TLS version reporting. fixes: #745

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

This file contains the PCRE man page that describes the regular expressions
supported by PCRE version 7.2. Note that not all of the features are relevant
in the context of Exim. In particular, the version of PCRE that is compiled
with Exim does not include UTF-8 support, there is no mechanism for changing
the options with which the PCRE functions are called, and features such as
callout are not accessible.
-----------------------------------------------------------------------------

PCREPATTERN(3)                                                  PCREPATTERN(3)


NAME
       PCRE - Perl-compatible regular expressions


PCRE REGULAR EXPRESSION DETAILS

       The  syntax  and semantics of the regular expressions supported by PCRE
       are described below. Regular expressions are also described in the Perl
       documentation  and  in  a  number  of books, some of which have copious
       examples.  Jeffrey Friedl's "Mastering Regular Expressions",  published
       by  O'Reilly, covers regular expressions in great detail. This descrip-
       tion of PCRE's regular expressions is intended as reference material.

       The original operation of PCRE was on strings of  one-byte  characters.
       However,  there is now also support for UTF-8 character strings. To use
       this, you must build PCRE to  include  UTF-8  support,  and  then  call
       pcre_compile()  with  the  PCRE_UTF8  option.  How this affects pattern
       matching is mentioned in several places below. There is also a  summary
       of  UTF-8  features  in  the  section on UTF-8 support in the main pcre
       page.

       The remainder of this document discusses the  patterns  that  are  sup-
       ported  by  PCRE when its main matching function, pcre_exec(), is used.
       From  release  6.0,   PCRE   offers   a   second   matching   function,
       pcre_dfa_exec(),  which matches using a different algorithm that is not
       Perl-compatible. Some of the features discussed below are not available
       when  pcre_dfa_exec()  is used. The advantages and disadvantages of the
       alternative function, and how it differs from the normal function,  are
       discussed in the pcrematching page.


CHARACTERS AND METACHARACTERS

       A  regular  expression  is  a pattern that is matched against a subject
       string from left to right. Most characters stand for  themselves  in  a
       pattern,  and  match  the corresponding characters in the subject. As a
       trivial example, the pattern

         The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless  matching is specified (the PCRE_CASELESS option), letters are
       matched independently of case. In UTF-8 mode, PCRE  always  understands
       the  concept  of case for characters whose values are less than 128, so
       caseless matching is always possible. For characters with  higher  val-
       ues,  the concept of case is supported if PCRE is compiled with Unicode
       property support, but not otherwise.   If  you  want  to  use  caseless
       matching  for  characters  128  and above, you must ensure that PCRE is
       compiled with Unicode property support as well as with UTF-8 support.

       The power of regular expressions comes  from  the  ability  to  include
       alternatives  and  repetitions in the pattern. These are encoded in the
       pattern by the use of metacharacters, which do not stand for themselves
       but instead are interpreted in some special way.

       There  are  two different sets of metacharacters: those that are recog-
       nized anywhere in the pattern except within square brackets, and  those
       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)
         $      assert end of string (or line, in multiline mode)
         .      match any character except newline (by default)
         [      start character class definition
         |      start of alternative branch
         (      start subpattern
         )      end subpattern
         ?      extends the meaning of (
                also 0 or 1 quantifier
                also quantifier minimizer
         *      0 or more quantifier
         +      1 or more quantifier
                also "possessive quantifier"
         {      start min/max quantifier

       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
         ^      negate the class, but only if the first character
         -      indicates character range
         [      POSIX character class (only if followed by POSIX
                  syntax)
         ]      terminates the character class

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


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

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

         \Qabc$xyz\E        abc$xyz        abc followed by the
                                             contents of $xyz
         \Qabc\$xyz\E       abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz

       The \Q...\E sequence is recognized both inside  and  outside  character
       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
       sequences than the binary character it represents:

         \a        alarm, that is, the BEL character (hex 07)
         \cx       "control-x", where x is any character
         \e        escape (hex 1B)
         \f        formfeed (hex 0C)
         \n        newline (hex 0A)
         \r        carriage return (hex 0D)
         \t        tab (hex 09)
         \ddd      character with octal code ddd, or backreference
         \xhh      character with hex code hh
         \x{hhh..} character with hex code hhh..

       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.

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

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

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

       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
         \40    is the same, provided there are fewer than 40
                   previous capturing subpatterns
         \7     is always a back reference
         \11    might be a back reference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a back reference, otherwise the
                   character with octal code 113
         \377   might be a back reference, otherwise
                   the byte consisting entirely of 1 bits
         \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
       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 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

       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
         \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
         \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
       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-
       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.

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

       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-
       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-
       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

       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-
       Symbols" are not currently supported by PCRE. Note  that  \P{Any}  does
       not match any characters, so always causes a match failure.

       Sets of Unicode characters are defined as belonging to certain scripts.
       A character from one of these sets can be matched using a script  name.
       For example:

         \p{Greek}
         \P{Han}

       Those  that are not part of an identified script are lumped together as
       "Common". The current list of scripts is:

       Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,
       Buhid,   Canadian_Aboriginal,   Cherokee,  Common,  Coptic,  Cuneiform,
       Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
       Gothic,  Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
       gana, Inherited, Kannada,  Katakana,  Kharoshthi,  Khmer,  Lao,  Latin,
       Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,
       Ogham, Old_Italic, Old_Persian, Oriya, Osmanya,  Phags_Pa,  Phoenician,
       Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,
       Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.

       Each character has exactly one general category property, specified  by
       a two-letter abbreviation. For compatibility with Perl, negation can be
       specified by including a circumflex between the opening brace  and  the
       property name. For example, \p{^Lu} is the same as \P{Lu}.

       If only one letter is specified with \p or \P, it includes all the gen-
       eral category properties that start with that letter. In this case,  in
       the  absence of negation, the curly brackets in the escape sequence are
       optional; these two examples have the same effect:

         \p{L}
         \pL

       The following general category property codes are supported:

         C     Other
         Cc    Control
         Cf    Format
         Cn    Unassigned
         Co    Private use
         Cs    Surrogate

         L     Letter
         Ll    Lower case letter
         Lm    Modifier letter
         Lo    Other letter
         Lt    Title case letter
         Lu    Upper case letter

         M     Mark
         Mc    Spacing mark
         Me    Enclosing mark
         Mn    Non-spacing mark

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

         P     Punctuation
         Pc    Connector punctuation
         Pd    Dash punctuation
         Pe    Close punctuation
         Pf    Final punctuation
         Pi    Initial punctuation
         Po    Other punctuation
         Ps    Open punctuation

         S     Symbol
         Sc    Currency symbol
         Sk    Modifier symbol
         Sm    Mathematical symbol
         So    Other symbol

         Z     Separator
         Zl    Line 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
       classified as a modifier or "other".

       The long synonyms for these properties  that  Perl  supports  (such  as
       \p{Letter})  are  not  supported by PCRE, nor is it permitted to prefix
       any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned) prop-
       erty.  Instead, this property is assumed for any code point that is not
       in the Unicode table.

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

       The  \X  escape  matches  any number of Unicode characters that form an
       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.
       None of them have codepoints less than 256, so  in  non-UTF-8  mode  \X
       matches any one character.

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

   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

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

         \b     matches at a word boundary
         \B     matches when not at a word boundary
         \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-
       acter class).

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

       The \A, \Z, and \z assertions differ from  the  traditional  circumflex
       and dollar (described in the next section) in that they only ever match
       at the very start and end of the subject string, whatever  options  are
       set.  Thus,  they are independent of multiline mode. These three asser-
       tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
       affect  only the behaviour of the circumflex and dollar metacharacters.
       However, if the startoffset argument of pcre_exec() is non-zero,  indi-
       cating that matching is to start at a point other than the beginning of
       the subject, \A can never match. The difference between \Z  and  \z  is
       that \Z matches before a newline at the end of the string as well as at
       the very end, whereas \z matches only at the end.

       The \G assertion is true only when the current matching position is  at
       the  start point of the match, as specified by the startoffset argument
       of pcre_exec(). It differs from \A when the  value  of  startoffset  is
       non-zero.  By calling pcre_exec() multiple times with appropriate argu-
       ments, you can mimic Perl's /g option, and it is in this kind of imple-
       mentation where \G can be useful.

       Note,  however,  that  PCRE's interpretation of \G, as the start of the
       current match, is subtly different from Perl's, which defines it as the
       end  of  the  previous  match. In Perl, these can be different when the
       previously matched string was empty. Because PCRE does just  one  match
       at a time, it cannot reproduce this behaviour.

       If  all  the alternatives of a pattern begin with \G, the expression is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.


CIRCUMFLEX AND DOLLAR

       Outside a character class, in the default matching mode, the circumflex
       character is an assertion that is true only  if  the  current  matching
       point  is  at the start of the subject string. If the startoffset argu-
       ment of pcre_exec() is non-zero, circumflex  can  never  match  if  the
       PCRE_MULTILINE  option  is  unset. Inside a character class, circumflex
       has an entirely different meaning (see below).

       Circumflex need not be the first character of the pattern if  a  number
       of  alternatives are involved, but it should be the first thing in each
       alternative in which it appears if the pattern is ever  to  match  that
       branch.  If all possible alternatives start with a circumflex, that is,
       if the pattern is constrained to match only at the start  of  the  sub-
       ject,  it  is  said  to be an "anchored" pattern. (There are also other
       constructs that can cause a pattern to be anchored.)

       A dollar character is an assertion that is true  only  if  the  current
       matching  point  is  at  the  end of the subject string, or immediately
       before a newline at the end of the string (by default). Dollar need not
       be  the  last  character of the pattern if a number of alternatives are
       involved, but it should be the last item in  any  branch  in  which  it
       appears. Dollar has no special meaning in a character class.

       The  meaning  of  dollar  can be changed so that it matches only at the
       very end of the string, by setting the  PCRE_DOLLAR_ENDONLY  option  at
       compile time. This does not affect the \Z assertion.

       The meanings of the circumflex and dollar characters are changed if the
       PCRE_MULTILINE option is set. When  this  is  the  case,  a  circumflex
       matches  immediately after internal newlines as well as at the start of
       the subject string. It does not match after a  newline  that  ends  the
       string.  A dollar matches before any newlines in the string, as well as
       at the very end, when PCRE_MULTILINE is set. When newline is  specified
       as  the  two-character  sequence CRLF, isolated CR and LF characters do
       not indicate newlines.

       For example, the pattern /^abc$/ matches the subject string  "def\nabc"
       (where  \n  represents a newline) in multiline mode, but not otherwise.
       Consequently, patterns that are anchored in single  line  mode  because
       all  branches  start  with  ^ are not anchored in multiline mode, and a
       match for circumflex is  possible  when  the  startoffset  argument  of
       pcre_exec()  is  non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
       PCRE_MULTILINE is set.

       Note that the sequences \A, \Z, and \z can be used to match  the  start
       and  end of the subject in both modes, and if all branches of a pattern
       start with \A it is always anchored, whether or not  PCRE_MULTILINE  is
       set.


FULL STOP (PERIOD, DOT)

       Outside a character class, a dot in the pattern matches any one charac-
       ter in the subject string except (by default) a character  that  signi-
       fies  the  end  of  a line. In UTF-8 mode, the matched character may be
       more than one byte long.

       When a line ending is defined as a single character, dot never  matches
       that  character; when the two-character sequence CRLF is used, dot does
       not match CR if it is immediately followed  by  LF,  but  otherwise  it
       matches  all characters (including isolated CRs and LFs). When any Uni-
       code line endings are being recognized, dot does not match CR or LF  or
       any of the other line ending characters.

       The  behaviour  of  dot  with regard to newlines can be changed. If the
       PCRE_DOTALL option is set, a dot matches  any  one  character,  without
       exception. If the two-character sequence CRLF is present in the subject
       string, it takes two dots to match it.

       The handling of dot is entirely independent of the handling of  circum-
       flex  and  dollar,  the  only relationship being that they both involve
       newlines. Dot has no special meaning in a character class.


MATCHING A SINGLE BYTE

       Outside a character class, the escape sequence \C matches any one byte,
       both  in  and  out  of  UTF-8 mode. Unlike a dot, it always matches any
       line-ending characters. The feature is provided in  Perl  in  order  to
       match  individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
       acters into individual bytes, what remains in the string may be a  mal-
       formed  UTF-8  string.  For this reason, the \C escape sequence is best
       avoided.

       PCRE does not allow \C to appear in  lookbehind  assertions  (described
       below),  because  in UTF-8 mode this would make it impossible to calcu-
       late the length of the lookbehind.


SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial. If a closing square bracket is required as a member of the class,
       it  should  be  the first data character in the class (after an initial
       circumflex, if present) or escaped with a backslash.

       A character class matches a single character in the subject.  In  UTF-8
       mode,  the character may occupy more than one byte. A matched character
       must be in the set of characters defined by the class, unless the first
       character  in  the  class definition is a circumflex, in which case the
       subject character must not be in the set defined by  the  class.  If  a
       circumflex  is actually required as a member of the class, ensure it is
       not the first character, or escape it with a backslash.

       For example, the character class [aeiou] matches any lower case  vowel,
       while  [^aeiou]  matches  any character that is not a lower case vowel.
       Note that a circumflex is just a convenient notation for specifying the
       characters  that  are in the class by enumerating those that are not. A
       class that starts with a circumflex is not an assertion: it still  con-
       sumes  a  character  from the subject string, and therefore it fails if
       the current pointer is at the end of the string.

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

       When caseless matching is set, any letters in a  class  represent  both
       their  upper  case  and lower case versions, so for example, a caseless
       [aeiou] matches "A" as well as "a", and a caseless  [^aeiou]  does  not
       match  "A", whereas a caseful version would. In UTF-8 mode, PCRE always
       understands the concept of case for characters whose  values  are  less
       than  128, so caseless matching is always possible. For characters with
       higher values, the concept of case is supported  if  PCRE  is  compiled
       with  Unicode  property support, but not otherwise.  If you want to use
       caseless matching for characters 128 and above, you  must  ensure  that
       PCRE  is  compiled  with Unicode property support as well as with UTF-8
       support.

       Characters that might indicate line breaks are  never  treated  in  any
       special  way  when  matching  character  classes,  whatever line-ending
       sequence is in  use,  and  whatever  setting  of  the  PCRE_DOTALL  and
       PCRE_MULTILINE options is used. A class such as [^a] always matches one
       of these characters.

       The minus (hyphen) character can be used to specify a range of  charac-
       ters  in  a  character  class.  For  example,  [d-m] matches any letter
       between d and m, inclusive. If a  minus  character  is  required  in  a
       class,  it  must  be  escaped  with a backslash or appear in a position
       where it cannot be interpreted as indicating a range, typically as  the
       first or last character in the class.

       It is not possible to have the literal character "]" as the end charac-
       ter of a range. A pattern such as [W-]46] is interpreted as a class  of
       two  characters ("W" and "-") followed by a literal string "46]", so it
       would match "W46]" or "-46]". However, if the "]"  is  escaped  with  a
       backslash  it is interpreted as the end of range, so [W-\]46] is inter-
       preted as a class containing a range followed by two other  characters.
       The  octal or hexadecimal representation of "]" can also be used to end
       a range.

       Ranges operate in the collating sequence of character values. They  can
       also   be  used  for  characters  specified  numerically,  for  example
       [\000-\037]. In UTF-8 mode, ranges can include characters whose  values
       are greater than 255, for example [\x{100}-\x{2ff}].

       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 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.

       The  character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
       in a character class, and add the characters that  they  match  to  the
       class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
       flex can conveniently be used with the upper case  character  types  to
       specify  a  more  restricted  set of characters than the matching lower
       case type. For example, the class [^\W_] matches any letter  or  digit,
       but not underscore.

       The  only  metacharacters  that are recognized in character classes are
       backslash, hyphen (only where it can be  interpreted  as  specifying  a
       range),  circumflex  (only  at the start), opening square bracket (only
       when it can be interpreted as introducing a POSIX class name - see  the
       next  section),  and  the  terminating closing square bracket. However,
       escaping other non-alphanumeric characters does no harm.


POSIX CHARACTER CLASSES

       Perl supports the POSIX notation for character classes. This uses names
       enclosed  by  [: and :] within the enclosing square brackets. PCRE also
       supports this notation. For example,

         [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class
       names are

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits
         space    white space (not quite the same as \s)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The  "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
       and space (32). Notice that this list includes the VT  character  (code
       11). This makes "space" different to \s, which does not include VT (for
       Perl compatibility).

       The name "word" is a Perl extension, and "blank"  is  a  GNU  extension
       from  Perl  5.8. Another Perl extension is negation, which is indicated
       by a ^ character after the colon. For example,

         [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE (and Perl) also recognize  the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

       In UTF-8 mode, characters with values greater than 128 do not match any
       of the POSIX character classes.


VERTICAL BAR

       Vertical bar characters are used to separate alternative patterns.  For
       example, the pattern

         gilbert|sullivan

       matches  either "gilbert" or "sullivan". Any number of alternatives may
       appear, and an empty  alternative  is  permitted  (matching  the  empty
       string). The matching process tries each alternative in turn, from left
       to right, and the first one that succeeds is used. If the  alternatives
       are  within a subpattern (defined below), "succeeds" means matching the
       rest of the main pattern as well as the alternative in the  subpattern.


INTERNAL OPTION SETTING

       The  settings  of  the  PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
       PCRE_EXTENDED options can be changed  from  within  the  pattern  by  a
       sequence  of  Perl  option  letters  enclosed between "(?" and ")". The
       option letters are

         i  for PCRE_CASELESS
         m  for PCRE_MULTILINE
         s  for PCRE_DOTALL
         x  for PCRE_EXTENDED

       For example, (?im) sets caseless, multiline matching. It is also possi-
       ble to unset these options by preceding the letter with a hyphen, and a
       combined setting and unsetting such as (?im-sx), which sets  PCRE_CASE-
       LESS  and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
       is also permitted. If a  letter  appears  both  before  and  after  the
       hyphen, the option is unset.

       When  an option change occurs at top level (that is, not inside subpat-
       tern parentheses), the change applies to the remainder of  the  pattern
       that follows.  If the change is placed right at the start of a pattern,
       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 (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
       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)

       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.

       The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and  PCRE_EXTRA
       can  be changed in the same way as the Perl-compatible options by using
       the characters J, U and X respectively.


SUBPATTERNS

       Subpatterns are delimited by parentheses (round brackets), which can be
       nested.  Turning part of a pattern into a subpattern does two things:

       1. It localizes a set of alternatives. For example, the pattern

         cat(aract|erpillar|)

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

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

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

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

       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
       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.

       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
       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".


DUPLICATE SUBPATTERN NUMBERS

       Perl 5.10 introduced a feature whereby each alternative in a subpattern
       uses the same numbers for its capturing parentheses. Such a  subpattern
       starts  with (?| and is itself a non-capturing subpattern. For example,
       consider this pattern:

         (?|(Sat)ur|(Sun))day

       Because the two alternatives are inside a (?| group, both sets of  cap-
       turing  parentheses  are  numbered one. Thus, when the pattern matches,
       you can look at captured substring number  one,  whichever  alternative
       matched.  This  construct  is useful when you want to capture part, but
       not all, of one of a number of alternatives. Inside a (?| group, paren-
       theses  are  numbered as usual, but the number is reset at the start of
       each branch. The numbers of any capturing buffers that follow the  sub-
       pattern  start after the highest number used in any branch. The follow-
       ing example is taken from the Perl documentation.  The  numbers  under-
       neath show in which buffer the captured content will be stored.

         # before  ---------------branch-reset----------- after
         / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
         # 1            2         2  3        2     3     4

       A  backreference  or  a  recursive call to a numbered subpattern always
       refers to the first one in the pattern with the given number.

       An alternative approach to using this "branch reset" feature is to  use
       duplicate named subpatterns, as described in the next section.


NAMED SUBPATTERNS

       Identifying  capturing  parentheses  by number is simple, but it can be
       very hard to keep track of the numbers in complicated  regular  expres-
       sions.  Furthermore,  if  an  expression  is  modified, the numbers may
       change. To help with this difficulty, PCRE supports the naming of  sub-
       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
       time.  This  can  be useful for patterns where only one instance of the
       named parentheses can match. Suppose you want to match the  name  of  a
       weekday,  either as a 3-letter abbreviation or as the full name, and in
       both cases you want to extract the abbreviation. This pattern (ignoring
       the line breaks) does the job:

         (?<DN>Mon|Fri|Sun)(?:day)?|
         (?<DN>Tue)(?:sday)?|
         (?<DN>Wed)(?:nesday)?|
         (?<DN>Thu)(?:rsday)?|
         (?<DN>Sat)(?:urday)?

       There  are  five capturing substrings, but only one is ever set after a
       match.  (An alternative way of solving this problem is to use a "branch
       reset" subpattern, as described in the previous section.)

       The  convenience  function  for extracting the data by name returns the
       substring for the first (and in this example, the only)  subpattern  of
       that  name  that  matched.  This saves searching to find which numbered
       subpattern it was. If you make a reference to a non-unique  named  sub-
       pattern  from elsewhere in the pattern, the one that corresponds to the
       lowest number is used. For further details of the interfaces  for  han-
       dling named subpatterns, see the pcreapi documentation.


REPETITION

       Repetition  is  specified  by  quantifiers, which can follow any of the
       following items:

         a literal data character
         the dot metacharacter
         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)
         a parenthesized subpattern (unless it is an assertion)

       The general repetition quantifier specifies a minimum and maximum  num-
       ber  of  permitted matches, by giving the two numbers in curly brackets
       (braces), separated by a comma. The numbers must be  less  than  65536,
       and the first must be less than or equal to the second. For example:

         z{2,4}

       matches  "zz",  "zzz",  or  "zzzz". A closing brace on its own is not a
       special character. If the second number is omitted, but  the  comma  is
       present,  there  is  no upper limit; if the second number and the comma
       are both omitted, the quantifier specifies an exact number of  required
       matches. Thus

         [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, while

         \d{8}

       matches  exactly  8  digits. An opening curly bracket that appears in a
       position where a quantifier is not allowed, or one that does not  match
       the  syntax of a quantifier, is taken as a literal character. For exam-
       ple, {,6} is not a quantifier, but a literal string of four characters.

       In  UTF-8  mode,  quantifiers  apply to UTF-8 characters rather than to
       individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
       acters, each of which is represented by a two-byte sequence. Similarly,
       when Unicode property support is available, \X{3} matches three Unicode
       extended  sequences,  each of which may be several bytes long (and they
       may be of different lengths).

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present.

       For  convenience, the three most common quantifiers have single-charac-
       ter abbreviations:

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

       It is possible to construct infinite loops by  following  a  subpattern
       that can match no characters with a quantifier that has no upper limit,
       for example:

         (a?)*

       Earlier versions of Perl and PCRE used to give an error at compile time
       for  such  patterns. However, because there are cases where this can be
       useful, such patterns are now accepted, but if any  repetition  of  the
       subpattern  does in fact match no characters, the loop is forcibly bro-
       ken.

       By default, the quantifiers are "greedy", that is, they match  as  much
       as  possible  (up  to  the  maximum number of permitted times), without
       causing the rest of the pattern to fail. The classic example  of  where
       this gives problems is in trying to match comments in C programs. These
       appear between /* and */ and within the comment,  individual  *  and  /
       characters  may  appear. An attempt to match C comments by applying the
       pattern

         /\*.*\*/

       to the string

         /* first comment */  not comment  /* second comment */

       fails, because it matches the entire string owing to the greediness  of
       the .*  item.

       However,  if  a quantifier is followed by a question mark, it ceases to
       be greedy, and instead matches the minimum number of times possible, so
       the pattern

         /\*.*?\*/

       does  the  right  thing with the C comments. The meaning of the various
       quantifiers is not otherwise changed,  just  the  preferred  number  of
       matches.   Do  not  confuse this use of question mark with its use as a
       quantifier in its own right. Because it has two uses, it can  sometimes
       appear doubled, as in

         \d??\d

       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 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.

       When  a  parenthesized  subpattern  is quantified with a minimum repeat
       count that is greater than 1 or with a limited maximum, more memory  is
       required  for  the  compiled  pattern, in proportion to the size of the
       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 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.

       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

       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-
       string that matched the final iteration. For example, after

         (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-
       tions. For example, after

         /(a|(b))+/

       matches "aba" the value of the second captured substring is "b".


ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

       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

         123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       action of the matcher is to try again with only 5 digits  matching  the
       \d+  item,  and  then  with  4,  and  so on, before ultimately failing.
       "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides
       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  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

       This  kind  of  parenthesis "locks up" the  part of the pattern it con-
       tains once it has matched, and a failure further into  the  pattern  is
       prevented  from  backtracking into it. Backtracking past it to previous
       items, however, works as normal.

       An alternative description is that a subpattern of  this  type  matches
       the  string  of  characters  that an identical standalone pattern would
       match, if anchored at the current point in the subject string.

       Atomic grouping subpatterns are not capturing subpatterns. Simple cases
       such as the above example can be thought of as a maximizing repeat that
       must swallow everything it can. So, while both \d+ and  \d+?  are  pre-
       pared  to  adjust  the number of digits they match in order to make the
       rest of the pattern match, (?>\d+) can only match an entire sequence of
       digits.

       Atomic  groups in general can of course contain arbitrarily complicated
       subpatterns, and can be nested. However, when  the  subpattern  for  an
       atomic group is just a single repeated item, as in the example above, a
       simpler notation, called a "possessive quantifier" can  be  used.  This
       consists  of  an  additional  + character following a quantifier. Using
       this notation, the previous example can be rewritten as

         \d++foo

       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  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+>)*[!?]

       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

       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+>)*[!?]

       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-
       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,
       it is always taken as a back reference, and causes  an  error  only  if
       there  are  not that many capturing left parentheses in the entire pat-
       tern. In other words, the parentheses that are referenced need  not  be
       to  the left of the reference for numbers less than 10. A "forward back
       reference" of this type can make sense when a  repetition  is  involved
       and  the  subpattern to the right has participated in an earlier itera-
       tion.

       It is not possible to have a numerical "forward back  reference"  to  a
       subpattern  whose  number  is  10  or  more using this syntax because a
       sequence such as \50 is interpreted as a character  defined  in  octal.
       See the subsection entitled "Non-printing characters" above for further
       details of the handling of digits following a backslash.  There  is  no
       such  problem  when named parentheses are used. A back reference to any
       subpattern is possible using named parentheses (see below).

       Another way of avoiding the ambiguity inherent in  the  use  of  digits
       following a backslash is to use the \g escape sequence, which is a fea-
       ture introduced in Perl 5.10. This escape must be followed by  a  posi-
       tive  or  a negative number, optionally enclosed in braces. These exam-
       ples are all identical:

         (ring), \1
         (ring), \g1
         (ring), \g{1}

       A positive number specifies an absolute reference without the ambiguity
       that  is  present  in  the older syntax. It is also useful when literal
       digits follow the reference. A negative number is a relative reference.
       Consider this example:

         (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a reference to the most recently started captur-
       ing subpattern before \g, that is, is it equivalent to  \2.  Similarly,
       \g{-2} would be equivalent to \1. The use of relative references can be
       helpful in long patterns, and also in  patterns  that  are  created  by
       joining together fragments that contain references within themselves.

       A  back  reference matches whatever actually matched the capturing sub-
       pattern in the current subject string, rather  than  anything  matching
       the subpattern itself (see "Subpatterns as subroutines" below for a way
       of doing that). So the pattern

         (sens|respons)e and \1ibility

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

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

       matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the
       original capturing subpattern is matched caselessly.

       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)
         (?<p1>(?i)rah)\s+\g{p1}

       A  subpattern  that  is  referenced  by  name may appear in the pattern
       before or after the reference.

       There may be more than one back reference to the same subpattern. If  a
       subpattern  has  not actually been used in a particular match, any back
       references to it always fail. For example, the pattern

         (a|(bc))\2

       always fails if it starts to match "a" rather than "bc". Because  there
       may  be  many  capturing parentheses in a pattern, all digits following
       the backslash are taken as part of a potential back  reference  number.
       If the pattern continues with a digit character, some delimiter must be
       used to terminate the back reference. If the  PCRE_EXTENDED  option  is
       set,  this  can  be  whitespace.  Otherwise an empty comment (see "Com-
       ments" below) can be used.

       A back reference that occurs inside the parentheses to which it  refers
       fails  when  the subpattern is first used, so, for example, (a\1) never
       matches.  However, such references can be useful inside  repeated  sub-
       patterns. For example, the pattern

         (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
       ation of the subpattern,  the  back  reference  matches  the  character
       string  corresponding  to  the previous iteration. In order for this to
       work, the pattern must be such that the first iteration does  not  need
       to  match the back reference. This can be done using alternation, as in
       the example above, or by a quantifier with a minimum of zero.


ASSERTIONS

       An assertion is a test on the characters  following  or  preceding  the
       current  matching  point that does not actually consume any characters.
       The simple assertions coded as \b, \B, \A, \G, \Z,  \z,  ^  and  $  are
       described above.

       More  complicated  assertions  are  coded as subpatterns. There are two
       kinds: those that look ahead of the current  position  in  the  subject
       string,  and  those  that  look  behind  it. An assertion subpattern is
       matched in the normal way, except that it does not  cause  the  current
       matching position to be changed.

       Assertion  subpatterns  are  not  capturing subpatterns, and may not be
       repeated, because it makes no sense to assert the  same  thing  several
       times.  If  any kind of assertion contains capturing subpatterns within
       it, these are counted for the purposes of numbering the capturing  sub-
       patterns in the whole pattern.  However, substring capturing is carried
       out only for positive assertions, because it does not  make  sense  for
       negative assertions.

   Lookahead assertions

       Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

         \w+(?=;)

       matches a word followed by a semicolon, but does not include the  semi-
       colon in the match, and

         foo(?!bar)

       matches  any  occurrence  of  "foo" that is not followed by "bar". Note
       that the apparently similar pattern

         (?!foo)bar

       does not find an occurrence of "bar"  that  is  preceded  by  something
       other  than "foo"; it finds any occurrence of "bar" whatsoever, because
       the assertion (?!foo) is always true when the next three characters are
       "bar". A lookbehind assertion is needed to achieve the other effect.

       If you want to force a matching failure at some point in a pattern, the
       most convenient way to do it is  with  (?!)  because  an  empty  string
       always  matches, so an assertion that requires there not to be an empty
       string must always fail.

   Lookbehind assertions

       Lookbehind assertions start with (?<= for positive assertions and  (?<!
       for negative assertions. For example,

         (?<!foo)bar

       does  find  an  occurrence  of "bar" that is not preceded by "foo". The
       contents of a lookbehind assertion are restricted  such  that  all  the
       strings it matches must have a fixed length. However, if there are sev-
       eral top-level alternatives, they do not all  have  to  have  the  same
       fixed length. Thus

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

       causes  an  error at compile time. Branches that match different length
       strings are permitted only at the top level of a lookbehind  assertion.
       This  is  an  extension  compared  with  Perl (at least for 5.8), which
       requires all branches to match the same length of string. An  assertion
       such as

         (?<=ab(c|de))

       is  not  permitted,  because  its single top-level branch can match two
       different lengths, but it is acceptable if rewritten to  use  two  top-
       level branches:

         (?<=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,
       to temporarily move the current position back by the fixed  length  and
       then try to match. If there are insufficient characters before the cur-
       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-
       ble to calculate the length of the lookbehind. The \X and  \R  escapes,
       which can match different numbers of bytes, are also not permitted.

       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$

       when  applied  to  a  long string that does not match. Because matching
       proceeds from left to right, PCRE will look for each "a" in the subject
       and  then  see  if what follows matches the rest of the pattern. If the
       pattern is specified as

         ^.*abcd$

       the initial .* matches the entire string at first, but when this  fails
       (because there is no following "a"), it backtracks to match all but the
       last character, then all but the last two characters, and so  on.  Once
       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)

       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
       processing time.

   Using multiple assertions

       Several assertions (of any sort) may occur in succession. For example,

         (?<=\d{3})(?<!999)foo

       matches "foo" preceded by three digits that are not "999". Notice  that
       each  of  the  assertions is applied independently at the same point in
       the subject string. First there is a  check  that  the  previous  three
       characters  are  all  digits,  and  then there is a check that the same
       three characters are not "999".  This pattern does not match "foo" pre-
       ceded  by  six  characters,  the first of which are digits and the last
       three of which are not "999". For example, it  doesn't  match  "123abc-
       foo". A pattern to do that is

         (?<=\d{3}...)(?<!999)foo

       This  time  the  first assertion looks at the preceding six characters,
       checking that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

       Assertions can be nested in any combination. For example,

         (?<=(?<!foo)bar)baz

       matches  an occurrence of "baz" that is preceded by "bar" which in turn
       is not preceded by "foo", while

         (?<=\d{3}(?!999)...)foo

       is another pattern that matches "foo" preceded by three digits and  any
       three characters that are not "999".


CONDITIONAL SUBPATTERNS

       It  is possible to cause the matching process to obey a subpattern con-
       ditionally or to choose between two alternative subpatterns,  depending
       on  the result of an assertion, or whether a previous capturing subpat-
       tern matched or not. The two possible forms of  conditional  subpattern
       are

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

       If  the  condition is satisfied, the yes-pattern is used; otherwise the
       no-pattern (if present) is used. If there are more  than  two  alterna-
       tives in the subpattern, a compile-time error occurs.

       There  are  four  kinds of condition: references to subpatterns, refer-
       ences to recursion, a pseudo-condition called DEFINE, and assertions.

   Checking for a used subpattern by number

       If the text between the parentheses consists of a sequence  of  digits,
       the  condition  is  true if the capturing subpattern of that number has
       previously matched. An alternative notation is to  precede  the  digits
       with a plus or minus sign. In this case, the subpattern number is rela-
       tive rather than absolute.  The most recently opened parentheses can be
       referenced  by  (?(-1),  the  next most recent by (?(-2), and so on. In
       looping constructs it can also make sense to refer to subsequent groups
       with constructs such as (?(+2).

       Consider  the  following  pattern, which contains non-significant white
       space to make it more readable (assume the PCRE_EXTENDED option) and to
       divide it into three parts for ease of discussion:

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

       The  first  part  matches  an optional opening parenthesis, and if that
       character is present, sets it as the first captured substring. The sec-
       ond  part  matches one or more characters that are not parentheses. The
       third part is a conditional subpattern that tests whether the first set
       of parentheses matched or not. If they did, that is, if subject started
       with an opening parenthesis, the condition is true, and so the yes-pat-
       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.

       If you were embedding this pattern in a larger one,  you  could  use  a
       relative reference:

         ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

       This  makes  the  fragment independent of the parentheses in the larger
       pattern.

   Checking for a used subpattern by name

       Perl uses the syntax (?(<name>)...) or (?('name')...)  to  test  for  a
       used  subpattern  by  name.  For compatibility with earlier versions of
       PCRE, which had this facility before Perl, the syntax  (?(name)...)  is
       also  recognized. However, there is a possible ambiguity with this syn-
       tax, because subpattern names may  consist  entirely  of  digits.  PCRE
       looks  first for a named subpattern; if it cannot find one and the name
       consists entirely of digits, PCRE looks for a subpattern of  that  num-
       ber,  which must be greater than zero. Using subpattern names that con-
       sist entirely of digits is not recommended.

       Rewriting the above example to use a named subpattern gives this:

         (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )


   Checking for pattern recursion

       If the condition is the string (R), and there is no subpattern with the
       name  R, the condition is true if a recursive call to the whole pattern
       or any subpattern has been made. If digits or a name preceded by amper-
       sand follow the letter R, for example:

         (?(R3)...) or (?(R&name)...)

       the  condition is true if the most recent recursion is into the subpat-
       tern whose number or name is given. This condition does not  check  the
       entire recursion stack.

       At  "top  level", all these recursion test conditions are false. Recur-
       sive patterns are described below.

   Defining subpatterns for use by reference only

       If the condition is the string (DEFINE), and  there  is  no  subpattern
       with  the  name  DEFINE,  the  condition is always false. In this case,
       there may be only one alternative  in  the  subpattern.  It  is  always
       skipped  if  control  reaches  this  point  in the pattern; the idea of
       DEFINE is that it can be used to define "subroutines" that can be  ref-
       erenced  from elsewhere. (The use of "subroutines" is described below.)
       For example, a pattern to match an IPv4 address could be  written  like
       this (ignore whitespace and line breaks):

         (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
         \b (?&byte) (\.(?&byte)){3} \b

       The  first part of the pattern is a DEFINE group inside which a another
       group named "byte" is defined. This matches an individual component  of
       an  IPv4  address  (a number less than 256). When matching takes place,
       this part of the pattern is skipped because DEFINE acts  like  a  false
       condition.

       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:

         (?(?=[^a-z]*[a-z])
         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The condition  is  a  positive  lookahead  assertion  that  matches  an
       optional  sequence of non-letters followed by a letter. In other words,
       it tests for the presence of at least one letter in the subject.  If  a
       letter  is found, the subject is matched against the first alternative;
       otherwise it is  matched  against  the  second.  This  pattern  matches
       strings  in  one  of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
       letters and dd are digits.


COMMENTS

       The sequence (?# marks the start of a comment that continues up to  the
       next  closing  parenthesis.  Nested  parentheses are not permitted. The
       characters that make up a comment play no part in the pattern  matching
       at all.

       If  the PCRE_EXTENDED option is set, an unescaped # character outside a
       character class introduces a  comment  that  continues  to  immediately
       after the next newline in the pattern.


RECURSIVE PATTERNS

       Consider  the problem of matching a string in parentheses, allowing for
       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.

       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
       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.

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

       In  PCRE (like Python, but unlike Perl), a recursive subpattern call is
       always treated as an atomic group. That is, once it has matched some of
       the subject string, it is never re-entered, even if it contains untried
       alternatives and there is a subsequent matching failure.

       This PCRE pattern solves the nested  parentheses  problem  (assume  the
       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-
       sized substring).  Finally there is a closing parenthesis.

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

       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()

       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
       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)

       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

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

       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,
       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.

       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)) * >

       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
       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

       matches "sense and sensibility" and "response and responsibility",  but
       not "sense and responsibility". If instead the pattern

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

       is  used, it does match "sense and responsibility" as well as the other
       two strings. Another example is  given  in  the  discussion  of  DEFINE
       above.

       Like recursive subpatterns, a "subroutine" call is always treated as an
       atomic group. That is, once it has matched some of the subject  string,
       it  is  never  re-entered, even if it contains untried alternatives and
       there is a subsequent matching failure.

       When a subpattern is used as a subroutine, processing options  such  as
       case-independence are fixed when the subpattern is defined. They cannot
       be changed for different calls. For example, consider this pattern:

         (abc)(?i:(?-1))

       It matches "abcabc". It does not match "abcABC" because the  change  of
       processing option does not affect the called subpattern.


CALLOUTS

       Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl code to be obeyed in the middle of matching a regular  expression.
       This makes it possible, amongst other things, to extract different sub-
       strings that match the same pair of parentheses when there is a repeti-
       tion.

       PCRE provides a similar feature, but of course it cannot obey arbitrary
       Perl code. The feature is called "callout". The caller of PCRE provides
       an  external function by putting its entry point in the global variable
       pcre_callout.  By default, this variable contains NULL, which  disables
       all calling out.

       Within  a  regular  expression,  (?C) indicates the points at which the
       external function is to be called. If you want  to  identify  different
       callout  points, you can put a number less than 256 after the letter C.
       The default value is zero.  For example, this pattern has  two  callout
       points:

         (?C1)abc(?C2)def

       If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
       automatically installed before each item in the pattern. They  are  all
       numbered 255.

       During matching, when PCRE reaches a callout point (and pcre_callout is
       set), the external function is called. It is provided with  the  number
       of  the callout, the position in the pattern, and, optionally, one item
       of data originally supplied by the caller of pcre_exec().  The  callout
       function  may cause matching to proceed, to backtrack, or to fail alto-
       gether. A complete description of the interface to the callout function
       is given in the pcrecallout documentation.


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.