Installed PCRE 6.0 sources, which involved adding a number of files and

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

This file contains the PCRE man page that describes the regular expressions
supported by PCRE version 6.0. 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.
-----------------------------------------------------------------------------



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. 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
       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 in 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 character are ignored.
       An escaping backslash can be used to include a whitespace or #  charac-
       ter 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... (UTF-8 mode only)

       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). In UTF-8 mode, any number of hexadecimal dig-
       its may appear between \x{ and }, but the value of the  character  code
       must  be  less  than  2**31  (that is, the maximum hexadecimal value is
       7FFFFFFF). If characters other than hexadecimal digits  appear  between
       \x{  and }, or if there is no terminating }, this form of escape is not
       recognized. Instead, the initial \x will  be  interpreted  as  a  basic
       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 when PCRE is in UTF-8 mode. There is no difference
       in the way they are handled. For example, \xdc is exactly the  same  as
       \x{dc}.

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

       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 generates a  sin-
       gle byte from the least significant 8 bits of the value. Any subsequent
       digits stand for themselves.  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 byte value or a single UTF-8
       character (in UTF-8 mode) can be used both inside and outside character
       classes.  In  addition,  inside  a  character class, the sequence \b is
       interpreted as the backspace character (hex 08), and the sequence \X is
       interpreted  as  the  character  "X".  Outside a character class, these
       sequences have different meanings (see below).

   Generic character types

       The third 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
         \s     any whitespace character
         \S     any character that is not a 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).

       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,
       \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
       code character property support is available.

   Unicode character properties

       When PCRE is built with Unicode character property support, three addi-
       tional  escape sequences to match generic character types are available
       when UTF-8 mode is selected. They are:

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

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

         \p{L}
         \pL

       The following 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

       Extended properties such as "Greek" or "InMusicalSymbols" are not  sup-
       ported by PCRE.

       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.

       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.

   Simple assertions

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

         \b     matches at a word boundary
         \B     matches when not at a word boundary
         \A     matches at start of subject
         \Z     matches at end of subject or before newline at end
         \z     matches at end of subject
         \G     matches at first matching position in subject

       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 that is the last character of the
       string as well as at the end of the string, whereas \z matches only  at
       the end.

       The  \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 character that is the last character in the string (by
       default). Dollar need not be the last character of  the  pattern  if  a
       number  of alternatives are involved, but it should be the last item in
       any branch in which it appears.  Dollar has no  special  meaning  in  a
       character class.

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

       The meanings of the circumflex and dollar characters are changed if the
       PCRE_MULTILINE option is set. When this is the case, they match immedi-
       ately  after  and  immediately  before  an  internal newline character,
       respectively, in addition to matching at the start and end of the  sub-
       ject  string.  For  example,  the  pattern  /^abc$/ matches the subject
       string "def\nabc" (where \n represents a newline character)  in  multi-
       line mode, but not otherwise.  Consequently, patterns that are anchored
       in single line mode because all branches start with ^ are not  anchored
       in  multiline  mode,  and  a  match for circumflex is possible when the
       startoffset  argument  of  pcre_exec()  is  non-zero.   The   PCRE_DOL-
       LAR_ENDONLY option is ignored if PCRE_MULTILINE is set.

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


FULL STOP (PERIOD, DOT)

       Outside a character class, a dot in the pattern matches any one charac-
       ter  in  the  subject,  including a non-printing character, but not (by
       default) newline.  In UTF-8 mode, a dot matches  any  UTF-8  character,
       which might be more than one byte long, except (by default) newline. If
       the PCRE_DOTALL option is set, dots match newlines as  well.  The  han-
       dling  of dot is entirely independent of the handling of circumflex and
       dollar, the only relationship being  that  they  both  involve  newline
       characters. Dot has no special meaning in a character class.


MATCHING A SINGLE BYTE

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

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

       The newline character is never treated in any special way in  character
       classes,  whatever  the  setting  of  the PCRE_DOTALL or PCRE_MULTILINE
       options is. A class such as [^a] will always match a newline.

       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 the "fr_FR" 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 alterna-
       tives  are within a subpattern (defined below), "succeeds" means match-
       ing the rest of the main pattern as well as the alternative in the sub-
       pattern.


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


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  the  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, and the
       maximum depth of nesting of all subpatterns, both  capturing  and  non-
       capturing, is 200.

       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".


NAMED SUBPATTERNS

       Identifying  capturing  parentheses  by number is simple, but it can be
       very hard to keep track of the numbers in complicated  regular  expres-
       sions.  Furthermore,  if  an  expression  is  modified, the numbers may
       change. To help with this difficulty, PCRE supports the naming of  sub-
       patterns,  something  that  Perl  does  not  provide. The Python syntax
       (?P<name>...) is used. Names consist  of  alphanumeric  characters  and
       underscores, and must be unique within a pattern.

       Named  capturing  parentheses  are  still  allocated numbers as well as
       names. The PCRE API provides function calls for extracting the name-to-
       number  translation table from a compiled pattern. There is also a con-
       venience function for extracting a captured substring by name. For fur-
       ther details see the pcreapi documentation.


REPETITION

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

         a literal data character
         the . metacharacter
         the \C escape sequence
         the \X escape sequence (in UTF-8 mode with Unicode properties)
         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  (and  historical compatibility) the three most common
       quantifiers have single-character 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 which 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 . 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,
       and a later one succeed. 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 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.

       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  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:

         (?>\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  or  processing  of  a possessive quantifier and the equivalent
       atomic group.

       The possessive quantifier syntax is an extension to the Perl syntax. It
       originates in Sun's Java package.

       When  a  pattern  contains an unlimited repeat inside a subpattern that
       can itself be repeated an unlimited number of  times,  the  use  of  an
       atomic  group  is  the  only way to avoid some failing matches taking a
       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. See the  subsec-
       tion  entitled  "Non-printing  characters" above for further details of
       the handling of digits following a backslash.

       A back reference matches whatever actually matched the  capturing  sub-
       pattern  in  the  current subject string, rather than anything matching
       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.

       Back  references  to named subpatterns use the Python syntax (?P=name).
       We could rewrite the above example as follows:

         (?<p1>(?i)rah)\s+(?P=p1)

       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 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)

       The  implementation  of lookbehind assertions is, for each alternative,
       to temporarily move the current position back by the  fixed  width  and
       then try to match. If there are insufficient characters before the cur-
       rent position, the match is deemed to fail.

       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.

       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

         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)

       or, equivalently, using the possessive quantifier syntax,

         ^.*+(?<=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 three kinds of condition. If the text between the parentheses
       consists of a sequence of digits, the condition  is  satisfied  if  the
       capturing  subpattern of that number has previously matched. The number
       must be greater than zero. Consider the following pattern,  which  con-
       tains  non-significant white space to make it more readable (assume the
       PCRE_EXTENDED option) and to divide it into three  parts  for  ease  of
       discussion:

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

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

       If  the  condition  is  not  a sequence of digits or (R), 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 up to the next new-
       line character 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.  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:

         $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.

       The  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) is a recursive call of the entire regular
       expression.

       For example, 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, 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:

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

       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

         (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 the next section and the pcrecallout documen-
       tation). 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. 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. Such references must, however, follow  the  subpattern  to
       which they refer.


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.

Last updated: 28 February 2005
Copyright (c) 1997-2005 University of Cambridge.