name'...)  as in Perl, or (?P<name>...) as in Python. References
+       to capturing parentheses from other parts of the pattern, such as back-
+       references,  recursion,  and conditions, can be made by name as well as
+       by number.
+
+       Names consist of up to  32  alphanumeric  characters  and  underscores.

        Named  capturing  parentheses  are  still  allocated numbers as well as
        Named  capturing  parentheses  are  still  allocated numbers as well as

-       names. The PCRE API provides function calls for extracting the name-to-
-       number  translation table from a compiled pattern. There is also a con-
-       venience function for extracting a captured substring by name. For fur-
-       ther details see the pcreapi documentation.
+       names, exactly as if the names were not present. The PCRE API  provides
+       function calls for extracting the name-to-number translation table from
+       a compiled pattern. There is also a convenience function for extracting
+       a captured substring by name.
+
+       By  default, a name must be unique within a pattern, but it is possible
+       to relax this constraint by setting the PCRE_DUPNAMES option at compile
+       time.  This  can  be useful for patterns where only one instance of the
+       named parentheses can match. Suppose you want to match the  name  of  a
+       weekday,  either as a 3-letter abbreviation or as the full name, and in
+       both cases you want to extract the abbreviation. This pattern (ignoring
+       the line breaks) does the job:
+
+         (?<DN>Mon|Fri|Sun)(?:day)?|
+         (?<DN>Tue)(?:sday)?|
+         (?<DN>Wed)(?:nesday)?|
+         (?<DN>Thu)(?:rsday)?|
+         (?<DN>Sat)(?:urday)?
+
+       There  are  five capturing substrings, but only one is ever set after a
+       match.  (An alternative way of solving this problem is to use a "branch
+       reset" subpattern, as described in the previous section.)
+
+       The  convenience  function  for extracting the data by name returns the
+       substring for the first (and in this example, the only)  subpattern  of
+       that  name  that  matched.  This saves searching to find which numbered
+       subpattern it was. If you make a reference to a non-unique  named  sub-
+       pattern  from elsewhere in the pattern, the one that corresponds to the
+       lowest number is used. For further details of the interfaces  for  han-
+       dling named subpatterns, see the pcreapi documentation.

 
 
 REPETITION
 
 
 REPETITION


@@ -732,9 +965,10 @@ REPETITION
        following items:
 
          a literal data character
        following items:
 
          a literal data character

-         the . metacharacter
+         the dot metacharacter

          the \C escape sequence
          the \X escape sequence (in UTF-8 mode with Unicode properties)
          the \C escape sequence
          the \X escape sequence (in UTF-8 mode with Unicode properties)

+         the \R escape sequence

          an escape such as \d that matches a single character
          a character class
          a back reference (see next section)
          an escape such as \d that matches a single character
          a character class
          a back reference (see next section)


@@ -774,8 +1008,8 @@ REPETITION
        The quantifier {0} is permitted, causing the expression to behave as if
        the previous item and the quantifier were not present.
 
        The quantifier {0} is permitted, causing the expression to behave as if
        the previous item and the quantifier were not present.
 

-       For  convenience  (and  historical compatibility) the three most common
-       quantifiers have single-character abbreviations:
+       For  convenience, the three most common quantifiers have single-charac-
+       ter abbreviations:

 
          *    is equivalent to {0,}
          +    is equivalent to {1,}
 
          *    is equivalent to {0,}
          +    is equivalent to {1,}


@@ -827,7 +1061,7 @@ REPETITION
        which matches one digit by preference, but can match two if that is the
        only way the rest of the pattern matches.
 
        which matches one digit by preference, but can match two if that is the
        only way the rest of the pattern matches.
 

-       If the PCRE_UNGREEDY option is set (an option which is not available in
+       If the PCRE_UNGREEDY option is set (an option that is not available  in

        Perl),  the  quantifiers are not greedy by default, but individual ones
        can be made greedy by following them with a  question  mark.  In  other
        words, it inverts the default behaviour.
        Perl),  the  quantifiers are not greedy by default, but individual ones
        can be made greedy by following them with a  question  mark.  In  other
        words, it inverts the default behaviour.


@@ -838,24 +1072,25 @@ REPETITION
        minimum or maximum.
 
        If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
        minimum or maximum.
 
        If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-

-       alent  to Perl's /s) is set, thus allowing the . to match newlines, the
-       pattern is implicitly anchored, because whatever follows will be  tried
-       against  every character position in the subject string, so there is no
-       point in retrying the overall match at any position  after  the  first.
-       PCRE normally treats such a pattern as though it were preceded by \A.
-
-       In  cases  where  it  is known that the subject string contains no new-
-       lines, it is worth setting PCRE_DOTALL in order to  obtain  this  opti-
+       alent  to  Perl's  /s) is set, thus allowing the dot to match newlines,
+       the pattern is implicitly anchored, because whatever  follows  will  be
+       tried  against every character position in the subject string, so there
+       is no point in retrying the overall match at  any  position  after  the
+       first.  PCRE  normally treats such a pattern as though it were preceded
+       by \A.
+
+       In cases where it is known that the subject  string  contains  no  new-
+       lines,  it  is  worth setting PCRE_DOTALL in order to obtain this opti-

        mization, or alternatively using ^ to indicate anchoring explicitly.
 
        mization, or alternatively using ^ to indicate anchoring explicitly.
 

-       However,  there is one situation where the optimization cannot be used.
-       When .*  is inside capturing parentheses that  are  the  subject  of  a
-       backreference  elsewhere in the pattern, a match at the start may fail,
-       and a later one succeed. Consider, for example:
+       However, there is one situation where the optimization cannot be  used.
+       When  .*   is  inside  capturing  parentheses that are the subject of a
+       backreference elsewhere in the pattern, a match at the start  may  fail
+       where a later one succeeds. Consider, for example:

 
          (.*)abc\1
 
 
          (.*)abc\1
 

-       If the subject is "xyz123abc123" the match point is the fourth  charac-
+       If  the subject is "xyz123abc123" the match point is the fourth charac-

        ter. For this reason, such a pattern is not implicitly anchored.
 
        When a capturing subpattern is repeated, the value captured is the sub-
        ter. For this reason, such a pattern is not implicitly anchored.
 
        When a capturing subpattern is repeated, the value captured is the sub-


@@ -864,8 +1099,8 @@ REPETITION
          (tweedle[dume]{3}\s*)+
 
        has matched "tweedledum tweedledee" the value of the captured substring
          (tweedle[dume]{3}\s*)+
 
        has matched "tweedledum tweedledee" the value of the captured substring

-       is  "tweedledee".  However,  if there are nested capturing subpatterns,
-       the corresponding captured values may have been set in previous  itera-
+       is "tweedledee". However, if there are  nested  capturing  subpatterns,
+       the  corresponding captured values may have been set in previous itera-

        tions. For example, after
 
          /(a|(b))+/
        tions. For example, after
 
          /(a|(b))+/


@@ -875,12 +1110,13 @@ REPETITION
 
 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
 
 
 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
 

-       With both maximizing and minimizing repetition, failure of what follows
-       normally causes the repeated item to be re-evaluated to see if  a  dif-
-       ferent number of repeats allows the rest of the pattern to match. Some-
-       times it is useful to prevent this, either to change the nature of  the
-       match,  or  to  cause it fail earlier than it otherwise might, when the
-       author of the pattern knows there is no point in carrying on.
+       With both maximizing ("greedy") and minimizing ("ungreedy"  or  "lazy")
+       repetition,  failure  of what follows normally causes the repeated item
+       to be re-evaluated to see if a different number of repeats  allows  the
+       rest  of  the pattern to match. Sometimes it is useful to prevent this,
+       either to change the nature of the match, or to cause it  fail  earlier
+       than  it otherwise might, when the author of the pattern knows there is
+       no point in carrying on.

 
        Consider, for example, the pattern \d+foo when applied to  the  subject
        line
 
        Consider, for example, the pattern \d+foo when applied to  the  subject
        line


@@ -894,10 +1130,9 @@ ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
        the  means for specifying that once a subpattern has matched, it is not
        to be re-evaluated in this way.
 
        the  means for specifying that once a subpattern has matched, it is not
        to be re-evaluated in this way.
 

-       If we use atomic grouping for the previous example, the  matcher  would
-       give up immediately on failing to match "foo" the first time. The nota-
-       tion is a kind of special parenthesis, starting with  (?>  as  in  this
-       example:
+       If we use atomic grouping for the previous example, the  matcher  gives
+       up  immediately  on failing to match "foo" the first time. The notation
+       is a kind of special parenthesis, starting with (?> as in this example:

 
          (?>\d+)foo
 
 
          (?>\d+)foo
 


@@ -929,76 +1164,127 @@ ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
        Possessive  quantifiers  are  always  greedy;  the   setting   of   the
        PCRE_UNGREEDY option is ignored. They are a convenient notation for the
        simpler forms of atomic group. However, there is no difference  in  the
        Possessive  quantifiers  are  always  greedy;  the   setting   of   the
        PCRE_UNGREEDY option is ignored. They are a convenient notation for the
        simpler forms of atomic group. However, there is no difference  in  the

-       meaning  or  processing  of  a possessive quantifier and the equivalent
-       atomic group.
-
-       The possessive quantifier syntax is an extension to the Perl syntax. It
-       originates in Sun's Java package.
-
-       When  a  pattern  contains an unlimited repeat inside a subpattern that
-       can itself be repeated an unlimited number of  times,  the  use  of  an
-       atomic  group  is  the  only way to avoid some failing matches taking a
+       meaning  of  a  possessive  quantifier and the equivalent atomic group,
+       though there may be a performance  difference;  possessive  quantifiers
+       should be slightly faster.
+
+       The  possessive  quantifier syntax is an extension to the Perl 5.8 syn-
+       tax.  Jeffrey Friedl originated the idea (and the name)  in  the  first
+       edition of his book. Mike McCloskey liked it, so implemented it when he
+       built Sun's Java package, and PCRE copied it from there. It  ultimately
+       found its way into Perl at release 5.10.
+
+       PCRE has an optimization that automatically "possessifies" certain sim-
+       ple pattern constructs. For example, the sequence  A+B  is  treated  as
+       A++B  because  there is no point in backtracking into a sequence of A's
+       when B must follow.
+
+       When a pattern contains an unlimited repeat inside  a  subpattern  that
+       can  itself  be  repeated  an  unlimited number of times, the use of an
+       atomic group is the only way to avoid some  failing  matches  taking  a

        very long time indeed. The pattern
 
          (\D+|<\d+>)*[!?]
 
        very long time indeed. The pattern
 
          (\D+|<\d+>)*[!?]
 

-       matches an unlimited number of substrings that either consist  of  non-
-       digits,  or  digits  enclosed in <>, followed by either ! or ?. When it
+       matches  an  unlimited number of substrings that either consist of non-
+       digits, or digits enclosed in <>, followed by either ! or  ?.  When  it

        matches, it runs quickly. However, if it is applied to
 
          aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
 
        matches, it runs quickly. However, if it is applied to
 
          aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
 

-       it takes a long time before reporting  failure.  This  is  because  the
-       string  can be divided between the internal \D+ repeat and the external
-       * repeat in a large number of ways, and all  have  to  be  tried.  (The
-       example  uses  [!?]  rather than a single character at the end, because
-       both PCRE and Perl have an optimization that allows  for  fast  failure
-       when  a single character is used. They remember the last single charac-
-       ter that is required for a match, and fail early if it is  not  present
-       in  the  string.)  If  the pattern is changed so that it uses an atomic
+       it  takes  a  long  time  before reporting failure. This is because the
+       string can be divided between the internal \D+ repeat and the  external
+       *  repeat  in  a  large  number of ways, and all have to be tried. (The
+       example uses [!?] rather than a single character at  the  end,  because
+       both  PCRE  and  Perl have an optimization that allows for fast failure
+       when a single character is used. They remember the last single  charac-
+       ter  that  is required for a match, and fail early if it is not present
+       in the string.) If the pattern is changed so that  it  uses  an  atomic

        group, like this:
 
          ((?>\D+)|<\d+>)*[!?]
 
        group, like this:
 
          ((?>\D+)|<\d+>)*[!?]
 

-       sequences of non-digits cannot be broken, and failure happens  quickly.
+       sequences  of non-digits cannot be broken, and failure happens quickly.

 
 
 BACK REFERENCES
 
        Outside a character class, a backslash followed by a digit greater than
        0 (and possibly further digits) is a back reference to a capturing sub-
 
 
 BACK REFERENCES
 
        Outside a character class, a backslash followed by a digit greater than
        0 (and possibly further digits) is a back reference to a capturing sub-

-       pattern  earlier  (that is, to its left) in the pattern, provided there
+       pattern earlier (that is, to its left) in the pattern,  provided  there

        have been that many previous capturing left parentheses.
 
        However, if the decimal number following the backslash is less than 10,
        have been that many previous capturing left parentheses.
 
        However, if the decimal number following the backslash is less than 10,

-       it  is  always  taken  as a back reference, and causes an error only if
-       there are not that many capturing left parentheses in the  entire  pat-
-       tern.  In  other words, the parentheses that are referenced need not be
-       to the left of the reference for numbers less than 10. See the  subsec-
-       tion  entitled  "Non-printing  characters" above for further details of
-       the handling of digits following a backslash.
-
-       A back reference matches whatever actually matched the  capturing  sub-
-       pattern  in  the  current subject string, rather than anything matching
+       it is always taken as a back reference, and causes  an  error  only  if
+       there  are  not that many capturing left parentheses in the entire pat-
+       tern. In other words, the parentheses that are referenced need  not  be
+       to  the left of the reference for numbers less than 10. A "forward back
+       reference" of this type can make sense when a  repetition  is  involved
+       and  the  subpattern to the right has participated in an earlier itera-
+       tion.
+
+       It is not possible to have a numerical "forward back  reference"  to  a
+       subpattern  whose  number  is  10  or  more using this syntax because a
+       sequence such as \50 is interpreted as a character  defined  in  octal.
+       See the subsection entitled "Non-printing characters" above for further
+       details of the handling of digits following a backslash.  There  is  no
+       such  problem  when named parentheses are used. A back reference to any
+       subpattern is possible using named parentheses (see below).
+
+       Another way of avoiding the ambiguity inherent in  the  use  of  digits
+       following a backslash is to use the \g escape sequence, which is a fea-
+       ture introduced in Perl 5.10. This escape must be followed by  a  posi-
+       tive  or  a negative number, optionally enclosed in braces. These exam-
+       ples are all identical:
+
+         (ring), \1
+         (ring), \g1
+         (ring), \g{1}
+
+       A positive number specifies an absolute reference without the ambiguity
+       that  is  present  in  the older syntax. It is also useful when literal
+       digits follow the reference. A negative number is a relative reference.
+       Consider this example:
+
+         (abc(def)ghi)\g{-1}
+
+       The sequence \g{-1} is a reference to the most recently started captur-
+       ing subpattern before \g, that is, is it equivalent to  \2.  Similarly,
+       \g{-2} would be equivalent to \1. The use of relative references can be
+       helpful in long patterns, and also in  patterns  that  are  created  by
+       joining together fragments that contain references within themselves.
+
+       A  back  reference matches whatever actually matched the capturing sub-
+       pattern in the current subject string, rather  than  anything  matching

        the subpattern itself (see "Subpatterns as subroutines" below for a way
        of doing that). So the pattern
 
          (sens|respons)e and \1ibility
 
        the subpattern itself (see "Subpatterns as subroutines" below for a way
        of doing that). So the pattern
 
          (sens|respons)e and \1ibility
 

-       matches  "sense and sensibility" and "response and responsibility", but
-       not "sense and responsibility". If caseful matching is in force at  the
-       time  of the back reference, the case of letters is relevant. For exam-
+       matches "sense and sensibility" and "response and responsibility",  but
+       not  "sense and responsibility". If caseful matching is in force at the
+       time of the back reference, the case of letters is relevant. For  exam-

        ple,
 
          ((?i)rah)\s+\1
 
        ple,
 
          ((?i)rah)\s+\1
 

-       matches "rah rah" and "RAH RAH", but not "RAH  rah",  even  though  the
+       matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the

        original capturing subpattern is matched caselessly.
 
        original capturing subpattern is matched caselessly.
 

-       Back  references  to named subpatterns use the Python syntax (?P=name).
-       We could rewrite the above example as follows:
+       There are several different ways of writing back  references  to  named
+       subpatterns.  The  .NET syntax \k{name} and the Perl syntax \k<name> or
+       \k'name' are supported, as is the Python syntax (?P=name). Perl  5.10's
+       unified back reference syntax, in which \g can be used for both numeric
+       and named references, is also supported. We  could  rewrite  the  above
+       example in any of the following ways:

 
 

-         (?<p1>(?i)rah)\s+(?P=p1)
+         (?<p1>(?i)rah)\s+\k<p1>
+         (?'p1'(?i)rah)\s+\k{p1}
+         (?P<p1>(?i)rah)\s+(?P=p1)
+         (?<p1>(?i)rah)\s+\g{p1}
+
+       A  subpattern  that  is  referenced  by  name may appear in the pattern
+       before or after the reference.

 
        There may be more than one back reference to the same subpattern. If  a
        subpattern  has  not actually been used in a particular match, any back
 
        There may be more than one back reference to the same subpattern. If  a
        subpattern  has  not actually been used in a particular match, any back


@@ -1087,8 +1373,8 @@ ASSERTIONS
        does  find  an  occurrence  of "bar" that is not preceded by "foo". The
        contents of a lookbehind assertion are restricted  such  that  all  the
        strings it matches must have a fixed length. However, if there are sev-
        does  find  an  occurrence  of "bar" that is not preceded by "foo". The
        contents of a lookbehind assertion are restricted  such  that  all  the
        strings it matches must have a fixed length. However, if there are sev-

-       eral alternatives, they do not all have to have the same fixed  length.
-       Thus
+       eral top-level alternatives, they do not all  have  to  have  the  same
+       fixed length. Thus

 
          (?<=bullock|donkey)
 
 
          (?<=bullock|donkey)
 


@@ -1110,19 +1396,23 @@ ASSERTIONS
 
          (?<=abc|abde)
 
 
          (?<=abc|abde)
 

+       In some cases, the Perl 5.10 escape sequence \K (see above) can be used
+       instead of a lookbehind assertion; this is not restricted to  a  fixed-
+       length.
+

        The  implementation  of lookbehind assertions is, for each alternative,
        The  implementation  of lookbehind assertions is, for each alternative,

-       to temporarily move the current position back by the  fixed  width  and
+       to temporarily move the current position back by the fixed  length  and

        then try to match. If there are insufficient characters before the cur-
        then try to match. If there are insufficient characters before the cur-

-       rent position, the match is deemed to fail.
+       rent position, the assertion fails.

 
        PCRE does not allow the \C escape (which matches a single byte in UTF-8
        mode)  to appear in lookbehind assertions, because it makes it impossi-
 
        PCRE does not allow the \C escape (which matches a single byte in UTF-8
        mode)  to appear in lookbehind assertions, because it makes it impossi-

-       ble to calculate the length of the lookbehind. The \X escape, which can
-       match different numbers of bytes, is also not permitted.
+       ble to calculate the length of the lookbehind. The \X and  \R  escapes,
+       which can match different numbers of bytes, are also not permitted.

 
 

-       Atomic  groups can be used in conjunction with lookbehind assertions to
-       specify efficient matching at the end of the subject string. Consider a
-       simple pattern such as
+       Possessive  quantifiers  can  be  used  in  conjunction with lookbehind
+       assertions to specify efficient matching at  the  end  of  the  subject
+       string. Consider a simple pattern such as

 
          abcd$
 
 
          abcd$
 


@@ -1139,13 +1429,9 @@ ASSERTIONS
        again  the search for "a" covers the entire string, from right to left,
        so we are no better off. However, if the pattern is written as
 
        again  the search for "a" covers the entire string, from right to left,
        so we are no better off. However, if the pattern is written as
 

-         ^(?>.*)(?<=abcd)
-
-       or, equivalently, using the possessive quantifier syntax,
-

          ^.*+(?<=abcd)
 
          ^.*+(?<=abcd)
 

-       there can be no backtracking for the .* item; it  can  match  only  the
+       there can be no backtracking for the .*+ item; it can  match  only  the

        entire  string.  The subsequent lookbehind assertion does a single test
        on the last four characters. If it fails, the match fails  immediately.
        For  long  strings, this approach makes a significant difference to the
        entire  string.  The subsequent lookbehind assertion does a single test
        on the last four characters. If it fails, the match fails  immediately.
        For  long  strings, this approach makes a significant difference to the


@@ -1200,13 +1486,23 @@ CONDITIONAL SUBPATTERNS
        no-pattern (if present) is used. If there are more  than  two  alterna-
        tives in the subpattern, a compile-time error occurs.
 
        no-pattern (if present) is used. If there are more  than  two  alterna-
        tives in the subpattern, a compile-time error occurs.
 

-       There are three kinds of condition. If the text between the parentheses
-       consists of a sequence of digits, the condition  is  satisfied  if  the
-       capturing  subpattern of that number has previously matched. The number
-       must be greater than zero. Consider the following pattern,  which  con-
-       tains  non-significant white space to make it more readable (assume the
-       PCRE_EXTENDED option) and to divide it into three  parts  for  ease  of
-       discussion:
+       There  are  four  kinds of condition: references to subpatterns, refer-
+       ences to recursion, a pseudo-condition called DEFINE, and assertions.
+
+   Checking for a used subpattern by number
+
+       If the text between the parentheses consists of a sequence  of  digits,
+       the  condition  is  true if the capturing subpattern of that number has
+       previously matched. An alternative notation is to  precede  the  digits
+       with a plus or minus sign. In this case, the subpattern number is rela-
+       tive rather than absolute.  The most recently opened parentheses can be
+       referenced  by  (?(-1),  the  next most recent by (?(-2), and so on. In
+       looping constructs it can also make sense to refer to subsequent groups
+       with constructs such as (?(+2).
+
+       Consider  the  following  pattern, which contains non-significant white
+       space to make it more readable (assume the PCRE_EXTENDED option) and to
+       divide it into three parts for ease of discussion:

 
          ( \( )?    [^()]+    (?(1) \) )
 
 
          ( \( )?    [^()]+    (?(1) \) )
 


@@ -1221,12 +1517,74 @@ CONDITIONAL SUBPATTERNS
        other  words,  this  pattern  matches  a  sequence  of non-parentheses,
        optionally enclosed in parentheses.
 
        other  words,  this  pattern  matches  a  sequence  of non-parentheses,
        optionally enclosed in parentheses.
 

-       If the condition is the string (R), it is satisfied if a recursive call
-       to  the pattern or subpattern has been made. At "top level", the condi-
-       tion is false.  This  is  a  PCRE  extension.  Recursive  patterns  are
-       described in the next section.
+       If you were embedding this pattern in a larger one,  you  could  use  a
+       relative reference:
+
+         ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...
+
+       This  makes  the  fragment independent of the parentheses in the larger
+       pattern.
+
+   Checking for a used subpattern by name
+
+       Perl uses the syntax (?(<name>)...) or (?('name')...)  to  test  for  a
+       used  subpattern  by  name.  For compatibility with earlier versions of
+       PCRE, which had this facility before Perl, the syntax  (?(name)...)  is
+       also  recognized. However, there is a possible ambiguity with this syn-
+       tax, because subpattern names may  consist  entirely  of  digits.  PCRE
+       looks  first for a named subpattern; if it cannot find one and the name
+       consists entirely of digits, PCRE looks for a subpattern of  that  num-
+       ber,  which must be greater than zero. Using subpattern names that con-
+       sist entirely of digits is not recommended.
+
+       Rewriting the above example to use a named subpattern gives this:
+
+         (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )
+
+
+   Checking for pattern recursion
+
+       If the condition is the string (R), and there is no subpattern with the
+       name  R, the condition is true if a recursive call to the whole pattern
+       or any subpattern has been made. If digits or a name preceded by amper-
+       sand follow the letter R, for example:
+
+         (?(R3)...) or (?(R&name)...)
+
+       the  condition is true if the most recent recursion is into the subpat-
+       tern whose number or name is given. This condition does not  check  the
+       entire recursion stack.
+
+       At  "top  level", all these recursion test conditions are false. Recur-
+       sive patterns are described below.
+
+   Defining subpatterns for use by reference only
+
+       If the condition is the string (DEFINE), and  there  is  no  subpattern
+       with  the  name  DEFINE,  the  condition is always false. In this case,
+       there may be only one alternative  in  the  subpattern.  It  is  always
+       skipped  if  control  reaches  this  point  in the pattern; the idea of
+       DEFINE is that it can be used to define "subroutines" that can be  ref-
+       erenced  from elsewhere. (The use of "subroutines" is described below.)
+       For example, a pattern to match an IPv4 address could be  written  like
+       this (ignore whitespace and line breaks):
+
+         (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
+         \b (?&byte) (\.(?&byte)){3} \b
+
+       The  first part of the pattern is a DEFINE group inside which a another
+       group named "byte" is defined. This matches an individual component  of
+       an  IPv4  address  (a number less than 256). When matching takes place,
+       this part of the pattern is skipped because DEFINE acts  like  a  false
+       condition.

 
 

-       If  the  condition  is  not  a sequence of digits or (R), it must be an
+       The rest of the pattern uses references to the named group to match the
+       four dot-separated components of an IPv4 address, insisting on  a  word
+       boundary at each end.
+
+   Assertion conditions
+
+       If  the  condition  is  not  in any of the above formats, it must be an

        assertion.  This may be a positive or negative lookahead or  lookbehind
        assertion.  Consider  this  pattern,  again  containing non-significant
        white space, and with the two alternatives on the second line:
        assertion.  This may be a positive or negative lookahead or  lookbehind
        assertion.  Consider  this  pattern,  again  containing non-significant
        white space, and with the two alternatives on the second line:


@@ -1251,8 +1609,8 @@ COMMENTS
        at all.
 
        If  the PCRE_EXTENDED option is set, an unescaped # character outside a
        at all.
 
        If  the PCRE_EXTENDED option is set, an unescaped # character outside a

-       character class introduces a comment that continues up to the next new-
-       line character in the pattern.
+       character class introduces a  comment  that  continues  to  immediately
+       after the next newline in the pattern.

 
 
 RECURSIVE PATTERNS
 
 
 RECURSIVE PATTERNS


@@ -1261,107 +1619,139 @@ RECURSIVE PATTERNS
        unlimited nested parentheses. Without the use of  recursion,  the  best
        that  can  be  done  is  to use a pattern that matches up to some fixed
        depth of nesting. It is not possible to  handle  an  arbitrary  nesting
        unlimited nested parentheses. Without the use of  recursion,  the  best
        that  can  be  done  is  to use a pattern that matches up to some fixed
        depth of nesting. It is not possible to  handle  an  arbitrary  nesting

-       depth.  Perl  provides  a  facility  that allows regular expressions to
-       recurse (amongst other things). It does this by interpolating Perl code
-       in the expression at run time, and the code can refer to the expression
-       itself. A Perl pattern to solve the parentheses problem can be  created
-       like this:
+       depth.
+
+       For some time, Perl has provided a facility that allows regular expres-
+       sions to recurse (amongst other things). It does this by  interpolating
+       Perl  code in the expression at run time, and the code can refer to the
+       expression itself. A Perl pattern using code interpolation to solve the
+       parentheses problem can be created like this:

 
          $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
 
        The (?p{...}) item interpolates Perl code at run time, and in this case
 
          $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
 
        The (?p{...}) item interpolates Perl code at run time, and in this case

-       refers recursively to the pattern in which it appears. Obviously,  PCRE
-       cannot  support  the  interpolation  of Perl code. Instead, it supports
-       some special syntax for recursion of the entire pattern, and  also  for
-       individual subpattern recursion.
+       refers recursively to the pattern in which it appears.

 
 

-       The  special item that consists of (? followed by a number greater than
+       Obviously, PCRE cannot support the interpolation of Perl code. Instead,
+       it  supports  special  syntax  for recursion of the entire pattern, and
+       also for individual subpattern recursion.  After  its  introduction  in
+       PCRE  and  Python,  this  kind of recursion was introduced into Perl at
+       release 5.10.
+
+       A special item that consists of (? followed by a  number  greater  than

        zero and a closing parenthesis is a recursive call of the subpattern of
        zero and a closing parenthesis is a recursive call of the subpattern of

-       the  given  number, provided that it occurs inside that subpattern. (If
-       not, it is a "subroutine" call, which is described  in  the  next  sec-
-       tion.)  The special item (?R) is a recursive call of the entire regular
-       expression.
+       the given number, provided that it occurs inside that  subpattern.  (If
+       not,  it  is  a  "subroutine" call, which is described in the next sec-
+       tion.) The special item (?R) or (?0) is a recursive call of the  entire
+       regular expression.
+
+       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.

 
 

-       For example, this PCRE pattern solves the  nested  parentheses  problem
-       (assume  the  PCRE_EXTENDED  option  is  set  so  that  white  space is
-       ignored):
+       This PCRE pattern solves the nested  parentheses  problem  (assume  the
+       PCRE_EXTENDED option is set so that white space is ignored):

 
          \( ( (?>[^()]+) | (?R) )* \)
 
 
          \( ( (?>[^()]+) | (?R) )* \)
 

-       First it matches an opening parenthesis. Then it matches any number  of
-       substrings  which  can  either  be  a sequence of non-parentheses, or a
-       recursive match of the pattern itself (that is  a  correctly  parenthe-
+       First  it matches an opening parenthesis. Then it matches any number of
+       substrings which can either be a  sequence  of  non-parentheses,  or  a
+       recursive  match  of the pattern itself (that is, a correctly parenthe-

        sized substring).  Finally there is a closing parenthesis.
 
        sized substring).  Finally there is a closing parenthesis.
 

-       If  this  were  part of a larger pattern, you would not want to recurse
+       If this were part of a larger pattern, you would not  want  to  recurse

        the entire pattern, so instead you could use this:
 
          ( \( ( (?>[^()]+) | (?1) )* \) )
 
        the entire pattern, so instead you could use this:
 
          ( \( ( (?>[^()]+) | (?1) )* \) )
 

-       We have put the pattern into parentheses, and caused the  recursion  to
-       refer  to them instead of the whole pattern. In a larger pattern, keep-
-       ing track of parenthesis numbers can be tricky. It may be  more  conve-
-       nient  to use named parentheses instead. For this, PCRE uses (?P>name),
-       which is an extension to the Python syntax that  PCRE  uses  for  named
-       parentheses (Perl does not provide named parentheses). We could rewrite
-       the above example as follows:
+       We  have  put the pattern into parentheses, and caused the recursion to
+       refer to them instead of the whole pattern.
+
+       In a larger pattern,  keeping  track  of  parenthesis  numbers  can  be
+       tricky.  This is made easier by the use of relative references. (A Perl
+       5.10 feature.)  Instead of (?1) in the  pattern  above  you  can  write
+       (?-2) to refer to the second most recently opened parentheses preceding
+       the recursion. In other  words,  a  negative  number  counts  capturing
+       parentheses leftwards from the point at which it is encountered.

 
 

-         (?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) )
+       It  is  also  possible  to refer to subsequently opened parentheses, by
+       writing references such as (?+2). However, these  cannot  be  recursive
+       because  the  reference  is  not inside the parentheses that are refer-
+       enced. They are always "subroutine" calls, as  described  in  the  next
+       section.

 
 

-       This particular example pattern contains nested unlimited repeats,  and
-       so  the  use of atomic grouping for matching strings of non-parentheses
-       is important when applying the pattern to strings that  do  not  match.
-       For example, when this pattern is applied to
+       An  alternative  approach is to use named parentheses instead. The Perl
+       syntax for this is (?&name); PCRE's earlier syntax  (?P>name)  is  also
+       supported. We could rewrite the above example as follows:
+
+         (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
+
+       If  there  is more than one subpattern with the same name, the earliest
+       one is used.
+
+       This particular example pattern that we have been looking  at  contains
+       nested  unlimited repeats, and so the use of atomic grouping for match-
+       ing strings of non-parentheses is important when applying  the  pattern
+       to strings that do not match. For example, when this pattern is applied
+       to

 
          (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
 
 
          (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
 

-       it  yields "no match" quickly. However, if atomic grouping is not used,
-       the match runs for a very long time indeed because there  are  so  many
-       different  ways  the  + and * repeats can carve up the subject, and all
+       it yields "no match" quickly. However, if atomic grouping is not  used,
+       the  match  runs  for a very long time indeed because there are so many
+       different ways the + and * repeats can carve up the  subject,  and  all

        have to be tested before failure can be reported.
 
        At the end of a match, the values set for any capturing subpatterns are
        those from the outermost level of the recursion at which the subpattern
        have to be tested before failure can be reported.
 
        At the end of a match, the values set for any capturing subpatterns are
        those from the outermost level of the recursion at which the subpattern

-       value is set.  If you want to obtain  intermediate  values,  a  callout
-       function can be used (see the next section and the pcrecallout documen-
-       tation). If the pattern above is matched against
+       value  is  set.   If  you want to obtain intermediate values, a callout
+       function can be used (see below and the pcrecallout documentation).  If
+       the pattern above is matched against

 
          (ab(cd)ef)
 
 
          (ab(cd)ef)
 

-       the value for the capturing parentheses is  "ef",  which  is  the  last
-       value  taken  on at the top level. If additional parentheses are added,
+       the  value  for  the  capturing  parentheses is "ef", which is the last
+       value taken on at the top level. If additional parentheses  are  added,

        giving
 
          \( ( ( (?>[^()]+) | (?R) )* ) \)
             ^                        ^
             ^                        ^
 
        giving
 
          \( ( ( (?>[^()]+) | (?R) )* ) \)
             ^                        ^
             ^                        ^
 

-       the string they capture is "ab(cd)ef", the contents of  the  top  level
-       parentheses.  If there are more than 15 capturing parentheses in a pat-
+       the  string  they  capture is "ab(cd)ef", the contents of the top level
+       parentheses. If there are more than 15 capturing parentheses in a  pat-

        tern, PCRE has to obtain extra memory to store data during a recursion,
        tern, PCRE has to obtain extra memory to store data during a recursion,

-       which  it  does  by  using pcre_malloc, freeing it via pcre_free after-
-       wards. If  no  memory  can  be  obtained,  the  match  fails  with  the
+       which it does by using pcre_malloc, freeing  it  via  pcre_free  after-
+       wards.  If  no  memory  can  be  obtained,  the  match  fails  with the

        PCRE_ERROR_NOMEMORY error.
 
        PCRE_ERROR_NOMEMORY error.
 

-       Do  not  confuse  the (?R) item with the condition (R), which tests for
-       recursion.  Consider this pattern, which matches text in  angle  brack-
-       ets,  allowing for arbitrary nesting. Only digits are allowed in nested
-       brackets (that is, when recursing), whereas any characters are  permit-
+       Do not confuse the (?R) item with the condition (R),  which  tests  for
+       recursion.   Consider  this pattern, which matches text in angle brack-
+       ets, allowing for arbitrary nesting. Only digits are allowed in  nested
+       brackets  (that is, when recursing), whereas any characters are permit-

        ted at the outer level.
 
          < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >
 
        ted at the outer level.
 
          < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >
 

-       In  this  pattern, (?(R) is the start of a conditional subpattern, with
-       two different alternatives for the recursive and  non-recursive  cases.
+       In this pattern, (?(R) is the start of a conditional  subpattern,  with
+       two  different  alternatives for the recursive and non-recursive cases.

        The (?R) item is the actual recursive call.
 
 
 SUBPATTERNS AS SUBROUTINES
 
        If the syntax for a recursive subpattern reference (either by number or
        The (?R) item is the actual recursive call.
 
 
 SUBPATTERNS AS SUBROUTINES
 
        If the syntax for a recursive subpattern reference (either by number or

-       by name) is used outside the parentheses to which it refers,  it  oper-
-       ates  like  a  subroutine in a programming language. An earlier example
-       pointed out that the pattern
+       by  name)  is used outside the parentheses to which it refers, it oper-
+       ates like a subroutine in a programming language. The "called"  subpat-
+       tern may be defined before or after the reference. A numbered reference
+       can be absolute or relative, as in these examples:
+
+         (...(absolute)...)...(?2)...
+         (...(relative)...)...(?-1)...
+         (...(?+1)...(relative)...
+
+       An earlier example pointed out that the pattern

 
          (sens|respons)e and \1ibility
 
 
          (sens|respons)e and \1ibility
 


@@ -1371,8 +1761,22 @@ SUBPATTERNS AS SUBROUTINES
          (sens|respons)e and (?1)ibility
 
        is  used, it does match "sense and responsibility" as well as the other
          (sens|respons)e and (?1)ibility
 
        is  used, it does match "sense and responsibility" as well as the other

-       two strings. Such references must, however, follow  the  subpattern  to
-       which they refer.
+       two strings. Another example is  given  in  the  discussion  of  DEFINE
+       above.
+
+       Like recursive subpatterns, a "subroutine" 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.
+
+       When a subpattern is used as a subroutine, processing options  such  as
+       case-independence are fixed when the subpattern is defined. They cannot
+       be changed for different calls. For example, consider this pattern:
+
+         (abc)(?i:(?-1))
+
+       It matches "abcabc". It does not match "abcABC" because the  change  of
+       processing option does not affect the called subpattern.

 
 
 CALLOUTS
 
 
 CALLOUTS


@@ -1409,5 +1813,20 @@ CALLOUTS
        gether. A complete description of the interface to the callout function
        is given in the pcrecallout documentation.
 
        gether. A complete description of the interface to the callout function
        is given in the pcrecallout documentation.
 

-Last updated: 09 September 2004
-Copyright (c) 1997-2004 University of Cambridge.
+
+SEE ALSO
+
+       pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
+
+
+AUTHOR
+
+       Philip Hazel
+       University Computing Service
+       Cambridge CB2 3QH, England.
+
+
+REVISION
+
+       Last updated: 19 June 2007
+       Copyright (c) 1997-2007 University of Cambridge.