Document 1041 merge (DCC fix).

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

diff --git a/doc/doc-txt/pcrepattern.txt b/doc/doc-txt/pcrepattern.txt
index 9712c86b41d24070ed528080610b078e8f507886..bfc1cab4cd9c6c7dfa60e6ab6b340ebfd6dc7c33 100644 (file)
--- a/doc/doc-txt/pcrepattern.txt
+++ b/doc/doc-txt/pcrepattern.txt
@@ -1,5 +1,5 @@
 This file contains the PCRE man page that describes the regular expressions
 This file contains the PCRE man page that describes the regular expressions

-supported by PCRE version 6.7. Note that not all of the features are relevant
+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
 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


@@ -34,36 +34,40 @@ PCRE REGULAR EXPRESSION DETAILS
        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
        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. The advantages and disadvantages  of  the  alternative
-       function, and how it differs from the normal function, are discussed in
-       the pcrematching page.
+       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.

 
 

-       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
+
+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
 
        matches a portion of a subject string that is identical to itself. When
        trivial example, the pattern
 
          The quick brown fox
 
        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
+       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.
 
        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
+       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.
 
        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
-       that are recognized in square brackets. Outside  square  brackets,  the
-       metacharacters are as follows:
+       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  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)


@@ -81,7 +85,7 @@ PCRE REGULAR EXPRESSION DETAILS
                 also "possessive quantifier"
          {      start min/max quantifier
 
                 also "possessive quantifier"
          {      start min/max quantifier
 

-       Part  of  a  pattern  that is in square brackets is called a "character
+       Part of a pattern that is in square brackets  is  called  a  "character

        class". In a character class the only metacharacters are:
 
          \      general escape character
        class". In a character class the only metacharacters are:
 
          \      general escape character


@@ -91,33 +95,33 @@ PCRE REGULAR EXPRESSION DETAILS
                   syntax)
          ]      terminates the character class
 
                   syntax)
          ]      terminates the character class
 

-       The following sections describe the use of each of the  metacharacters.
+       The  following sections describe the use of each of the metacharacters.

 
 
 BACKSLASH
 
        The backslash character has several uses. Firstly, if it is followed by
 
 
 BACKSLASH
 
        The backslash character has several uses. Firstly, if it is followed by

-       a non-alphanumeric character, it takes away any  special  meaning  that
-       character  may  have.  This  use  of  backslash  as an escape character
+       a  non-alphanumeric  character,  it takes away any special meaning that
+       character may have. This  use  of  backslash  as  an  escape  character

        applies both inside and outside character classes.
 
        applies both inside and outside character classes.
 

-       For example, if you want to match a * character, you write  \*  in  the
-       pattern.   This  escaping  action  applies whether or not the following
-       character would otherwise be interpreted as a metacharacter, so  it  is
-       always  safe  to  precede  a non-alphanumeric with backslash to specify
-       that it stands for itself. In particular, if you want to match a  back-
+       For  example,  if  you want to match a * character, you write \* in the
+       pattern.  This escaping action applies whether  or  not  the  following
+       character  would  otherwise be interpreted as a metacharacter, so it is
+       always safe to precede a non-alphanumeric  with  backslash  to  specify
+       that  it stands for itself. In particular, if you want to match a back-

        slash, you write \\.
 
        slash, you write \\.
 

-       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 are ignored. An escap-
        # outside a character class and the next newline are ignored. An escap-

-       ing backslash can be used to include a whitespace  or  #  character  as
+       ing  backslash  can  be  used to include a whitespace or # character as

        part of the pattern.
 
        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-
-       ent  from  Perl  in  that  $  and  @ are handled as literals in \Q...\E
-       sequences in PCRE, whereas in Perl, $ and @ cause  variable  interpola-
+       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-
+       ent from Perl in that $ and  @  are  handled  as  literals  in  \Q...\E
+       sequences  in  PCRE, whereas in Perl, $ and @ cause variable interpola-

        tion. Note the following examples:
 
          Pattern            PCRE matches   Perl matches
        tion. Note the following examples:
 
          Pattern            PCRE matches   Perl matches


@@ -127,16 +131,16 @@ BACKSLASH
          \Qabc\$xyz\E       abc\$xyz       abc\$xyz
          \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
 
          \Qabc\$xyz\E       abc\$xyz       abc\$xyz
          \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
 

-       The  \Q...\E  sequence  is recognized both inside and outside character
+       The \Q...\E sequence is recognized both inside  and  outside  character

        classes.
 
    Non-printing characters
 
        A second use of backslash provides a way of encoding non-printing char-
        classes.
 
    Non-printing characters
 
        A second use of backslash provides a way of encoding non-printing char-

-       acters  in patterns in a visible manner. There is no restriction on the
-       appearance of non-printing characters, apart from the binary zero  that
-       terminates  a  pattern,  but  when  a pattern is being prepared by text
-       editing, it is usually easier  to  use  one  of  the  following  escape
+       acters in patterns in a visible manner. There is no restriction on  the
+       appearance  of non-printing characters, apart from the binary zero that
+       terminates a pattern, but when a pattern  is  being  prepared  by  text
+       editing,  it  is  usually  easier  to  use  one of the following escape

        sequences than the binary character it represents:
 
          \a        alarm, that is, the BEL character (hex 07)
        sequences than the binary character it represents:
 
          \a        alarm, that is, the BEL character (hex 07)


@@ -150,45 +154,45 @@ BACKSLASH
          \xhh      character with hex code hh
          \x{hhh..} character with hex code hhh..
 
          \xhh      character with hex code hh
          \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
-       inverted.   Thus  \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
+       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
+       inverted.  Thus \cz becomes hex 1A, but \c{ becomes hex 3B,  while  \c;

        becomes hex 7B.
 
        becomes hex 7B.
 

-       After \x, from zero to two hexadecimal digits are read (letters can  be
-       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
+       After  \x, from zero to two hexadecimal digits are read (letters can be
+       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,
        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
+       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
        \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

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

-       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
+       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
        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
+       (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-
        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-

-       its as a decimal number. If the number is less than  10,  or  if  there
+       its  as  a  decimal  number. If the number is less than 10, or if there

        have been at least that many previous capturing left parentheses in the
        have been at least that many previous capturing left parentheses in the

-       expression, the entire  sequence  is  taken  as  a  back  reference.  A
-       description  of how this works is given later, following the discussion
+       expression,  the  entire  sequence  is  taken  as  a  back reference. A
+       description of how this works is given later, following the  discussion

        of parenthesized subpatterns.
 
        of parenthesized subpatterns.
 

-       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, ane 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
+       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 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
        example:
 
          \040   is another way of writing a space


@@ -206,99 +210,171 @@ BACKSLASH
          \81    is either a back reference, or a binary zero
                    followed by the two characters "8" and "1"
 
          \81    is either a back reference, or a binary zero
                    followed by the two characters "8" and "1"
 

-       Note that octal values of 100 or greater must not be  introduced  by  a
+       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 character value can be used both
        leading zero, because no more than three octal digits are ever read.
 
        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 sequence \X is interpreted as the character "X". Outside a
-       character class, these sequences have different meanings (see below).
+       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). (If
+       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-
        "use locale;" is included in a Perl script, \s may match the VT charac-

-       ter. In PCRE, it never does.)
+       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. The use of locales with
-       Unicode is discouraged.
+       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 character properties 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

        script names, the general category properties, and "Any", which matches
        any character (including newline). Other properties such as "InMusical-
        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
+       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.
        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.
+       A character from one of these sets can be matched using a script  name.

        For example:
 
          \p{Greek}
          \P{Han}
 
        For example:
 
          \p{Greek}
          \P{Han}
 

-       Those that are not part of an identified script are lumped together  as
+       Those  that are not part of an identified script are lumped together as

        "Common". The current list of scripts is:
 
        "Common". The current list of scripts is:
 

-       Arabic,  Armenian,  Bengali,  Bopomofo, Braille, Buginese, Buhid, Cana-
-       dian_Aboriginal, Cherokee, Common, Coptic, Cypriot, Cyrillic,  Deseret,
-       Devanagari,  Ethiopic,  Georgian,  Glagolitic, Gothic, Greek, Gujarati,
-       Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana,  Inherited,  Kannada,
-       Katakana,  Kharoshthi,  Khmer,  Lao, Latin, Limbu, Linear_B, Malayalam,
-       Mongolian, Myanmar, New_Tai_Lue, Ogham, Old_Italic, Old_Persian, Oriya,
-       Osmanya,  Runic,  Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tag-
-       banwa,  Tai_Le,  Tamil,  Telugu,  Thaana,  Thai,   Tibetan,   Tifinagh,
-       Ugaritic, Yi.
-
-       Each  character has exactly one general category property, specified by
+       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
        a two-letter abbreviation. For compatibility with Perl, negation can be

-       specified  by  including a circumflex between the opening brace and the
+       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-
        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
+       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}
        optional; these two examples have the same effect:
 
          \p{L}


@@ -350,39 +426,60 @@ BACKSLASH
          Zp    Paragraph separator
          Zs    Space separator
 
          Zp    Paragraph separator
          Zs    Space separator
 

-       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
+       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".
 
        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
+       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.
 
        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.
+       Specifying caseless matching does not affect  these  escape  sequences.

        For example, \p{Lu} always matches only upper case letters.
 
        For example, \p{Lu} always matches only upper case letters.
 

-       The \X escape matches any number of Unicode  characters  that  form  an
+       The  \X  escape  matches  any number of Unicode characters that form an

        extended Unicode sequence. \X is equivalent to
 
          (?>\PM\pM*)
 
        extended Unicode sequence. \X is equivalent to
 
          (?>\PM\pM*)
 

-       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.
+       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.
+       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
+       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.
 
        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-
+       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.
        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.


@@ -390,10 +487,11 @@ BACKSLASH
 
          \b     matches at a word boundary
          \B     matches when not at a word boundary
 
          \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
        has a different meaning, namely the backspace character, inside a char-
 
        These  assertions may not appear in character classes (but note that \b
        has a different meaning, namely the backspace character, inside a char-


@@ -490,30 +588,34 @@ FULL STOP (PERIOD, DOT)
        Outside a character class, a dot in the pattern matches any one charac-
        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
        Outside a character class, a dot in the pattern matches any one charac-
        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  (CR  or LF), dot never matches that character; when the two-
-       character sequence CRLF is used, dot does not match CR if it is immedi-
-       ately  followed by LF, but otherwise it matches all characters (includ-
-       ing isolated CRs and LFs).
-
-       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 newline is defined as the two-character sequence CRLF, 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
+       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,
        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,

-       both in and out of UTF-8 mode. Unlike a dot, it always matches  CR  and
-       LF.  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.
+       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-
 
        PCRE does not allow \C to appear in  lookbehind  assertions  (described
        below),  because  in UTF-8 mode this would make it impossible to calcu-


@@ -560,11 +662,11 @@ SQUARE BRACKETS AND CHARACTER CLASSES
        PCRE  is  compiled  with Unicode property support as well as with UTF-8
        support.
 
        PCRE  is  compiled  with Unicode property support as well as with UTF-8
        support.
 

-       Characters that might indicate  line  breaks  (CR  and  LF)  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.
+       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


@@ -590,7 +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.


@@ -696,26 +798,27 @@ 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_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
-       can be changed in the same way as the Perl-compatible options by  using
+       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.
 
 
        the characters J, U and X respectively.
 
 


@@ -728,18 +831,18 @@ SUBPATTERNS
 
          cat(aract|erpillar|)
 
 
          cat(aract|erpillar|)
 

-       matches one of the words "cat", "cataract", or  "caterpillar".  Without
-       the  parentheses,  it  would  match "cataract", "erpillar" or the empty
+       matches  one  of the words "cat", "cataract", or "caterpillar". Without
+       the parentheses, it would match  "cataract",  "erpillar"  or  an  empty

        string.
 
        string.
 

-       2. It sets up the subpattern as  a  capturing  subpattern.  This  means
-       that,  when  the  whole  pattern  matches,  that portion of the subject
+       2.  It  sets  up  the  subpattern as a capturing subpattern. This means
+       that, when the whole pattern  matches,  that  portion  of  the  subject

        string that matched the subpattern is passed back to the caller via the
        string that matched the subpattern is passed back to the caller via the

-       ovector  argument  of pcre_exec(). Opening parentheses are counted from
-       left to right (starting from 1) to obtain  numbers  for  the  capturing
+       ovector argument of pcre_exec(). Opening parentheses are  counted  from
+       left  to  right  (starting  from 1) to obtain numbers for the capturing

        subpatterns.
 
        subpatterns.
 

-       For  example,  if the string "the red king" is matched against the pat-
+       For example, if the string "the red king" is matched against  the  pat-

        tern
 
          the ((red|white) (king|queen))
        tern
 
          the ((red|white) (king|queen))


@@ -747,51 +850,87 @@ SUBPATTERNS
        the captured substrings are "red king", "red", and "king", and are num-
        bered 1, 2, and 3, respectively.
 
        the captured substrings are "red king", "red", and "king", and are num-
        bered 1, 2, and 3, respectively.
 

-       The  fact  that  plain  parentheses  fulfil two functions is not always
-       helpful.  There are often times when a grouping subpattern is  required
-       without  a capturing requirement. If an opening parenthesis is followed
-       by a question mark and a colon, the subpattern does not do any  captur-
-       ing,  and  is  not  counted when computing the number of any subsequent
-       capturing subpatterns. For example, if the string "the white queen"  is
+       The fact that plain parentheses fulfil  two  functions  is  not  always
+       helpful.   There are often times when a grouping subpattern is required
+       without a capturing requirement. If an opening parenthesis is  followed
+       by  a question mark and a colon, the subpattern does not do any captur-
+       ing, and is not counted when computing the  number  of  any  subsequent
+       capturing  subpatterns. For example, if the string "the white queen" is

        matched against the pattern
 
          the ((?:red|white) (king|queen))
 
        the captured substrings are "white queen" and "queen", and are numbered
        matched against the pattern
 
          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

        between the "?" and the ":". Thus the two patterns
 
          (?i:saturday|sunday)
          (?:(?i)saturday|sunday)
 
        match exactly the same set of strings. Because alternative branches are
        between the "?" and the ":". Thus the two patterns
 
          (?i:saturday|sunday)
          (?:(?i)saturday|sunday)
 
        match exactly the same set of strings. Because alternative branches are

-       tried  from  left  to right, and options are not reset until the end of
-       the subpattern is reached, an option setting in one branch does  affect
-       subsequent  branches,  so  the above patterns match "SUNDAY" as well as
+       tried from left to right, and options are not reset until  the  end  of
+       the  subpattern is reached, an option setting in one branch does affect
+       subsequent branches, so the above patterns match "SUNDAY"  as  well  as

        "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
 
 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. References to capturing parentheses from other
-       parts of the pattern, such as  backreferences,  recursion,  and  condi-
-       tions, 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
-       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.
+       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. 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
+       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
 
        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


@@ -801,21 +940,23 @@ NAMED SUBPATTERNS
        both cases you want to extract the abbreviation. This pattern (ignoring
        the line breaks) does the job:
 
        both cases you want to extract the abbreviation. This pattern (ignoring
        the line breaks) does the job:
 

-         (?P<DN>Mon|Fri|Sun)(?:day)?|
-         (?P<DN>Tue)(?:sday)?|
-         (?P<DN>Wed)(?:nesday)?|
-         (?P<DN>Thu)(?:rsday)?|
-         (?P<DN>Sat)(?:urday)?
+         (?<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
 
        There  are  five capturing substrings, but only one is ever set after a

-       match.  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 subpattern from elsewhere in the  pattern,  the  one  that
-       corresponds  to  the  lowest number is used. For further details of the
-       interfaces for handling named subpatterns, see the  pcreapi  documenta-
-       tion.
+       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


@@ -824,9 +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)


@@ -866,8 +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,}


@@ -919,7 +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.


@@ -930,24 +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-


@@ -956,8 +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))+/


@@ -967,12 +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


@@ -986,10 +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
 


@@ -1021,63 +1164,95 @@ 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.
-       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.
-
-       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. 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-
+       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.
 
        tion.
 

-       It is not possible to have a numerical "forward back reference" to sub-
-       pattern whose number is 10 or more. However, a back  reference  to  any
-       subpattern  is  possible  using named parentheses (see below). See also
-       the subsection entitled "Non-printing  characters"  above  for  further
-       details of the handling of digits following a backslash.
+       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
 
        A  back  reference matches whatever actually matched the capturing sub-
        pattern in the current subject string, rather  than  anything  matching


@@ -1096,10 +1271,17 @@ BACK REFERENCES
        matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the
        original capturing subpattern is matched caselessly.
 
        matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the
        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+\k<p1>
+         (?'p1'(?i)rah)\s+\k{p1}

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


@@ -1214,19 +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$
 


@@ -1243,13 +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


@@ -1304,15 +1486,19 @@ 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, or a sequence of alphanumeric charac-
-       ters  and underscores, the condition is satisfied if the capturing sub-
-       pattern of that number or name has previously matched. There is a  pos-
-       sible  ambiguity here, because subpattern names may consist entirely of
-       digits. PCRE looks first for a named subpattern; if it cannot find  one
-       and  the text consists entirely of digits, it looks for a subpattern of
-       that number, which must be greater than zero.  Using  subpattern  names
-       that consist entirely of digits is not recommended.
+       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
 
        Consider  the  following  pattern, which contains non-significant white
        space to make it more readable (assume the PCRE_EXTENDED option) and to


@@ -1329,18 +1515,76 @@ CONDITIONAL SUBPATTERNS
        tern  is  executed  and  a  closing parenthesis is required. Otherwise,
        since no-pattern is not present, the  subpattern  matches  nothing.  In
        other  words,  this  pattern  matches  a  sequence  of non-parentheses,
        tern  is  executed  and  a  closing parenthesis is required. Otherwise,
        since no-pattern is not present, the  subpattern  matches  nothing.  In
        other  words,  this  pattern  matches  a  sequence  of non-parentheses,

-       optionally enclosed in parentheses. Rewriting it to use a named subpat-
-       tern gives this:
+       optionally enclosed in parentheses.
+
+       If you were embedding this pattern in a larger one,  you  could  use  a
+       relative reference:

 
 

-         (?P<OPEN> \( )?    [^()]+    (?(OPEN) \) )
+         ...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
 
        If the condition is the string (R), and there is no subpattern with the

-       name R, the condition is satisfied if a recursive call to  the  pattern
-       or  subpattern  has  been made. At "top level", the condition is false.
-       This is a PCRE extension.  Recursive patterns are described in the next
-       section.
+       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.

 
 

-       If  the  condition  is  not  a sequence of digits or (R), it must be an
+       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.
+
+       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:


@@ -1375,111 +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.
+
+       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.

 
 

-       The  special item that consists of (? followed by a number greater than
+       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.

 
 

-       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 subse-
-       quent matching failure.
+       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.

 
 

-       This  PCRE  pattern  solves  the nested parentheses problem (assume the
+       This PCRE pattern solves the nested  parentheses  problem  (assume  the

        PCRE_EXTENDED option is set so that white space is ignored):
 
          \( ( (?>[^()]+) | (?R) )* \)
 
        PCRE_EXTENDED option is set so that white space is ignored):
 
          \( ( (?>[^()]+) | (?R) )* \)
 

-       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.

 
 

-         (?P<pn> \( ( (?>[^()]+) | (?P>pn) )* \) )
+       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.

 
 

-       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
+       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.
+
+       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
 


@@ -1489,15 +1761,23 @@ 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, if given  numerically,  must  follow  the
-       subpattern  to which they refer. However, named references can refer to
-       later subpatterns.
+       two strings. Another example is  given  in  the  discussion  of  DEFINE
+       above.

 
        Like recursive subpatterns, a "subroutine" call is always treated as an
 
        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
+       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.
 
        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
 


@@ -1533,5 +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: 06 June 2006
-Copyright (c) 1997-2006 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.