NAME | PCRE REGULAR EXPRESSION DETAILS | SPECIAL START-OF-PATTERN ITEMS | EBCDIC CHARACTER CODES | CHARACTERS AND METACHARACTERS | BACKSLASH | CIRCUMFLEX AND DOLLAR | FULL STOP (PERIOD, DOT) AND \N | MATCHING A SINGLE DATA UNIT | SQUARE BRACKETS AND CHARACTER CLASSES | POSIX CHARACTER CLASSES | COMPATIBILITY FEATURE FOR WORD BOUNDARIES | VERTICAL BAR | INTERNAL OPTION SETTING | SUBPATTERNS | DUPLICATE SUBPATTERN NUMBERS | NAMED SUBPATTERNS | REPETITION | ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS | BACK REFERENCES | ASSERTIONS | CONDITIONAL SUBPATTERNS | COMMENTS | RECURSIVE PATTERNS | SUBPATTERNS AS SUBROUTINES | ONIGURUMA SUBROUTINE SYNTAX | CALLOUTS | BACKTRACKING CONTROL | SEE ALSO | AUTHOR | REVISION | COLOPHON

PCREPATTERN(3)            Library Functions Manual            PCREPATTERN(3)

NAME         top

       PCRE - Perl-compatible regular expressions

PCRE REGULAR EXPRESSION DETAILS         top


       The syntax and semantics of the regular expressions that are
       supported by PCRE are described in detail below. There is a quick-
       reference syntax summary in the pcresyntax page. PCRE tries to match
       Perl syntax and semantics as closely as it can. PCRE also supports
       some alternative regular expression syntax (which does not conflict
       with the Perl syntax) in order to provide some compatibility with
       regular expressions in Python, .NET, and Oniguruma.

       Perl's regular expressions are described in its own documentation,
       and regular expressions in general are covered 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 description of PCRE's regular
       expressions is intended as reference material.

       This document discusses the patterns that are supported by PCRE when
       one its main matching functions, pcre_exec() (8-bit) or
       pcre[16|32]_exec() (16- or 32-bit), is used. PCRE also has
       alternative matching functions, pcre_dfa_exec() and
       pcre[16|32_dfa_exec(), which match using a different algorithm that
       is not Perl-compatible. Some of the features discussed below are not
       available when DFA matching is used. The advantages and disadvantages
       of the alternative functions, and how they differ from the normal
       functions, are discussed in the pcrematching page.

SPECIAL START-OF-PATTERN ITEMS         top


       A number of options that can be passed to pcre_compile() can also be
       set by special items at the start of a pattern. These are not Perl-
       compatible, but are provided to make these options accessible to
       pattern writers who are not able to change the program that processes
       the pattern. Any number of these items may appear, but they must all
       be together right at the start of the pattern string, and the letters
       must be in upper case.

   UTF support

       The original operation of PCRE was on strings of one-byte characters.
       However, there is now also support for UTF-8 strings in the original
       library, an extra library that supports 16-bit and UTF-16 character
       strings, and a third library that supports 32-bit and UTF-32
       character strings. To use these features, PCRE must be built to
       include appropriate support. When using UTF strings you must either
       call the compiling function with the PCRE_UTF8, PCRE_UTF16, or
       PCRE_UTF32 option, or the pattern must start with one of these
       special sequences:

         (*UTF8)
         (*UTF16)
         (*UTF32)
         (*UTF)

       (*UTF) is a generic sequence that can be used with any of the
       libraries.  Starting a pattern with such a sequence is equivalent to
       setting the relevant option. How setting a UTF mode affects pattern
       matching is mentioned in several places below. There is also a
       summary of features in the pcreunicode page.

       Some applications that allow their users to supply patterns may wish
       to restrict them to non-UTF data for security reasons. If the
       PCRE_NEVER_UTF option is set at compile time, (*UTF) etc. are not
       allowed, and their appearance causes an error.

   Unicode property support

       Another special sequence that may appear at the start of a pattern is
       (*UCP).  This has the same effect as setting the PCRE_UCP option: it
       causes sequences such as \d and \w to use Unicode properties to
       determine character types, instead of recognizing only characters
       with codes less than 128 via a lookup table.

   Disabling auto-possessification

       If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect
       as setting the PCRE_NO_AUTO_POSSESS option at compile time. This
       stops PCRE from making quantifiers possessive when what follows
       cannot match the repeated item. For example, by default a+b is
       treated as a++b. For more details, see the pcreapi documentation.

   Disabling start-up optimizations

       If a pattern starts with (*NO_START_OPT), it has the same effect as
       setting the PCRE_NO_START_OPTIMIZE option either at compile or
       matching time. This disables several optimizations for quickly
       reaching "no match" results. For more details, see the pcreapi
       documentation.

   Newline conventions

       PCRE supports five different conventions for indicating line breaks
       in strings: a single CR (carriage return) character, a single LF
       (linefeed) character, the two-character sequence CRLF, any of the
       three preceding, or any Unicode newline sequence. The pcreapi page
       has further discussion about newlines, and shows how to set the
       newline convention in the options arguments for the compiling and
       matching functions.

       It is also possible to specify a newline convention by starting a
       pattern string with one of the following five sequences:

         (*CR)        carriage return
         (*LF)        linefeed
         (*CRLF)      carriage return, followed by linefeed
         (*ANYCRLF)   any of the three above
         (*ANY)       all Unicode newline sequences

       These override the default and the options given to the compiling
       function. For example, on a Unix system where LF is the default
       newline sequence, the pattern

         (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF
       is no longer a newline. If more than one of these settings is
       present, the last one is used.

       The newline convention affects where the circumflex and dollar
       assertions are true. It also affects the interpretation of the dot
       metacharacter when PCRE_DOTALL is not set, and the behaviour of \N.
       However, it does not affect what the \R escape sequence matches. By
       default, this is any Unicode newline sequence, for Perl
       compatibility. However, this can be changed; see the description of
       \R in the section entitled "Newline sequences" below. A change of \R
       setting can be combined with a change of newline convention.

   Setting match and recursion limits

       The caller of pcre_exec() can set a limit on the number of times the
       internal match() function is called and on the maximum depth of
       recursive calls. These facilities are provided to catch runaway
       matches that are provoked by patterns with huge matching trees (a
       typical example is a pattern with nested unlimited repeats) and to
       avoid running out of system stack by too much recursion. When one of
       these limits is reached, pcre_exec() gives an error return. The
       limits can also be set by items at the start of the pattern of the
       form

         (*LIMIT_MATCH=d)
         (*LIMIT_RECURSION=d)

       where d is any number of decimal digits. However, the value of the
       setting must be less than the value set (or defaulted) by the caller
       of pcre_exec() for it to have any effect. In other words, the pattern
       writer can lower the limits set by the programmer, but not raise
       them. If there is more than one setting of one of these limits, the
       lower value is used.

EBCDIC CHARACTER CODES         top


       PCRE can be compiled to run in an environment that uses EBCDIC as its
       character code rather than ASCII or Unicode (typically a mainframe
       system). In the sections below, character code values are ASCII or
       Unicode; in an EBCDIC environment these characters may have different
       code values, and there are no code points greater than 255.

CHARACTERS AND METACHARACTERS         top


       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 a UTF 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 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
       recognized 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         top


       The backslash character has several uses. Firstly, if it is followed
       by a character that is not a number or a letter, 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
       backslash, you write \\.

       In a UTF mode, only ASCII numbers and letters have any special
       meaning after a backslash. All other characters (in particular, those
       whose codepoints are greater than 127) are treated as literals.

       If a pattern is compiled with the PCRE_EXTENDED option, most white
       space in the pattern (other than in a character class), and
       characters between a # outside a character class and the next
       newline, inclusive, are ignored. An escaping backslash can be used to
       include a white space or # character as part of the pattern.

       If you want to remove the special meaning from a sequence of
       characters, you can do so by putting them between \Q and \E. This is
       different from Perl in that $ and @ are handled as literals in
       \Q...\E sequences in PCRE, whereas in Perl, $ and @ cause variable
       interpolation. 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.  An isolated \E that is not preceded by \Q is ignored. If \Q
       is not followed by \E later in the pattern, the literal
       interpretation continues to the end of the pattern (that is, \E is
       assumed at the end). If the isolated \Q is inside a character class,
       this causes an error, because the character class is not terminated.

   Non-printing characters

       A second use of backslash provides a way of encoding non-printing
       characters 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 often easier to use one of the following
       escape sequences than the binary character it represents.  In an
       ASCII or Unicode environment, these escapes are as follows:

         \a        alarm, that is, the BEL character (hex 07)
         \cx       "control-x", where x is any ASCII character
         \e        escape (hex 1B)
         \f        form feed (hex 0C)
         \n        linefeed (hex 0A)
         \r        carriage return (hex 0D)
         \t        tab (hex 09)
         \0dd      character with octal code 0dd
         \ddd      character with octal code ddd, or back reference
         \o{ddd..} character with octal code ddd..
         \xhh      character with hex code hh
         \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
         \uhhhh    character with hex code hhhh (JavaScript mode only)

       The precise effect of \cx on ASCII characters 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 \cA to \cZ become hex 01 to hex
       1A (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c;
       becomes hex 7B (; is 3B). If the data item (byte or 16-bit value)
       following \c has a value greater than 127, a compile-time error
       occurs. This locks out non-ASCII characters in all modes.

       When PCRE is compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t
       generate the appropriate EBCDIC code values. The \c escape is
       processed as specified for Perl in the perlebcdic document. The only
       characters that are allowed after \c are A-Z, a-z, or one of @, [, \,
       ], ^, _, or ?. Any other character provokes a compile-time error. The
       sequence \c@ encodes character code 0; after \c the letters (in
       either case) encode characters 1-26 (hex 01 to hex 1A); [, \, ], ^,
       and _ encode characters 27-31 (hex 1B to hex 1F), and \c? becomes
       either 255 (hex FF) or 95 (hex 5F).

       Thus, apart from \c?, these escapes generate the same character code
       values as they do in an ASCII environment, though the meanings of the
       values mostly differ. For example, \cG always generates code value 7,
       which is BEL in ASCII but DEL in EBCDIC.

       The sequence \c? generates DEL (127, hex 7F) in an ASCII environment,
       but because 127 is not a control character in EBCDIC, Perl makes it
       generate the APC character. Unfortunately, there are several variants
       of EBCDIC. In most of them the APC character has the value 255 (hex
       FF), but in the one Perl calls POSIX-BC its value is 95 (hex 5F). If
       certain other characters have POSIX-BC values, PCRE makes \c?
       generate 95; otherwise it generates 255.

       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\015 specifies two binary zeros followed by a CR
       character (code value 13). Make sure you supply two digits after the
       initial zero if the pattern character that follows is itself an octal
       digit.

       The escape \o must be followed by a sequence of octal digits,
       enclosed in braces. An error occurs if this is not the case. This
       escape is a recent addition to Perl; it provides way of specifying
       character code points as octal numbers greater than 0777, and it also
       allows octal numbers and back references to be unambiguously
       specified.

       For greater clarity and unambiguity, it is best to avoid following \
       by a digit greater than zero. Instead, use \o{} or \x{} to specify
       character numbers, and \g{} to specify back references. The following
       paragraphs describe the old, ambiguous syntax.

       The handling of a backslash followed by a digit other than 0 is
       complicated, and Perl has changed in recent releases, causing PCRE
       also to change. Outside a character class, PCRE reads the digit and
       any following digits as a decimal number. If the number is less than
       8, 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 following \ is
       greater than 7 and there have not been that many capturing
       subpatterns, PCRE handles \8 and \9 as the literal characters "8" and
       "9", and otherwise re-reads up to three octal digits following the
       backslash, using them to generate a data character.  Any subsequent
       digits stand for themselves. For example:

         \040   is another way of writing an ASCII 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 value 255 (decimal)
         \81    is either a back reference, or the two
                   characters "8" and "1"

       Note that octal values of 100 or greater that are specified using
       this syntax must not be introduced by a leading zero, because no more
       than three octal digits are ever read.

       By default, after \x that is not followed by {, 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 }. If a
       character other than a hexadecimal digit appears between \x{ and },
       or if there is no terminating }, an error occurs.

       If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
       is as just described only when it is followed by two hexadecimal
       digits.  Otherwise, it matches a literal "x" character. In JavaScript
       mode, support for code points greater than 256 is provided by \u,
       which must be followed by four hexadecimal digits; otherwise it
       matches a literal "u" character.

       Characters whose value is less than 256 can be defined by either of
       the two syntaxes for \x (or by \u in JavaScript mode). There is no
       difference in the way they are handled. For example, \xdc is exactly
       the same as \x{dc} (or \u00dc in JavaScript mode).

   Constraints on character values

       Characters that are specified using octal or hexadecimal numbers are
       limited to certain values, as follows:

         8-bit non-UTF mode    less than 0x100
         8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
         16-bit non-UTF mode   less than 0x10000
         16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
         32-bit non-UTF mode   less than 0x100000000
         32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint

       Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-
       called "surrogate" codepoints), and 0xffef.

   Escape sequences in character classes

       All the sequences that define a single character value can be used
       both inside and outside character classes. In addition, inside a
       character class, \b is interpreted as the backspace character (hex
       08).

       \N is not allowed in a character class. \B, \R, and \X are not
       special inside a character class. Like other unrecognized escape
       sequences, they are treated as the literal characters "B", "R", and
       "X" by default, but cause an error if the PCRE_EXTRA option is set.
       Outside a character class, these sequences have different meanings.

   Unsupported escape sequences

       In Perl, the sequences \l, \L, \u, and \U are recognized by its
       string handler and used to modify the case of following characters.
       By default, PCRE does not support these escape sequences. However, if
       the PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character,
       and \u can be used to define a character by code point, as described
       in the previous section.

   Absolute and relative back references

       The sequence \g followed by an unsigned or a 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.

   Absolute and relative subroutine calls

       For compatibility with Oniguruma, the non-Perl syntax \g followed by
       a name or a number enclosed either in angle brackets or single
       quotes, is an alternative syntax for referencing a subpattern as a
       "subroutine". Details are discussed later.  Note that \g{...} (Perl
       syntax) and \g<...> (Oniguruma syntax) are not synonymous. The former
       is a back reference; the latter is a subroutine call.

   Generic character types

       Another use of backslash is for specifying generic character types:

         \d     any decimal digit
         \D     any character that is not a decimal digit
         \h     any horizontal white space character
         \H     any character that is not a horizontal white space character
         \s     any white space character
         \S     any character that is not a white space character
         \v     any vertical white space character
         \V     any character that is not a vertical white space character
         \w     any "word" character
         \W     any "non-word" character

       There is also the single sequence \N, which matches a non-newline
       character.  This is the same as the "." metacharacter when
       PCRE_DOTALL is not set. Perl also uses \N to match characters by
       name; PCRE does not support this.

       Each pair of lower and upper case escape sequences partitions the
       complete set of characters into two disjoint sets. Any given
       character matches one, and only one, of each pair. The sequences can
       appear both inside and outside character 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, because there is
       no character to match.

       For compatibility with Perl, \s did not used to match the VT
       character (code 11), which made it different from the the POSIX
       "space" class. However, Perl added VT at release 5.18, and PCRE
       followed suit at release 8.34. The default \s characters are now HT
       (9), LF (10), VT (11), FF (12), CR (13), and space (32), which are
       defined as white space in the "C" locale. This list may vary if
       locale-specific matching is taking place. For example, in some
       locales the "non-breaking space" character (\xA0) is recognized as
       white space, and in others the VT character is not.

       A "word" character is an underscore or any character that is a letter
       or digit.  By default, the definition of letters and digits is
       controlled 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 127 are used for accented letters, and these are then
       matched by \w. The use of locales with Unicode is discouraged.

       By default, characters whose code points are greater than 127 never
       match \d, \s, or \w, and always match \D, \S, and \W, although this
       may vary for characters in the range 128-255 when locale-specific
       matching is happening.  These escape sequences retain their original
       meanings from before Unicode support was available, mainly for
       efficiency reasons. If PCRE is compiled with Unicode property
       support, and the PCRE_UCP option is set, the behaviour is changed so
       that Unicode properties are used to determine character types, as
       follows:

         \d  any character that matches \p{Nd} (decimal digit)
         \s  any character that matches \p{Z} or \h or \v
         \w  any character that matches \p{L} or \p{N}, plus underscore

       The upper case escapes match the inverse sets of characters. Note
       that \d matches only decimal digits, whereas \w matches any Unicode
       digit, as well as any Unicode letter, and underscore. Note also that
       PCRE_UCP affects \b, and \B because they are defined in terms of \w
       and \W. Matching these sequences is noticeably slower when PCRE_UCP
       is set.

       The sequences \h, \H, \v, and \V are features that were added to Perl
       at release 5.10. In contrast to the other sequences, which match only
       ASCII characters by default, these always match certain high-valued
       code points, whether or not PCRE_UCP is set. The horizontal space
       characters are:

         U+0009     Horizontal tab (HT)
         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 (LF)
         U+000B     Vertical tab (VT)
         U+000C     Form feed (FF)
         U+000D     Carriage return (CR)
         U+0085     Next line (NEL)
         U+2028     Line separator
         U+2029     Paragraph separator

       In 8-bit, non-UTF-8 mode, only the characters with codepoints less
       than 256 are relevant.

   Newline sequences

       Outside a character class, by default, the escape sequence \R matches
       any Unicode newline sequence. In 8-bit 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 (form feed,
       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 other modes, two additional characters whose codepoints are
       greater than 255 are added: LS (line separator, U+2028) and PS
       (paragraph separator, U+2029).  Unicode character property support is
       not needed for these characters to be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead
       of the complete set of Unicode line endings) by setting the option
       PCRE_BSR_ANYCRLF either at compile time or when the pattern is
       matched. (BSR is an abbrevation for "backslash R".) This can be made
       the default when PCRE is built; if this is the case, the other
       behaviour can be requested via the PCRE_BSR_UNICODE option.  It is
       also possible to specify these settings by starting a pattern string
       with one of the following sequences:

         (*BSR_ANYCRLF)   CR, LF, or CRLF only
         (*BSR_UNICODE)   any Unicode newline sequence

       These override the default and the options given to the compiling
       function, but they can themselves be overridden by options given to a
       matching function. Note that these special settings, which are not
       Perl-compatible, are recognized only at the very start of a pattern,
       and that they must be in upper case. If more than one of them is
       present, the last one is used. They can be combined with a change of
       newline convention; for example, a pattern can start with:

         (*ANY)(*BSR_ANYCRLF)

       They can also be combined with the (*UTF8), (*UTF16), (*UTF32),
       (*UTF) or (*UCP) special sequences. Inside a character class, \R is
       treated as an unrecognized escape sequence, and so matches the letter
       "R" by default, but causes an error if PCRE_EXTRA is set.

   Unicode character properties

       When PCRE is built with Unicode character property support, three
       additional escape sequences that match characters with specific
       properties are available.  When in 8-bit non-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       a Unicode extended grapheme cluster

       The property names represented by xx above are limited to the Unicode
       script names, the general category properties, "Any", which matches
       any character (including newline), and some special PCRE properties
       (described in the next section).  Other Perl properties such as
       "InMusicalSymbols" 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, Avestan, Balinese, Bamum, Bassa_Vah, Batak,
       Bengali, Bopomofo, Brahmi, Braille, Buginese, Buhid,
       Canadian_Aboriginal, Carian, Caucasian_Albanian, Chakma, Cham,
       Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret,
       Devanagari, Duployan, Egyptian_Hieroglyphs, Elbasan, Ethiopic,
       Georgian, Glagolitic, Gothic, Grantha, Greek, Gujarati, Gurmukhi,
       Han, Hangul, Hanunoo, Hebrew, Hiragana, Imperial_Aramaic, Inherited,
       Inscriptional_Pahlavi, Inscriptional_Parthian, Javanese, Kaithi,
       Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Khojki, Khudawadi,
       Lao, Latin, Lepcha, Limbu, Linear_A, Linear_B, Lisu, Lycian, Lydian,
       Mahajani, Malayalam, Mandaic, Manichaean, Meetei_Mayek,
       Mende_Kikakui, Meroitic_Cursive, Meroitic_Hieroglyphs, Miao, Modi,
       Mongolian, Mro, Myanmar, Nabataean, New_Tai_Lue, Nko, Ogham,
       Ol_Chiki, Old_Italic, Old_North_Arabian, Old_Permic, Old_Persian,
       Old_South_Arabian, Old_Turkic, Oriya, Osmanya, Pahawh_Hmong,
       Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician, Psalter_Pahlavi,
       Rejang, Runic, Samaritan, Saurashtra, Sharada, Shavian, Siddham,
       Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, Tagalog,
       Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, Thaana,
       Thai, Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi.

       Each character has exactly one Unicode 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
       general 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 Cs (Surrogate) property applies only to characters in the range
       U+D800 to U+DFFF. Such characters are not valid in Unicode strings
       and so cannot be tested by PCRE, unless UTF validity checking has
       been turned off (see the discussion of PCRE_NO_UTF8_CHECK,
       PCRE_NO_UTF16_CHECK and PCRE_NO_UTF32_CHECK in the pcreapi page).
       Perl does not support the Cs property.

       The long synonyms for property names 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)
       property.  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. This is
       different from the behaviour of current versions of Perl.

       Matching characters by Unicode property is not fast, because PCRE has
       to do a multistage table lookup in order to find a character's
       property. That is why the traditional escape sequences such as \d and
       \w do not use Unicode properties in PCRE by default, though you can
       make them do so by setting the PCRE_UCP option or by starting the
       pattern with (*UCP).

   Extended grapheme clusters

       The \X escape matches any number of Unicode characters that form an
       "extended grapheme cluster", and treats the sequence as an atomic
       group (see below).  Up to and including release 8.31, PCRE matched an
       earlier, simpler definition that was equivalent to

         (?>\PM\pM*)

       That is, it matched a character without the "mark" property, followed
       by zero or more characters with the "mark" property. Characters with
       the "mark" property are typically non-spacing accents that affect the
       preceding character.

       This simple definition was extended in Unicode to include more
       complicated kinds of composite character by giving each character a
       grapheme breaking property, and creating rules that use these
       properties to define the boundaries of extended grapheme clusters. In
       releases of PCRE later than 8.31, \X matches one of these clusters.

       \X always matches at least one character. Then it decides whether to
       add additional characters according to the following rules for ending
       a cluster:

       1. End at the end of the subject string.

       2. Do not end between CR and LF; otherwise end after any control
       character.

       3. Do not break Hangul (a Korean script) syllable sequences. Hangul
       characters are of five types: L, V, T, LV, and LVT. An L character
       may be followed by an L, V, LV, or LVT character; an LV or V
       character may be followed by a V or T character; an LVT or T
       character may be follwed only by a T character.

       4. Do not end before extending characters or spacing marks.
       Characters with the "mark" property always have the "extend" grapheme
       breaking property.

       5. Do not end after prepend characters.

       6. Otherwise, end the cluster.

   PCRE's additional properties

       As well as the standard Unicode properties described above, PCRE
       supports four more that make it possible to convert traditional
       escape sequences such as \w and \s to use Unicode properties. PCRE
       uses these non-standard, non-Perl properties internally when PCRE_UCP
       is set. However, they may also be used explicitly. These properties
       are:

         Xan   Any alphanumeric character
         Xps   Any POSIX space character
         Xsp   Any Perl space character
         Xwd   Any Perl "word" character

       Xan matches characters that have either the L (letter) or the N
       (number) property. Xps matches the characters tab, linefeed, vertical
       tab, form feed, or carriage return, and any other character that has
       the Z (separator) property.  Xsp is the same as Xps; it used to
       exclude vertical tab, for Perl compatibility, but Perl changed, and
       so PCRE followed at release 8.34. Xwd matches the same characters as
       Xan, plus underscore.

       There is another non-standard property, Xuc, which matches any
       character that can be represented by a Universal Character Name in
       C++ and other programming languages. These are the characters $, @, `
       (grave accent), and all characters with Unicode code points greater
       than or equal to U+00A0, except for the surrogates U+D800 to U+DFFF.
       Note that most base (ASCII) characters are excluded. (Universal
       Character Names are of the form \uHHHH or \UHHHHHHHH where H is a
       hexadecimal digit. Note that the Xuc property does not match these
       sequences but the characters that they represent.)

   Resetting the match start

       The escape sequence \K causes any previously 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".

       Perl documents that the use of \K within assertions is "not well
       defined". In PCRE, \K is acted upon when it occurs inside positive
       assertions, but is ignored in negative assertions. Note that when a
       pattern such as (?=ab\K) matches, the reported start of the match can
       be greater than the end of the match.

   Simple assertions

       The final use of backslash is for certain simple assertions. An
       assertion 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

       Inside a character class, \b has a different meaning; it matches the
       backspace character. If any other of these assertions appears in a
       character class, by default it matches the corresponding literal
       character (for example, \B matches the letter B). However, if the
       PCRE_EXTRA option is set, an "invalid escape sequence" error is
       generated instead.

       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. In a
       UTF mode, the meanings of \w and \W can be changed by setting the
       PCRE_UCP option. When this is done, it also affects \b and \B.
       Neither PCRE nor Perl has a separate "start of word" or "end of word"
       metasequence. However, whatever follows \b normally determines which
       it is. For example, the fragment \ba matches "a" at the start of a
       word.

       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 assertions 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, indicating 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 arguments, you can mimic Perl's /g option, and it is in
       this kind of implementation 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         top


       The circumflex and dollar metacharacters are zero-width assertions.
       That is, they test for a particular condition being true without
       consuming any characters from the subject string.

       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 argument 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 subject, it is said to be an "anchored" pattern. (There are also
       other constructs that can cause a pattern to be anchored.)

       The 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). Note,
       however, that it does not actually match the newline. 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) AND \N         top


       Outside a character class, a dot in the pattern matches any one
       character in the subject string except (by default) a character that
       signifies the end of a line.

       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 Unicode 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
       circumflex and dollar, the only relationship being that they both
       involve newlines. Dot has no special meaning in a character class.

       The escape sequence \N behaves like a dot, except that it is not
       affected by the PCRE_DOTALL option. In other words, it matches any
       character except one that signifies the end of a line. Perl also uses
       \N to match characters by name; PCRE does not support this.

MATCHING A SINGLE DATA UNIT         top


       Outside a character class, the escape sequence \C matches any one
       data unit, whether or not a UTF mode is set. In the 8-bit library,
       one data unit is one byte; in the 16-bit library it is a 16-bit unit;
       in the 32-bit library it is a 32-bit unit. Unlike a dot, \C always
       matches line-ending characters. The feature is provided in Perl in
       order to match individual bytes in UTF-8 mode, but it is unclear how
       it can usefully be used. Because \C breaks up characters into
       individual data units, matching one unit with \C in a UTF mode means
       that the rest of the string may start with a malformed UTF character.
       This has undefined results, because PCRE assumes that it is dealing
       with valid UTF strings (and by default it checks this at the start of
       processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
       PCRE_NO_UTF32_CHECK option is used).

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

       In general, the \C escape sequence is best avoided. However, one way
       of using it that avoids the problem of malformed UTF characters is to
       use a lookahead to check the length of the next character, as in this
       pattern, which could be used with a UTF-8 string (ignore white space
       and line breaks):

         (?| (?=[\x00-\x7f])(\C) |
             (?=[\x80-\x{7ff}])(\C)(\C) |
             (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
             (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

       A group that starts with (?| resets the capturing parentheses numbers
       in each alternative (see "Duplicate Subpattern Numbers" below). The
       assertions at the start of each branch check the next UTF-8 character
       for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
       character's individual bytes are then captured by the appropriate
       number of groups.

SQUARE BRACKETS AND CHARACTER CLASSES         top


       An opening square bracket introduces a character class, terminated by
       a closing square bracket. A closing square bracket on its own is not
       special by default.  However, if the PCRE_JAVASCRIPT_COMPAT option is
       set, a lone closing square bracket causes a compile-time error. 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 a UTF
       mode, the character may be more than one data unit long. 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 consumes a character from the subject string, and
       therefore it fails if the current pointer is at the end of the
       string.

       In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than
       255 (0xffff) can be included in a class as a literal string of data
       units, 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 a UTF 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 in a UTF mode for characters 128
       and above, you must ensure that PCRE is compiled with Unicode
       property support as well as with UTF 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
       characters 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, or immediately
       after a range. For example, [b-d-z] matches letters in the range b to
       d, a hyphen character, or z.

       It is not possible to have the literal character "]" as the end
       character 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 interpreted 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.

       An error is generated if a POSIX character class (see below) or an
       escape sequence other than one that defines a single character
       appears at a point where a range ending character is expected. For
       example, [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not.

       Ranges operate in the collating sequence of character values. They
       can also be used for characters specified numerically, for example
       [\000-\037]. Ranges can include any characters that are valid for the
       current mode.

       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 a non-UTF
       mode, if character tables for a French locale are in use, [\xc8-\xcb]
       matches accented E characters in both cases. In UTF modes, PCRE
       supports the concept of case for characters with values greater than
       128 only when it is compiled with Unicode property support.

       The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v,
       \V, \w, and \W may appear in a character class, and add the
       characters that they match to the class. For example, [\dABCDEF]
       matches any hexadecimal digit. In UTF modes, the PCRE_UCP option
       affects the meanings of \d, \s, \w and their upper case partners,
       just as it does when they appear outside a character class, as
       described in the section entitled "Generic character types" above.
       The escape sequence \b has a different meaning inside a character
       class; it matches the backspace character. The sequences \B, \N, \R,
       and \X are not special inside a character class. Like any other
       unrecognized escape sequences, they are treated as the literal
       characters "B", "N", "R", and "X" by default, but cause an error if
       the PCRE_EXTRA option is set.

       A circumflex 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, whereas [\w] includes
       underscore. A positive character class should be read as "something
       OR something OR ..." and a negative class as "NOT something AND NOT
       something AND NOT ...".

       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, or for
       a special compatibility feature - see the next two sections), and the
       terminating closing square bracket. However, escaping other non-
       alphanumeric characters does no harm.

POSIX CHARACTER CLASSES         top


       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 and
       space
         space    white space (the same as \s from PCRE 8.34)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The default "space" characters are HT (9), LF (10), VT (11), FF (12),
       CR (13), and space (32). If locale-specific matching is taking place,
       the list of space characters may be different; there may be fewer or
       more of them. "Space" used to be different to \s, which did not
       include VT, for Perl compatibility.  However, Perl changed at release
       5.18, and PCRE followed at release 8.34.  "Space" and \s now match
       the same set of characters.

       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.

       By default, characters with values greater than 128 do not match any
       of the POSIX character classes. However, if the PCRE_UCP option is
       passed to pcre_compile(), some of the classes are changed so that
       Unicode character properties are used. This is achieved by replacing
       certain POSIX classes by other sequences, as follows:

         [:alnum:]  becomes  \p{Xan}
         [:alpha:]  becomes  \p{L}
         [:blank:]  becomes  \h
         [:digit:]  becomes  \p{Nd}
         [:lower:]  becomes  \p{Ll}
         [:space:]  becomes  \p{Xps}
         [:upper:]  becomes  \p{Lu}
         [:word:]   becomes  \p{Xwd}

       Negated versions, such as [:^alpha:] use \P instead of \p. Three
       other POSIX classes are handled specially in UCP mode:

       [:graph:] This matches characters that have glyphs that mark the page
                 when printed. In Unicode property terms, it matches all
                 characters with the L, M, N, P, S, or Cf properties, except
                 for:

                   U+061C           Arabic Letter Mark
                   U+180E           Mongolian Vowel Separator
                   U+2066 - U+2069  Various "isolate"s

       [:print:] This matches the same characters as [:graph:] plus space
                 characters that are not controls, that is, characters with
                 the Zs property.

       [:punct:] This matches all characters that have the Unicode P
                 (punctuation) property, plus those characters whose code
                 points are less than 128 that have the S (Symbol) property.

       The other POSIX classes are unchanged, and match only characters with
       code points less than 128.

COMPATIBILITY FEATURE FOR WORD BOUNDARIES         top


       In the POSIX.2 compliant library that was included in 4.4BSD Unix,
       the ugly syntax [[:<:]] and [[:>:]] is used for matching "start of
       word" and "end of word". PCRE treats these items as follows:

         [[:<:]]  is converted to  \b(?=\w)
         [[:>:]]  is converted to  \b(?<=\w)

       Only these exact character sequences are recognized. A sequence such
       as [a[:<:]b] provokes error for an unrecognized POSIX class name.
       This support is not compatible with Perl. It is provided to help
       migrations from other environments, and is best not used in any new
       patterns. Note that \b matches at the start and the end of a word
       (see "Simple assertions" above), and in a Perl-style pattern the
       preceding or following character normally shows which is wanted,
       without the need for the assertions that are used above in order to
       give exactly the POSIX behaviour.

VERTICAL BAR         top


       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         top


       The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
       PCRE_EXTENDED options (which are Perl-compatible) 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
       possible to unset these options by preceding the letter with a
       hyphen, and a combined setting and unsetting such as (?im-sx), which
       sets PCRE_CASELESS 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.

       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.

       When one of these option changes occurs at top level (that is, not
       inside subpattern parentheses), the change applies to the remainder
       of the pattern that follows. An option change within a subpattern
       (see below for a description of subpatterns) affects only that part
       of the subpattern 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.

       Note: There are other PCRE-specific options that can be set by the
       application when the compiling or matching functions are called. In
       some cases the pattern can contain special leading sequences such as
       (*CRLF) to override what the application has set or what has been
       defaulted. Details are given in the section entitled "Newline
       sequences" above. There are also the (*UTF8), (*UTF16),(*UTF32), and
       (*UCP) leading sequences that can be used to set UTF and Unicode
       property modes; they are equivalent to setting the PCRE_UTF8,
       PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP options, respectively. The
       (*UTF) sequence is a generic version that can be used with any of the
       libraries. However, the application can set the PCRE_NEVER_UTF
       option, which locks out the use of the (*UTF) sequences.

SUBPATTERNS         top


       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 "cataract", "caterpillar", or "cat". 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 the matching function. (This applies only to
       the traditional matching functions; the DFA matching functions do not
       support capturing.)

       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 pattern

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

       the captured substrings are "red king", "red", and "king", and are
       numbered 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 capturing, 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         top


       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
       capturing 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, parentheses are numbered as usual, but the number is
       reset at the start of each branch. The numbers of any capturing
       parentheses that follow the subpattern start after the highest number
       used in any branch. The following example is taken from the Perl
       documentation. The numbers underneath 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 back reference to a numbered subpattern uses the most recent value
       that is set for that number by any subpattern. The following pattern
       matches "abcabc" or "defdef":

         /(?|(abc)|(def))\1/

       In contrast, a subroutine call to a numbered subpattern always refers
       to the first one in the pattern with the given number. The following
       pattern matches "abcabc" or "defabc":

         /(?|(abc)|(def))(?1)/

       If a condition test for a subpattern's having matched refers to a
       non-unique number, the test is true if any of the subpatterns of that
       number have matched.

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

NAMED SUBPATTERNS         top


       Identifying capturing parentheses by number is simple, but it can be
       very hard to keep track of the numbers in complicated regular
       expressions. Furthermore, if an expression is modified, the numbers
       may change. To help with this difficulty, PCRE supports the naming of
       subpatterns. 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 syntax. Perl allows identically numbered subpatterns to have
       different names, but PCRE does not.

       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,
       but must start with a non-digit. 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. (Duplicate names are also always permitted for
       subpatterns with the same number, set up as described in the previous
       section.) Duplicate names 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 back reference to a non-unique named subpattern from
       elsewhere in the pattern, the subpatterns to which the name refers
       are checked in the order in which they appear in the overall pattern.
       The first one that is set is used for the reference. For example,
       this pattern matches both "foofoo" and "barbar" but not "foobar" or
       "barfoo":

         (?:(?<n>foo)|(?<n>bar))\k<n>

       If you make a subroutine call to a non-unique named subpattern, the
       one that corresponds to the first occurrence of the name is used. In
       the absence of duplicate numbers (see the previous section) this is
       the one with the lowest number.

       If you use a named reference in a condition test (see the section
       about conditions below), either to check whether a subpattern has
       matched, or to check for recursion, all subpatterns with the same
       name are tested. If the condition is true for any one of them, the
       overall condition is true. This is the same behaviour as testing by
       number. For further details of the interfaces for handling named
       subpatterns, see the pcreapi documentation.

       Warning: You cannot use different names to distinguish between two
       subpatterns with the same number because PCRE uses only the numbers
       when matching. For this reason, an error is given at compile time if
       different names are given to subpatterns with the same number.
       However, you can always give the same name to subpatterns with the
       same number, even when PCRE_DUPNAMES is not set.

REPETITION         top


       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
         the \R escape sequence
         an escape such as \d or \pL that matches a single character
         a character class
         a back reference (see next section)
         a parenthesized subpattern (including assertions)
         a subroutine call to a subpattern (recursive or otherwise)

       The general repetition quantifier specifies a minimum and maximum
       number 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 example, {,6} is not a quantifier, but a literal string of four
       characters.

       In UTF modes, quantifiers apply to characters rather than to
       individual data units. Thus, for example, \x{100}{2} matches two
       characters, each of which is represented by a two-byte sequence in a
       UTF-8 string. Similarly, \X{3} matches three Unicode extended
       grapheme clusters, each of which may be several data units 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. This may be
       useful for subpatterns that are referenced as subroutines from
       elsewhere in the pattern (but see also the section entitled "Defining
       subpatterns for use by reference only" below). Items other than
       subpatterns that have a {0} quantifier are omitted from the compiled
       pattern.

       For convenience, 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 broken.

       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
       (equivalent 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
       newlines, it is worth setting PCRE_DOTALL in order to obtain this
       optimization, or alternatively using ^ to indicate anchoring
       explicitly.

       However, there are some cases where the optimization cannot be used.
       When .*  is inside capturing parentheses that are the subject of a
       back reference 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
       character. For this reason, such a pattern is not implicitly
       anchored.

       Another case where implicit anchoring is not applied is when the
       leading .* is inside an atomic group. Once again, a match at the
       start may fail where a later one succeeds. Consider this pattern:

         (?>.*?a)b

       It matches "ab" in the subject "aab". The use of the backtracking
       control verbs (*PRUNE) and (*SKIP) also disable this optimization.

       When a capturing subpattern is repeated, the value captured is the
       substring 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 iterations. For example, after

         /(a|(b))+/

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

ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS         top


       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
       contains 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 prepared 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

       Note that a possessive quantifier can be used with an entire group,
       for example:

         (abc|xyz){2,3}+

       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
       syntax.  Jeffrey Friedl originated the idea (and the name) in the
       first edition of his book. Mike McCloskey liked it, so implemented it
       when he built Sun's Java package, and PCRE copied it from there. It
       ultimately found its way into Perl at release 5.10.

       PCRE has an optimization that automatically "possessifies" certain
       simple 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 character 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         top


       Outside a character class, a backslash followed by a digit greater
       than 0 (and possibly further digits) is a back reference to a
       capturing subpattern 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
       pattern. 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 iteration.

       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. This escape
       must be followed by an unsigned number or a negative number,
       optionally enclosed in braces. These examples are all identical:

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

       An unsigned 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
       capturing subpattern before \g, that is, is it equivalent to \2 in
       this example.  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
       subpattern 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 example,

         ((?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 by default. For example, the
       pattern

         (a|(bc))\2

       always fails if it starts to match "a" rather than "bc". However, if
       the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back
       reference to an unset value matches an empty string.

       Because there may be many capturing parentheses in a pattern, all
       digits following a 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 white space. Otherwise, the
       \g{ syntax or an empty comment (see "Comments" below) can be used.

   Recursive back references

       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 subpatterns. For example, the pattern

         (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each
       iteration 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.

       Back references of this type cause the group that they reference to
       be treated as an atomic group.  Once the whole group has been
       matched, a subsequent matching failure cannot cause backtracking into
       the middle of the group.

ASSERTIONS         top


       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. If such an
       assertion contains capturing subpatterns within it, these are counted
       for the purposes of numbering the capturing subpatterns in the whole
       pattern. However, substring capturing is carried out only for
       positive assertions. (Perl sometimes, but not always, does do
       capturing in negative assertions.)

       WARNING: If a positive assertion containing one or more capturing
       subpatterns succeeds, but failure to match later in the pattern
       causes backtracking over this assertion, the captures within the
       assertion are reset only if no higher numbered captures are already
       set. This is, unfortunately, a fundamental limitation of the current
       implementation, and as PCRE1 is now in maintenance-only status, it is
       unlikely ever to change.

       For compatibility with Perl, assertion subpatterns may be repeated;
       though it makes no sense to assert the same thing several times, the
       side effect of capturing parentheses may occasionally be useful. In
       practice, there only three cases:

       (1) If the quantifier is {0}, the assertion is never obeyed during
       matching.  However, it may contain internal capturing parenthesized
       groups that are called from elsewhere via the subroutine mechanism.

       (2) If quantifier is {0,n} where n is greater than zero, it is
       treated as if it were {0,1}. At run time, the rest of the pattern
       match is tried with and without the assertion, the order depending on
       the greediness of the quantifier.

       (3) If the minimum repetition is greater than zero, the quantifier is
       ignored.  The assertion is obeyed just once when encountered during
       matching.

   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
       semicolon 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.  The backtracking control verb (*FAIL)
       or (*F) is a synonym for (?!).

   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
       several 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, 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 to PCRE if rewritten to use
       two top-level branches:

         (?<=abc|abde)

       In some cases, the escape sequence \K (see above) can be used instead
       of a lookbehind assertion to get round the fixed-length restriction.

       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
       current position, the assertion fails.

       In a UTF mode, PCRE does not allow the \C escape (which matches a
       single data unit even in a UTF mode) to appear in lookbehind
       assertions, because it makes it impossible to calculate the length of
       the lookbehind. The \X and \R escapes, which can match different
       numbers of data units, are also not permitted.

       "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
       lookbehinds, as long as the subpattern matches a fixed-length string.
       Recursion, however, is not supported.

       Possessive quantifiers can be used in conjunction with lookbehind
       assertions to specify efficient matching of fixed-length strings at
       the end of subject strings. 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" preceded by six characters, the first of which are digits and
       the last three of which are not "999". For example, it doesn't match
       "123abcfoo". 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         top


       It is possible to cause the matching process to obey a subpattern
       conditionally or to choose between two alternative subpatterns,
       depending on the result of an assertion, or whether a specific
       capturing subpattern has already been matched. 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
       alternatives in the subpattern, a compile-time error occurs. Each of
       the two alternatives may itself contain nested subpatterns of any
       form, including conditional subpatterns; the restriction to two
       alternatives applies only at the level of the condition. This pattern
       fragment is an example where the alternatives are complex:

         (?(1) (A|B|C) | (D | (?(2)E|F) | E) )

       There are four kinds of condition: references to subpatterns,
       references 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 a capturing subpattern of that number has
       previously matched. If there is more than one capturing subpattern
       with the same number (see the earlier section about duplicate
       subpattern numbers), the condition is true if any of them have
       matched. An alternative notation is to precede the digits with a plus
       or minus sign. In this case, the subpattern number is relative rather
       than absolute. The most recently opened parentheses can be referenced
       by (?(-1), the next most recent by (?(-2), and so on. Inside loops it
       can also make sense to refer to subsequent groups. The next
       parentheses to be opened can be referenced as (?(+1), and so on. (The
       value zero in any of these forms is not used; it provokes a compile-
       time error.)

       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
       second part matches one or more characters that are not parentheses.
       The third part is a conditional subpattern that tests whether or not
       the first set of parentheses matched. If they did, that is, if
       subject started with an opening parenthesis, the condition is true,
       and so the yes-pattern 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.

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

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

       If the name used in a condition of this kind is a duplicate, the test
       is applied to all subpatterns of the same name, and is true if any
       one of them has matched.

   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 ampersand follow the letter R, for example:

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

       the condition is true if the most recent recursion is into a
       subpattern whose number or name is given. This condition does not
       check the entire recursion stack. If the name used in a condition of
       this kind is a duplicate, the test is applied to all subpatterns of
       the same name, and is true if any one of them is the most recent
       recursion.

       At "top level", all these recursion test conditions are false.  The
       syntax for recursive patterns is 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
       referenced from elsewhere. (The use of subroutines is described
       below.) For example, a pattern to match an IPv4 address such as
       "192.168.23.245" could be written like this (ignore white space 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         top


       There are two ways of including comments in patterns that are
       processed by PCRE. In both cases, the start of the comment must not
       be in a character class, nor in the middle of any other sequence of
       related characters such as (?: or a subpattern name or number. The
       characters that make up a comment play no part in the pattern
       matching.

       The sequence (?# marks the start of a comment that continues up to
       the next closing parenthesis. Nested parentheses are not permitted.
       If the PCRE_EXTENDED option is set, an unescaped # character also
       introduces a comment, which in this case continues to immediately
       after the next newline character or character sequence in the
       pattern. Which characters are interpreted as newlines is controlled
       by the options passed to a compiling function or by a special
       sequence at the start of the pattern, as described in the section
       entitled "Newline conventions" above. Note that the end of this type
       of comment is a literal newline sequence in the pattern; escape
       sequences that happen to represent a newline do not count. For
       example, consider this pattern when PCRE_EXTENDED is set, and the
       default newline convention is in force:

         abc #comment \n still comment

       On encountering the # character, pcre_compile() skips along, looking
       for a newline in the pattern. The sequence \n is still literal at
       this stage, so it does not terminate the comment. Only an actual
       character with the code value 0x0a (the default newline) does so.

RECURSIVE PATTERNS         top


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

       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
       parenthesized substring).  Finally there is a closing parenthesis.
       Note the use of a possessive quantifier to avoid backtracking into
       sequences of non-parentheses.

       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.
       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
       referenced. They are always non-recursive 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 a possessive quantifier
       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 a possessive quantifier 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 of capturing parentheses are those
       from the outermost level. 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 inner capturing parentheses (numbered 2) is "ef",
       which is the last value taken on at the top level. If a capturing
       subpattern is not matched at the top level, its final captured value
       is unset, even if it was (temporarily) set at a deeper level during
       the matching process.

       If there are more than 15 capturing parentheses in a pattern, PCRE
       has to obtain extra memory to store data during a recursion, which it
       does by using pcre_malloc, freeing it via pcre_free afterwards. 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
       brackets, allowing for arbitrary nesting. Only digits are allowed in
       nested brackets (that is, when recursing), whereas any characters are
       permitted 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.

   Differences in recursion processing between PCRE and Perl

       Recursion processing in PCRE differs from Perl in two important ways.
       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 can be illustrated by the following pattern, which
       purports to match a palindromic string that contains an odd number of
       characters (for example, "a", "aba", "abcba", "abcdcba"):

         ^(.|(.)(?1)\2)$

       The idea is that it either matches a single character, or two
       identical characters surrounding a sub-palindrome. In Perl, this
       pattern works; in PCRE it does not if the pattern is longer than
       three characters. Consider the subject string "abcba":

       At the top level, the first character is matched, but as it is not at
       the end of the string, the first alternative fails; the second
       alternative is taken and the recursion kicks in. The recursive call
       to subpattern 1 successfully matches the next character ("b"). (Note
       that the beginning and end of line tests are not part of the
       recursion).

       Back at the top level, the next character ("c") is compared with what
       subpattern 2 matched, which was "a". This fails. Because the
       recursion is treated as an atomic group, there are now no
       backtracking points, and so the entire match fails. (Perl is able, at
       this point, to re-enter the recursion and try the second
       alternative.) However, if the pattern is written with the
       alternatives in the other order, things are different:

         ^((.)(?1)\2|.)$

       This time, the recursing alternative is tried first, and continues to
       recurse until it runs out of characters, at which point the recursion
       fails. But this time we do have another alternative to try at the
       higher level. That is the big difference: in the previous case the
       remaining alternative is at a deeper recursion level, which PCRE
       cannot use.

       To change the pattern so that it matches all palindromic strings, not
       just those with an odd number of characters, it is tempting to change
       the pattern to this:

         ^((.)(?1)\2|.?)$

       Again, this works in Perl, but not in PCRE, and for the same reason.
       When a deeper recursion has matched a single character, it cannot be
       entered again in order to match an empty string. The solution is to
       separate the two cases, and write out the odd and even cases as
       alternatives at the higher level:

         ^(?:((.)(?1)\2|)|((.)(?3)\4|.))

       If you want to match typical palindromic phrases, the pattern has to
       ignore all non-word characters, which can be done like this:

         ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$

       If run with the PCRE_CASELESS option, this pattern matches phrases
       such as "A man, a plan, a canal: Panama!" and it works well in both
       PCRE and Perl. Note the use of the possessive quantifier *+ to avoid
       backtracking into sequences of non-word characters. Without this,
       PCRE takes a great deal longer (ten times or more) to match typical
       phrases, and Perl takes so long that you think it has gone into a
       loop.

       WARNING: The palindrome-matching patterns above work only if the
       subject string does not start with a palindrome that is shorter than
       the entire string.  For example, although "abcba" is correctly
       matched, if the subject is "ababa", PCRE finds the palindrome "aba"
       at the start, then fails at top level because the end of the string
       does not follow. Once again, it cannot jump back into the recursion
       to try other alternatives, so the entire match fails.

       The second way in which PCRE and Perl differ in their recursion
       processing is in the handling of captured values. In Perl, when a
       subpattern is called recursively or as a subpattern (see the next
       section), it has no access to any values that were captured outside
       the recursion, whereas in PCRE these values can be referenced.
       Consider this pattern:

         ^(.)(\1|a(?2))

       In PCRE, this pattern matches "bab". The first capturing parentheses
       match "b", then in the second group, when the back reference \1 fails
       to match "b", the second alternative matches "a" and then recurses.
       In the recursion, \1 does now match "b" and so the whole match
       succeeds. In Perl, the pattern fails to match because inside the
       recursive call \1 cannot access the externally set value.

SUBPATTERNS AS SUBROUTINES         top


       If the syntax for a recursive subpattern call (either by number or by
       name) is used outside the parentheses to which it refers, it operates
       like a subroutine in a programming language. The called subpattern
       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.

       All subroutine calls, whether recursive or not, are always treated as
       atomic groups. That is, once a subroutine 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. Any
       capturing parentheses that are set during the subroutine call revert
       to their previous values afterwards.

       Processing options such as case-independence are fixed when a
       subpattern is defined, so if it is used as a subroutine, such options
       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.

ONIGURUMA SUBROUTINE SYNTAX         top


       For compatibility with Oniguruma, the non-Perl syntax \g followed by
       a name or a number enclosed either in angle brackets or single
       quotes, is an alternative syntax for referencing a subpattern as a
       subroutine, possibly recursively. Here are two of the examples used
       above, rewritten using this syntax:

         (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
         (sens|respons)e and \g'1'ibility

       PCRE supports an extension to Oniguruma: if a number is preceded by a
       plus or a minus sign it is taken as a relative reference. For
       example:

         (abc)(?i:\g<-1>)

       Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are
       not synonymous. The former is a back reference; the latter is a
       subroutine call.

CALLOUTS         top


       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 substrings that match the same pair of parentheses when
       there is a repetition.

       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 (8-bit library) or pcre[16|32]_callout
       (16-bit or 32-bit library).  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 a compiling function,
       callouts are automatically installed before each item in the pattern.
       They are all numbered 255. If there is a conditional group in the
       pattern whose condition is an assertion, an additional callout is
       inserted just before the condition. An explicit callout may also be
       set at this position, as in this example:

         (?(?C9)(?=a)abc|def)

       Note that this applies only to assertion conditions, not to other
       types of condition.

       During matching, when PCRE reaches a callout point, 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 the matching function. The
       callout function may cause matching to proceed, to backtrack, or to
       fail altogether.

       By default, PCRE implements a number of optimizations at compile time
       and matching time, and one side-effect is that sometimes callouts are
       skipped. If you need all possible callouts to happen, you need to set
       options that disable the relevant optimizations. More details, and a
       complete description of the interface to the callout function, are
       given in the pcrecallout documentation.

BACKTRACKING CONTROL         top


       Perl 5.10 introduced a number of "Special Backtracking Control
       Verbs", which are still described in the Perl documentation as
       "experimental and subject to change or removal in a future version of
       Perl". It goes on to say: "Their usage in production code should be
       noted to avoid problems during upgrades." The same remarks apply to
       the PCRE features described in this section.

       The new verbs make use of what was previously invalid syntax: an
       opening parenthesis followed by an asterisk. They are generally of
       the form (*VERB) or (*VERB:NAME). Some may take either form, possibly
       behaving differently depending on whether or not a name is present. A
       name is any sequence of characters that does not include a closing
       parenthesis. The maximum length of name is 255 in the 8-bit library
       and 65535 in the 16-bit and 32-bit libraries. If the name is empty,
       that is, if the closing parenthesis immediately follows the colon,
       the effect is as if the colon were not there.  Any number of these
       verbs may occur in a pattern.

       Since these verbs are specifically related to backtracking, most of
       them can be used only when the pattern is to be matched using one of
       the traditional matching functions, because these use a backtracking
       algorithm. With the exception of (*FAIL), which behaves like a
       failing negative assertion, the backtracking control verbs cause an
       error if encountered by a DFA matching function.

       The behaviour of these verbs in repeated groups, assertions, and in
       subpatterns called as subroutines (whether or not recursively) is
       documented below.

   Optimizations that affect backtracking verbs

       PCRE contains some optimizations that are used to speed up matching
       by running some checks at the start of each match attempt. For
       example, it may know the minimum length of matching subject, or that
       a particular character must be present. When one of these
       optimizations bypasses the running of a match, any included
       backtracking verbs will not, of course, be processed. You can
       suppress the start-of-match optimizations by setting the
       PCRE_NO_START_OPTIMIZE option when calling pcre_compile() or
       pcre_exec(), or by starting the pattern with (*NO_START_OPT). There
       is more discussion of this option in the section entitled "Option
       bits for pcre_exec()" in the pcreapi documentation.

       Experiments with Perl suggest that it too has similar optimizations,
       sometimes leading to anomalous results.

   Verbs that act immediately

       The following verbs act as soon as they are encountered. They may not
       be followed by a name.

          (*ACCEPT)

       This verb causes the match to end successfully, skipping the
       remainder of the pattern. However, when it is inside a subpattern
       that is called as a subroutine, only that subpattern is ended
       successfully. Matching then continues at the outer level. If
       (*ACCEPT) in triggered in a positive assertion, the assertion
       succeeds; in a negative assertion, the assertion fails.

       If (*ACCEPT) is inside capturing parentheses, the data so far is
       captured. For example:

         A((?:A|B(*ACCEPT)|C)D)

       This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is
       captured by the outer parentheses.

         (*FAIL) or (*F)

       This verb causes a matching failure, forcing backtracking to occur.
       It is equivalent to (?!) but easier to read. The Perl documentation
       notes that it is probably useful only when combined with (?{}) or
       (??{}). Those are, of course, Perl features that are not present in
       PCRE. The nearest equivalent is the callout feature, as for example
       in this pattern:

         a+(?C)(*FAIL)

       A match with the string "aaaa" always fails, but the callout is taken
       before each backtrack happens (in this example, 10 times).

   Recording which path was taken

       There is one verb whose main purpose is to track how a match was
       arrived at, though it also has a secondary use in conjunction with
       advancing the match starting point (see (*SKIP) below).

         (*MARK:NAME) or (*:NAME)

       A name is always required with this verb. There may be as many
       instances of (*MARK) as you like in a pattern, and their names do not
       have to be unique.

       When a match succeeds, the name of the last-encountered (*MARK:NAME),
       (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to
       the caller as described in the section entitled "Extra data for
       pcre_exec()" in the pcreapi documentation. Here is an example of
       pcretest output, where the /K modifier requests the retrieval and
       outputting of (*MARK) data:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/K
         data> XY
          0: XY
         MK: A
         XZ
          0: XZ
         MK: B

       The (*MARK) name is tagged with "MK:" in this output, and in this
       example it indicates which of the two alternatives matched. This is a
       more efficient way of obtaining this information than putting each
       alternative in its own capturing parentheses.

       If a verb with a name is encountered in a positive assertion that is
       true, the name is recorded and passed back if it is the last-
       encountered. This does not happen for negative assertions or failing
       positive assertions.

       After a partial match or a failed match, the last encountered name in
       the entire match process is returned. For example:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/K
         data> XP
         No match, mark = B

       Note that in this unanchored example the mark is retained from the
       match attempt that started at the letter "X" in the subject.
       Subsequent match attempts starting at "P" and then with an empty
       string do not get as far as the (*MARK) item, but nevertheless do not
       reset it.

       If you are interested in (*MARK) values after failed matches, you
       should probably set the PCRE_NO_START_OPTIMIZE option (see above) to
       ensure that the match is always attempted.

   Verbs that act after backtracking

       The following verbs do nothing when they are encountered. Matching
       continues with what follows, but if there is no subsequent match,
       causing a backtrack to the verb, a failure is forced. That is,
       backtracking cannot pass to the left of the verb. However, when one
       of these verbs appears inside an atomic group or an assertion that is
       true, its effect is confined to that group, because once the group
       has been matched, there is never any backtracking into it. In this
       situation, backtracking can "jump back" to the left of the entire
       atomic group or assertion. (Remember also, as stated above, that this
       localization also applies in subroutine calls.)

       These verbs differ in exactly what kind of failure occurs when
       backtracking reaches them. The behaviour described below is what
       happens when the verb is not in a subroutine or an assertion.
       Subsequent sections cover these special cases.

         (*COMMIT)

       This verb, which may not be followed by a name, causes the whole
       match to fail outright if there is a later matching failure that
       causes backtracking to reach it. Even if the pattern is unanchored,
       no further attempts to find a match by advancing the starting point
       take place. If (*COMMIT) is the only backtracking verb that is
       encountered, once it has been passed pcre_exec() is committed to
       finding a match at the current starting point, or not at all. For
       example:

         a+(*COMMIT)b

       This matches "xxaab" but not "aacaab". It can be thought of as a kind
       of dynamic anchor, or "I've started, so I must finish." The name of
       the most recently passed (*MARK) in the path is passed back when
       (*COMMIT) forces a match failure.

       If there is more than one backtracking verb in a pattern, a different
       one that follows (*COMMIT) may be triggered first, so merely passing
       (*COMMIT) during a match does not always guarantee that a match must
       be at this starting point.

       Note that (*COMMIT) at the start of a pattern is not the same as an
       anchor, unless PCRE's start-of-match optimizations are turned off, as
       shown in this output from pcretest:

           re> /(*COMMIT)abc/
         data> xyzabc
          0: abc
         data> xyzabc\Y
         No match

       For this pattern, PCRE knows that any match must start with "a", so
       the optimization skips along the subject to "a" before applying the
       pattern to the first set of data. The match attempt then succeeds. In
       the second set of data, the escape sequence \Y is interpreted by the
       pcretest program. It causes the PCRE_NO_START_OPTIMIZE option to be
       set when pcre_exec() is called.  This disables the optimization that
       skips along to the first character. The pattern is now applied
       starting at "x", and so the (*COMMIT) causes the match to fail
       without trying any other starting points.

         (*PRUNE) or (*PRUNE:NAME)

       This verb causes the match to fail at the current starting position
       in the subject if there is a later matching failure that causes
       backtracking to reach it. If the pattern is unanchored, the normal
       "bumpalong" advance to the next starting character then happens.
       Backtracking can occur as usual to the left of (*PRUNE), before it is
       reached, or when matching to the right of (*PRUNE), but if there is
       no match to the right, backtracking cannot cross (*PRUNE). In simple
       cases, the use of (*PRUNE) is just an alternative to an atomic group
       or possessive quantifier, but there are some uses of (*PRUNE) that
       cannot be expressed in any other way. In an anchored pattern (*PRUNE)
       has the same effect as (*COMMIT).

       The behaviour of (*PRUNE:NAME) is the not the same as
       (*MARK:NAME)(*PRUNE).  It is like (*MARK:NAME) in that the name is
       remembered for passing back to the caller. However, (*SKIP:NAME)
       searches only for names set with (*MARK).

         (*SKIP)

       This verb, when given without a name, is like (*PRUNE), except that
       if the pattern is unanchored, the "bumpalong" advance is not to the
       next character, but to the position in the subject where (*SKIP) was
       encountered. (*SKIP) signifies that whatever text was matched leading
       up to it cannot be part of a successful match. Consider:

         a+(*SKIP)b

       If the subject is "aaaac...", after the first match attempt fails
       (starting at the first character in the string), the starting point
       skips on to start the next attempt at "c". Note that a possessive
       quantifer does not have the same effect as this example; although it
       would suppress backtracking during the first match attempt, the
       second attempt would start at the second character instead of
       skipping on to "c".

         (*SKIP:NAME)

       When (*SKIP) has an associated name, its behaviour is modified. When
       it is triggered, the previous path through the pattern is searched
       for the most recent (*MARK) that has the same name. If one is found,
       the "bumpalong" advance is to the subject position that corresponds
       to that (*MARK) instead of to where (*SKIP) was encountered. If no
       (*MARK) with a matching name is found, the (*SKIP) is ignored.

       Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME).
       It ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME).

         (*THEN) or (*THEN:NAME)

       This verb causes a skip to the next innermost alternative when
       backtracking reaches it. That is, it cancels any further backtracking
       within the current alternative. Its name comes from the observation
       that it can be used for a pattern-based if-then-else block:

         ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       If the COND1 pattern matches, FOO is tried (and possibly further
       items after the end of the group if FOO succeeds); on failure, the
       matcher skips to the second alternative and tries COND2, without
       backtracking into COND1. If that succeeds and BAR fails, COND3 is
       tried. If subsequently BAZ fails, there are no more alternatives, so
       there is a backtrack to whatever came before the entire group. If
       (*THEN) is not inside an alternation, it acts like (*PRUNE).

       The behaviour of (*THEN:NAME) is the not the same as
       (*MARK:NAME)(*THEN).  It is like (*MARK:NAME) in that the name is
       remembered for passing back to the caller. However, (*SKIP:NAME)
       searches only for names set with (*MARK).

       A subpattern that does not contain a | character is just a part of
       the enclosing alternative; it is not a nested alternation with only
       one alternative. The effect of (*THEN) extends beyond such a
       subpattern to the enclosing alternative. Consider this pattern, where
       A, B, etc. are complex pattern fragments that do not contain any |
       characters at this level:

         A (B(*THEN)C) | D

       If A and B are matched, but there is a failure in C, matching does
       not backtrack into A; instead it moves to the next alternative, that
       is, D.  However, if the subpattern containing (*THEN) is given an
       alternative, it behaves differently:

         A (B(*THEN)C | (*FAIL)) | D

       The effect of (*THEN) is now confined to the inner subpattern. After
       a failure in C, matching moves to (*FAIL), which causes the whole
       subpattern to fail because there are no more alternatives to try. In
       this case, matching does now backtrack into A.

       Note that a conditional subpattern is not considered as having two
       alternatives, because only one is ever used. In other words, the |
       character in a conditional subpattern has a different meaning.
       Ignoring white space, consider:

         ^.*? (?(?=a) a | b(*THEN)c )

       If the subject is "ba", this pattern does not match. Because .*? is
       ungreedy, it initially matches zero characters. The condition (?=a)
       then fails, the character "b" is matched, but "c" is not. At this
       point, matching does not backtrack to .*? as might perhaps be
       expected from the presence of the | character. The conditional
       subpattern is part of the single alternative that comprises the whole
       pattern, and so the match fails. (If there was a backtrack into .*?,
       allowing it to match "b", the match would succeed.)

       The verbs just described provide four different "strengths" of
       control when subsequent matching fails. (*THEN) is the weakest,
       carrying on the match at the next alternative. (*PRUNE) comes next,
       failing the match at the current starting position, but allowing an
       advance to the next character (for an unanchored pattern). (*SKIP) is
       similar, except that the advance may be more than one character.
       (*COMMIT) is the strongest, causing the entire match to fail.

   More than one backtracking verb

       If more than one backtracking verb is present in a pattern, the one
       that is backtracked onto first acts. For example, consider this
       pattern, where A, B, etc. are complex pattern fragments:

         (A(*COMMIT)B(*THEN)C|ABD)

       If A matches but B fails, the backtrack to (*COMMIT) causes the
       entire match to fail. However, if A and B match, but C fails, the
       backtrack to (*THEN) causes the next alternative (ABD) to be tried.
       This behaviour is consistent, but is not always the same as Perl's.
       It means that if two or more backtracking verbs appear in succession,
       all the the last of them has no effect. Consider this example:

         ...(*COMMIT)(*PRUNE)...

       If there is a matching failure to the right, backtracking onto
       (*PRUNE) causes it to be triggered, and its action is taken. There
       can never be a backtrack onto (*COMMIT).

   Backtracking verbs in repeated groups

       PCRE differs from Perl in its handling of backtracking verbs in
       repeated groups. For example, consider:

         /(a(*COMMIT)b)+ac/

       If the subject is "abac", Perl matches, but PCRE fails because the
       (*COMMIT) in the second repeat of the group acts.

   Backtracking verbs in assertions

       (*FAIL) in an assertion has its normal effect: it forces an immediate
       backtrack.

       (*ACCEPT) in a positive assertion causes the assertion to succeed
       without any further processing. In a negative assertion, (*ACCEPT)
       causes the assertion to fail without any further processing.

       The other backtracking verbs are not treated specially if they appear
       in a positive assertion. In particular, (*THEN) skips to the next
       alternative in the innermost enclosing group that has alternations,
       whether or not this is within the assertion.

       Negative assertions are, however, different, in order to ensure that
       changing a positive assertion into a negative assertion changes its
       result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a
       negative assertion to be true, without considering any further
       alternative branches in the assertion.  Backtracking into (*THEN)
       causes it to skip to the next enclosing alternative within the
       assertion (the normal behaviour), but if the assertion does not have
       such an alternative, (*THEN) behaves like (*PRUNE).

   Backtracking verbs in subroutines

       These behaviours occur whether or not the subpattern is called
       recursively.  Perl's treatment of subroutines is different in some
       cases.

       (*FAIL) in a subpattern called as a subroutine has its normal effect:
       it forces an immediate backtrack.

       (*ACCEPT) in a subpattern called as a subroutine causes the
       subroutine match to succeed without any further processing. Matching
       then continues after the subroutine call.

       (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a
       subroutine cause the subroutine match to fail.

       (*THEN) skips to the next alternative in the innermost enclosing
       group within the subpattern that has alternatives. If there is no
       such group within the subpattern, (*THEN) causes the subroutine match
       to fail.

SEE ALSO         top


       pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3),
       pcre16(3), pcre32(3).

AUTHOR         top


       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION         top


       Last updated: 23 October 2016
       Copyright (c) 1997-2016 University of Cambridge.

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular Expressions)
       project.  Information about the project can be found at 
       ⟨http://www.pcre.org/⟩.  If you have a bug report for this manual
       page, see ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This
       page was obtained from the tarball pcre-8.41.tar.gz fetched from 
       ⟨ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/⟩ on
       2017-09-15.  If you discover any rendering problems in this HTML ver‐
       sion of the page, or you believe there is a better or more up-to-date
       source for the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original man‐
       ual page), send a mail to man-pages@man7.org

PCRE 8.40                      23 October 2016                PCREPATTERN(3)

Pages that refer to this page: pcregrep(1)pcretest(1)pcresyntax(3)