pcrepattern(3) — Linux manual page


PCREPATTERN(3)          Library Functions Manual          PCREPATTERN(3)

NAME         top

       PCRE - Perl-compatible regular expressions


       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

       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


       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:


       (*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

   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

   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

   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


       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


       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.


       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.


       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

       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

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

       The following sections describe the use of each of the

BACKSLASH         top

       The backslash character has several uses. Firstly, if it is
       followed by a character that is not a number or a letter, it
       takes away any special meaning that character may have. This use
       of backslash as an escape character applies both inside and
       outside character classes.

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

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

       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

       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

       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

       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

       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

   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

         \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
         \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
         \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:


       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 abbreviation 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

         (*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:


       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:


       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


       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
       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


       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

       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 followed 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:


       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


       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

       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.


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

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

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

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

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

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

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

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

FULL STOP (PERIOD, DOT) AND \N         top

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

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

       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

       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


       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
       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) |

       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.


       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

       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.


       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,


       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    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

       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,


       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:

              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

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

              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)

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


       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

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

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

VERTICAL BAR         top

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


       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.


       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


       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,


       matches "ab", "aB", "c", and "C", even though when matching "C"
       the first branch is abandoned before the option setting. This is
       because the effects of option settings happen at compile time.
       There would be some very weird behaviour otherwise.

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

SUBPATTERNS         top

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

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


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

       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

         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

       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


       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".


       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:


       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":


       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":


       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


       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:


       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

       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":


       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

REPETITION         top

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

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

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


       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


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


       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:


       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


       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

       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


       If the subject is "xyz123abc123" the match point is the fourth
       character. For this reason, such a pattern is not implicitly

       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:


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

       When a capturing subpattern is repeated, the value captured is
       the substring that matched the final iteration. For example,


       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


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


       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


       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:


       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


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


       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

       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


       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


       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:


       sequences of non-digits cannot be broken, and failure happens


       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

         (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:


       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,


       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:


       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


       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


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

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

ASSERTIONS         top

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

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

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

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

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

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

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

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

   Lookahead assertions

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


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


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


       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,


       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


       is permitted, but


       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


       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:


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

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


       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


       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


       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


       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


       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,


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


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


       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:


       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

         (?(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

   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

       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 "" 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:

         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

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

COMMENTS         top

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

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

         abc #comment \n still comment

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


       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


       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

       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


       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"):


       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:


       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:


       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:


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


       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:


       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.


       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


       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:


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


       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:


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

CALLOUTS         top

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

       PCRE provides a similar feature, but of course it cannot obey
       arbitrary Perl code. The feature is called "callout". The caller
       of PCRE provides an external function by putting its entry point
       in the global variable pcre_callout (8-bit library) or
       pcre[16|32]_callout (16-bit or 32-bit library).  By default, this
       variable contains NULL, which disables all calling out.

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


       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


       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


       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

       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.


       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:


       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 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
          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.


       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:


       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).


       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.


       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 quantifier 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".


       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) 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:


       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:


       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:


       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

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

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

   Backtracking verbs in subroutines

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

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

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

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

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

SEE ALSO         top

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

AUTHOR         top

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

REVISION         top

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

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular
       Expressions) project.  Information about the project can be found
       at ⟨http://www.pcre.org/⟩.  If you have a bug report for this
       manual page, see
       ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This page was
       obtained from the tarball pcre-8.45.tar.gz fetched from
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       to-date source for the page, or you have corrections or
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PCRE 8.40                    23 October 2016              PCREPATTERN(3)

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