pcrepattern

PCREPATTERN(3)             Library Functions Manual             PCREPATTERN(3)



NAME
       PCRE - Perl-compatible regular expressions

PCRE REGULAR EXPRESSION DETAILS

       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

       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

       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

       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

       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

       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

       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

       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

       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

       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

       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

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

         gilbert|sullivan

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

INTERNAL OPTION SETTING

       The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
       PCRE_EXTENDED options (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

       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

       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

       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

       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

       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

       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

       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

       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

       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

       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

       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

       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

       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

       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

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

AUTHOR

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

REVISION

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



PCRE 8.40                       23 October 2016                 PCREPATTERN(3)