pcre2syntax man page

PCRE2 — Perl-compatible regular expressions (revised API)

Pcre2 Regular Expression Syntax Summary

The full syntax and semantics of the regular expressions that are supported by PCRE2 are described in the pcre2pattern documentation. This document contains a quick-reference summary of the syntax.

Quoting

 \x         where x is non-alphanumeric is a literal x
 \Q...\E    treat enclosed characters as literal

Escaped Characters

This table applies to ASCII and Unicode environments.

 \a         alarm, that is, the BEL character (hex 07)
 \cx        "control-x", where x is any ASCII printing character
 \e         escape (hex 1B)
 \f         form feed (hex 0C)
 \n         newline (hex 0A)
 \r         carriage return (hex 0D)
 \t         tab (hex 09)
 \0dd       character with octal code 0dd
 \ddd       character with octal code ddd, or backreference
 \o{ddd..}  character with octal code ddd..
 \U         "U" if PCRE2_ALT_BSUX is set (otherwise is an error)
 \uhhhh     character with hex code hhhh (if PCRE2_ALT_BSUX is set)
 \xhh       character with hex code hh
 \x{hhh..}  character with hex code hhh..

Note that \0dd is always an octal code. The treatment of backslash followed by a non-zero digit is complicated; for details see the section "Non-printing characters" in the pcre2pattern documentation, where details of escape processing in EBCDIC environments are also given.

When \x is not followed by {, from zero to two hexadecimal digits are read, but if PCRE2_ALT_BSUX is set, \x must be followed by two hexadecimal digits to be recognized as a hexadecimal escape; otherwise it matches a literal "x". Likewise, if \u (in ALT_BSUX mode) is not followed by four hexadecimal digits, it matches a literal "u".

Character Types

 .          any character except newline;
              in dotall mode, any character whatsoever
 \C         one code unit, even in UTF mode (best avoided)
 \d         a decimal digit
 \D         a character that is not a decimal digit
 \h         a horizontal white space character
 \H         a character that is not a horizontal white space character
 \N         a character that is not a newline
 \p{xx}     a character with the xx property
 \P{xx}     a character without the xx property
 \R         a newline sequence
 \s         a white space character
 \S         a character that is not a white space character
 \v         a vertical white space character
 \V         a character that is not a vertical white space character
 \w         a "word" character
 \W         a "non-word" character
 \X         a Unicode extended grapheme cluster

\C is dangerous because it may leave the current matching point in the middle of a UTF-8 or UTF-16 character. The application can lock out the use of \C by setting the PCRE2_NEVER_BACKSLASH_C option. It is also possible to build PCRE2 with the use of \C permanently disabled.

By default, \d, \s, and \w match only ASCII characters, even in UTF-8 mode or in the 16-bit and 32-bit libraries. However, if locale-specific matching is happening, \s and \w may also match characters with code points in the range 128-255. If the PCRE2_UCP option is set, the behaviour of these escape sequences is changed to use Unicode properties and they match many more characters.

GENERAL CATEGORY PROPERTIES FOR \p and \P

 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
 L&         Ll, Lu, or Lt

 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

PCRE2 SPECIAL CATEGORY PROPERTIES FOR \p and \P

 Xan        Alphanumeric: union of properties L and N
 Xps        POSIX space: property Z or tab, NL, VT, FF, CR
 Xsp        Perl space: property Z or tab, NL, VT, FF, CR
 Xuc        Univerally-named character: one that can be
              represented by a Universal Character Name
 Xwd        Perl word: property Xan or underscore

Perl and POSIX space are now the same. Perl added VT to its space character set at release 5.18.

SCRIPT NAMES FOR \p AND \P

Ahom, Anatolian_Hieroglyphs, 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, Hatran, 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, Multani, Myanmar, Nabataean, New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Hungarian, 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, SignWriting, 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.

Character Classes

 [...]       positive character class
 [^...]      negative character class
 [x-y]       range (can be used for hex characters)
 [[:xxx:]]   positive POSIX named set
 [[:^xxx:]]  negative POSIX named set

 alnum       alphanumeric
 alpha       alphabetic
 ascii       0-127
 blank       space or tab
 cntrl       control character
 digit       decimal digit
 graph       printing, excluding space
 lower       lower case letter
 print       printing, including space
 punct       printing, excluding alphanumeric
 space       white space
 upper       upper case letter
 word        same as \w
 xdigit      hexadecimal digit

In PCRE2, POSIX character set names recognize only ASCII characters by default, but some of them use Unicode properties if PCRE2_UCP is set. You can use \Q...\E inside a character class.

Quantifiers

 ?           0 or 1, greedy
 ?+          0 or 1, possessive
 ??          0 or 1, lazy
 *           0 or more, greedy
 *+          0 or more, possessive
 *?          0 or more, lazy
 +           1 or more, greedy
 ++          1 or more, possessive
 +?          1 or more, lazy
 {n}         exactly n
 {n,m}       at least n, no more than m, greedy
 {n,m}+      at least n, no more than m, possessive
 {n,m}?      at least n, no more than m, lazy
 {n,}        n or more, greedy
 {n,}+       n or more, possessive
 {n,}?       n or more, lazy

Anchors and Simple Assertions

 \b          word boundary
 \B          not a word boundary
 ^           start of subject
               also after an internal newline in multiline mode
               (after any newline if PCRE2_ALT_CIRCUMFLEX is set)
 \A          start of subject
 $           end of subject
               also before newline at end of subject
               also before internal newline in multiline mode
 \Z          end of subject
               also before newline at end of subject
 \z          end of subject
 \G          first matching position in subject

Match Point Reset

 \K          reset start of match

\K is honoured in positive assertions, but ignored in negative ones.

Alternation

 expr|expr|expr...

Capturing

 (...)           capturing group
 (?<name>...)    named capturing group (Perl)
 (?'name'...)    named capturing group (Perl)
 (?P<name>...)   named capturing group (Python)
 (?:...)         non-capturing group
 (?|...)         non-capturing group; reset group numbers for
                  capturing groups in each alternative

Atomic Groups

 (?>...)         atomic, non-capturing group

Comment

 (?#....)        comment (not nestable)

Option Setting

 (?i)            caseless
 (?J)            allow duplicate names
 (?m)            multiline
 (?s)            single line (dotall)
 (?U)            default ungreedy (lazy)
 (?x)            extended (ignore white space)
 (?-...)         unset option(s)

The following are recognized only at the very start of a pattern or after one of the newline or \R options with similar syntax. More than one of them may appear.

 (*LIMIT_MATCH=d) set the match limit to d (decimal number)
 (*LIMIT_RECURSION=d) set the recursion limit to d (decimal number)
 (*NOTEMPTY)     set PCRE2_NOTEMPTY when matching
 (*NOTEMPTY_ATSTART) set PCRE2_NOTEMPTY_ATSTART when matching
 (*NO_AUTO_POSSESS) no auto-possessification (PCRE2_NO_AUTO_POSSESS)
 (*NO_DOTSTAR_ANCHOR) no .* anchoring (PCRE2_NO_DOTSTAR_ANCHOR)
 (*NO_JIT)       disable JIT optimization
 (*NO_START_OPT) no start-match optimization (PCRE2_NO_START_OPTIMIZE)
 (*UTF)          set appropriate UTF mode for the library in use
 (*UCP)          set PCRE2_UCP (use Unicode properties for \d etc)

Note that LIMIT_MATCH and LIMIT_RECURSION can only reduce the value of the limits set by the caller of pcre2_match() or pcre2_dfa_match(), not increase them. The application can lock out the use of (*UTF) and (*UCP) by setting the PCRE2_NEVER_UTF or PCRE2_NEVER_UCP options, respectively, at compile time.

Newline Convention

These are recognized only at the very start of the pattern or after option settings with a similar syntax.

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

What \R Matches

These are recognized only at the very start of the pattern or after option setting with a similar syntax.

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

Lookahead and Lookbehind Assertions

 (?=...)         positive look ahead
 (?!...)         negative look ahead
 (?<=...)        positive look behind
 (?<!...)        negative look behind

Each top-level branch of a look behind must be of a fixed length.

Backreferences

 \n              reference by number (can be ambiguous)
 \gn             reference by number
 \g{n}           reference by number
 \g+n            relative reference by number (PCRE2 extension)
 \g-n            relative reference by number
 \g{+n}          relative reference by number (PCRE2 extension)
 \g{-n}          relative reference by number
 \k<name>        reference by name (Perl)
 \k'name'        reference by name (Perl)
 \g{name}        reference by name (Perl)
 \k{name}        reference by name (.NET)
 (?P=name)       reference by name (Python)

Subroutine References (Possibly Recursive)

 (?R)            recurse whole pattern
 (?n)            call subpattern by absolute number
 (?+n)           call subpattern by relative number
 (?-n)           call subpattern by relative number
 (?&name)        call subpattern by name (Perl)
 (?P>name)       call subpattern by name (Python)
 \g<name>        call subpattern by name (Oniguruma)
 \g'name'        call subpattern by name (Oniguruma)
 \g<n>           call subpattern by absolute number (Oniguruma)
 \g'n'           call subpattern by absolute number (Oniguruma)
 \g<+n>          call subpattern by relative number (PCRE2 extension)
 \g'+n'          call subpattern by relative number (PCRE2 extension)
 \g<-n>          call subpattern by relative number (PCRE2 extension)
 \g'-n'          call subpattern by relative number (PCRE2 extension)

Conditional Patterns

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

 (?(n)               absolute reference condition
 (?(+n)              relative reference condition
 (?(-n)              relative reference condition
 (?(<name>)          named reference condition (Perl)
 (?('name')          named reference condition (Perl)
 (?(name)            named reference condition (PCRE2, deprecated)
 (?(R)               overall recursion condition
 (?(Rn)              specific numbered group recursion condition
 (?(R&name)          specific named group recursion condition
 (?(DEFINE)          define subpattern for reference
 (?(VERSION[>]=n.m)  test PCRE2 version
 (?(assert)          assertion condition

Note the ambiguity of (?(R) and (?(Rn) which might be named reference conditions or recursion tests. Such a condition is interpreted as a reference condition if the relevant named group exists.

Backtracking Control

The following act immediately they are reached:

 (*ACCEPT)       force successful match
 (*FAIL)         force backtrack; synonym (*F)
 (*MARK:NAME)    set name to be passed back; synonym (*:NAME)

The following act only when a subsequent match failure causes a backtrack to reach them. They all force a match failure, but they differ in what happens afterwards. Those that advance the start-of-match point do so only if the pattern is not anchored.

 (*COMMIT)       overall failure, no advance of starting point
 (*PRUNE)        advance to next starting character
 (*PRUNE:NAME)   equivalent to (*MARK:NAME)(*PRUNE)
 (*SKIP)         advance to current matching position
 (*SKIP:NAME)    advance to position corresponding to an earlier
                 (*MARK:NAME); if not found, the (*SKIP) is ignored
 (*THEN)         local failure, backtrack to next alternation
 (*THEN:NAME)    equivalent to (*MARK:NAME)(*THEN)

Callouts

 (?C)            callout (assumed number 0)
 (?Cn)           callout with numerical data n
 (?C"text")      callout with string data

The allowed string delimiters are ` ' " ^ % # $ (which are the same for the start and the end), and the starting delimiter { matched with the ending delimiter }. To encode the ending delimiter within the string, double it.

See Also

pcre2pattern(3), pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2(3).

Author

Philip Hazel
University Computing Service
Cambridge, England.

Revision

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

Referenced By

pcre2grep(1), pcre2pattern(3).

23 December 2016 PCRE2 10.23