tools/RE2Syntax.md

In re2: R Interface to Google RE2 (C++) Regular Expression Library

The simplest regular expression is a single literal character. Except for the metacharacters like *+?()|, characters match themselves. To match a metacharacter, escape it with a backslash: \+ matches a literal plus character.

Two regular expressions can be alternated or concatenated to form a new regular expression: if e1 matches s and e2 matches t, then e1|e2 matches s or t, and e1e2 matches st.

The metacharacters *, +, and ? are repetition operators: e1* matches a sequence of zero or more (possibly different) strings, each of which match e1; e1+ matches one or more; e1? matches zero or one.

The operator precedence, from weakest to strongest binding, is first alternation, then concatenation, and finally the repetition operators. Explicit parentheses can be used to force different meanings, just as in arithmetic expressions. Some examples: ab|cd is equivalent to (ab)|(cd); ab* is equivalent to a(b*).

The syntax described so far is most of the traditional Unix egrep regular expression syntax. This subset suffices to describe all regular languages: loosely speaking, a regular language is a set of strings that can be matched in a single pass through the text using only a fixed amount of memory. Newer regular expression facilities (notably Perl and those that have copied it) have added many new operators and escape sequences, which make the regular expressions more concise, and sometimes more cryptic, but usually not more powerful.

This page lists the regular expression syntax accepted by RE2.

It also lists some syntax accepted by PCRE, PERL, and VIM.

Grayed out expressions are not supported by RE2.

kinds of single-character expressionsexamples any character, possibly including newline (s=true). character class[xyz] negated character class[^xyz] Perl character class (link)\d negated Perl character class\D ASCII character class (link)[[:alpha:]] negated ASCII character class[[:^alpha:]] Unicode character class (one-letter name)\pN Unicode character class\p{Greek} negated Unicode character class (one-letter name)\PN negated Unicode character class\P{Greek} Composites xyx followed by y x|yx or y (prefer x) Repetitions x*zero or more x, prefer more x+one or more x, prefer more x?zero or one x, prefer one x{n,m}n or n+1 or ... or m x, prefer more x{n,}n or more x, prefer more x{n}exactly n x x*?zero or more x, prefer fewer x+?one or more x, prefer fewer x??zero or one x, prefer zero x{n,m}?n or n+1 or ... or m x, prefer fewer x{n,}?n or more x, prefer fewer x{n}?exactly n x x{}(≡ x*) (NOT SUPPORTED) VIM x{-}(≡ x*?) (NOT SUPPORTED) VIM x{-n}(≡ x{n}?) (NOT SUPPORTED) VIM x=(≡ x?) (NOT SUPPORTED) VIM

Implementation restriction: The counting forms x{n,m}, x{n,}, and x{n} reject forms that create a minimum or maximum repetition count above 1000. Unlimited repetitions are not subject to this restriction.

Possessive repetitions x*+zero or more x, possessive (NOT SUPPORTED) x++one or more x, possessive (NOT SUPPORTED) x?+zero or one x, possessive (NOT SUPPORTED) x{n,m}+n or ... or m x, possessive (NOT SUPPORTED) x{n,}+n or more x, possessive (NOT SUPPORTED) x{n}+exactly n x, possessive (NOT SUPPORTED) Grouping (re)numbered capturing group (submatch) (?P<name>re)named & numbered capturing group (submatch) (?<name>re)named & numbered capturing group (submatch) (NOT SUPPORTED) (?'name're)named & numbered capturing group (submatch) (NOT SUPPORTED) (?:re)non-capturing group (?flags)set flags within current group; non-capturing (?flags:re)set flags during re; non-capturing (?#text)comment (NOT SUPPORTED) (?|x|y|z)branch numbering reset (NOT SUPPORTED) (?>re)possessive match of re (NOT SUPPORTED) re@>possessive match of re (NOT SUPPORTED) VIM %(re)non-capturing group (NOT SUPPORTED) VIM Flags icase-insensitive (default false) mmulti-line mode: ^ and $ match begin/end line in addition to begin/end text (default false) slet . match \n (default false) Uungreedy: swap meaning of x* and x*?, x+ and x+?, etc (default false)

Flag syntax is xyz (set) or -xyz (clear) or xy-z (set xy, clear z).

Empty strings ^at beginning of text or line (m=true) $at end of text (like \z not \Z) or line (m=true) \Aat beginning of text \bat ASCII word boundary (\w on one side and \W, \A, or \z on the other) \Bnot at ASCII word boundary \gat beginning of subtext being searched (NOT SUPPORTED) PCRE \Gat end of last match (NOT SUPPORTED) PERL \Zat end of text, or before newline at end of text (NOT SUPPORTED) \zat end of text (?=re)before text matching re (NOT SUPPORTED) (?!re)before text not matching re (NOT SUPPORTED) (?<=re)after text matching re (NOT SUPPORTED) (?<!re)after text not matching re (NOT SUPPORTED) re&before text matching re (NOT SUPPORTED) VIM re@=before text matching re (NOT SUPPORTED) VIM re@!before text not matching re (NOT SUPPORTED) VIM re@<=after text matching re (NOT SUPPORTED) VIM re@<!after text not matching re (NOT SUPPORTED) VIM \zssets start of match (= \K) (NOT SUPPORTED) VIM \zesets end of match (NOT SUPPORTED) VIM \%^beginning of file (NOT SUPPORTED) VIM \%$end of file (NOT SUPPORTED) VIM \%Von screen (NOT SUPPORTED) VIM \%#cursor position (NOT SUPPORTED) VIM \%'mmark m position (NOT SUPPORTED) VIM \%23lin line 23 (NOT SUPPORTED) VIM \%23cin column 23 (NOT SUPPORTED) VIM \%23vin virtual column 23 (NOT SUPPORTED) VIM Escape sequences \abell (≡ \007) \fform feed (≡ \014) \thorizontal tab (≡ \011) \nnewline (≡ \012) \rcarriage return (≡ \015) \vvertical tab character (≡ \013) \*literal *, for any punctuation character * \123octal character code (up to three digits) \x7Fhex character code (exactly two digits) \x{10FFFF}hex character code \Cmatch a single byte even in UTF-8 mode \Q...\Eliteral text ... even if ... has punctuation \1backreference (NOT SUPPORTED) \bbackspace (NOT SUPPORTED) (use \010) \cKcontrol char ^K (NOT SUPPORTED) (use \001 etc) \eescape (NOT SUPPORTED) (use \033) \g1backreference (NOT SUPPORTED) \g{1}backreference (NOT SUPPORTED) \g{+1}backreference (NOT SUPPORTED) \g{-1}backreference (NOT SUPPORTED) \g{name}named backreference (NOT SUPPORTED) \g<name>subroutine call (NOT SUPPORTED) \g'name'subroutine call (NOT SUPPORTED) \k<name>named backreference (NOT SUPPORTED) \k'name'named backreference (NOT SUPPORTED) \lXlowercase X (NOT SUPPORTED) \uxuppercase x (NOT SUPPORTED) \L...\Elowercase text ... (NOT SUPPORTED) \Kreset beginning of $0 (NOT SUPPORTED) \N{name}named Unicode character (NOT SUPPORTED) \Rline break (NOT SUPPORTED) \U...\Eupper case text ... (NOT SUPPORTED) \Xextended Unicode sequence (NOT SUPPORTED) \%d123decimal character 123 (NOT SUPPORTED) VIM \%xFFhex character FF (NOT SUPPORTED) VIM \%o123octal character 123 (NOT SUPPORTED) VIM \%u1234Unicode character 0x1234 (NOT SUPPORTED) VIM \%U12345678Unicode character 0x12345678 (NOT SUPPORTED) VIM Character class elements xsingle character A-Zcharacter range (inclusive) \dPerl character class [:foo:]ASCII character class foo \p{Foo}Unicode character class Foo \pFUnicode character class F (one-letter name) Named character classes as character class elements [\d]digits (≡ \d) [^\d]not digits (≡ \D) [\D]not digits (≡ \D) [^\D]not not digits (≡ \d) [[:name:]]named ASCII class inside character class (≡ [:name:]) [^[:name:]]named ASCII class inside negated character class (≡ [:^name:]) [\p{Name}]named Unicode property inside character class (≡ \p{Name}) [^\p{Name}]named Unicode property inside negated character class (≡ \P{Name}) Perl character classes (all ASCII-only) \ddigits (≡ [0-9]) \Dnot digits (≡ [^0-9]) \swhitespace (≡ [\t\n\f\r ]) \Snot whitespace (≡ [^\t\n\f\r ]) \wword characters (≡ [0-9A-Za-z_]) \Wnot word characters (≡ [^0-9A-Za-z_]) \hhorizontal space (NOT SUPPORTED) \Hnot horizontal space (NOT SUPPORTED) \vvertical space (NOT SUPPORTED) \Vnot vertical space (NOT SUPPORTED) ASCII character classes [[:alnum:]]alphanumeric (≡ [0-9A-Za-z]) [[:alpha:]]alphabetic (≡ [A-Za-z]) [[:ascii:]]ASCII (≡ [\x00-\x7F]) [[:blank:]]blank (≡ [\t ]) [[:cntrl:]]control (≡ [\x00-\x1F\x7F]) [[:digit:]]digits (≡ [0-9]) [[:graph:]]graphical (≡ [!-~] ≡ [A-Za-z0-9!"#$%&'()*+,\-./:;<=>?@[\\\]^_`{|}~]) [[:lower:]]lower case (≡ [a-z]) [[:print:]]printable (≡ [ -~] ≡ [ [:graph:]]) [[:punct:]]punctuation (≡ [!-/:-@[-`{-~]) [[:space:]]whitespace (≡ [\t\n\v\f\r ]) [[:upper:]]upper case (≡ [A-Z]) [[:word:]]word characters (≡ [0-9A-Za-z_]) [[:xdigit:]]hex digit (≡ [0-9A-Fa-f]) Unicode character class names--general category Cother Cccontrol Cfformat Cnunassigned code points (NOT SUPPORTED) Coprivate use Cssurrogate Lletter LCcased letter (NOT SUPPORTED) L&cased letter (NOT SUPPORTED) Lllowercase letter Lmmodifier letter Loother letter Lttitlecase letter Luuppercase letter Mmark Mcspacing mark Meenclosing mark Mnnon-spacing mark Nnumber Nddecimal number Nlletter number Noother number Ppunctuation Pcconnector punctuation Pddash punctuation Peclose punctuation Pffinal punctuation Piinitial punctuation Poother punctuation Psopen punctuation Ssymbol Sccurrency symbol Skmodifier symbol Smmath symbol Soother symbol Zseparator Zlline separator Zpparagraph separator Zsspace separator Unicode character class names--scripts Adlam Ahom Anatolian_Hieroglyphs Arabic Armenian Avestan Balinese Bamum Bassa_Vah Batak Bengali Bhaiksuki Bopomofo Brahmi Braille Buginese Buhid Canadian_Aboriginal Carian Caucasian_Albanian Chakma Cham Cherokee Chorasmian Common Coptic Cuneiform Cypriot Cyrillic Deseret Devanagari Dives_Akuru Dogra Duployan Egyptian_Hieroglyphs Elbasan Elymaic Ethiopic Georgian Glagolitic Gothic Grantha Greek Gujarati Gunjala_Gondi Gurmukhi Han Hangul Hanifi_Rohingya Hanunoo Hatran Hebrew Hiragana Imperial_Aramaic Inherited Inscriptional_Pahlavi Inscriptional_Parthian Javanese Kaithi Kannada Katakana Kayah_Li Kharoshthi Khitan_Small_Script Khmer Khojki Khudawadi Lao Latin Lepcha Limbu Linear_A Linear_B Lisu Lycian Lydian Mahajani Makasar Malayalam Mandaic Manichaean Marchen Masaram_Gondi Medefaidrin Meetei_Mayek Mende_Kikakui Meroitic_Cursive Meroitic_Hieroglyphs Miao Modi Mongolian Mro Multani Myanmar Nabataean Nandinagari New_Tai_Lue Newa Nko Nushu Nyiakeng_Puachue_Hmong Ogham Ol_Chiki Old_Hungarian Old_Italic Old_North_Arabian Old_Permic Old_Persian Old_Sogdian Old_South_Arabian Old_Turkic Oriya Osage Osmanya Pahawh_Hmong Palmyrene Pau_Cin_Hau Phags_Pa Phoenician Psalter_Pahlavi Rejang Runic Samaritan Saurashtra Sharada Shavian Siddham SignWriting Sinhala Sogdian Sora_Sompeng Soyombo Sundanese Syloti_Nagri Syriac Tagalog Tagbanwa Tai_Le Tai_Tham Tai_Viet Takri Tamil Tangut Telugu Thaana Thai Tibetan Tifinagh Tirhuta Ugaritic Vai Wancho Warang_Citi Yezidi Yi Zanabazar_Square Vim character classes \iidentifier character (NOT SUPPORTED) VIM \I\i except digits (NOT SUPPORTED) VIM \kkeyword character (NOT SUPPORTED) VIM \K\k except digits (NOT SUPPORTED) VIM \ffile name character (NOT SUPPORTED) VIM \F\f except digits (NOT SUPPORTED) VIM \pprintable character (NOT SUPPORTED) VIM \P\p except digits (NOT SUPPORTED) VIM \swhitespace character (≡ [ \t]) (NOT SUPPORTED) VIM \Snon-white space character (≡ [^ \t]) (NOT SUPPORTED) VIM \ddigits (≡ [0-9]) VIM \Dnot \d VIM \xhex digits (≡ [0-9A-Fa-f]) (NOT SUPPORTED) VIM \Xnot \x (NOT SUPPORTED) VIM \ooctal digits (≡ [0-7]) (NOT SUPPORTED) VIM \Onot \o (NOT SUPPORTED) VIM \wword character VIM \Wnot \w VIM \hhead of word character (NOT SUPPORTED) VIM \Hnot \h (NOT SUPPORTED) VIM \aalphabetic (NOT SUPPORTED) VIM \Anot \a (NOT SUPPORTED) VIM \llowercase (NOT SUPPORTED) VIM \Lnot lowercase (NOT SUPPORTED) VIM \uuppercase (NOT SUPPORTED) VIM \Unot uppercase (NOT SUPPORTED) VIM \_x\x plus newline, for any x (NOT SUPPORTED) VIM \cignore case (NOT SUPPORTED) VIM \Cmatch case (NOT SUPPORTED) VIM \mmagic (NOT SUPPORTED) VIM \Mnomagic (NOT SUPPORTED) VIM \vverymagic (NOT SUPPORTED) VIM \Vverynomagic (NOT SUPPORTED) VIM \Zignore differences in Unicode combining characters (NOT SUPPORTED) VIM Magic (?{code})arbitrary Perl code (NOT SUPPORTED) PERL (??{code})postponed arbitrary Perl code (NOT SUPPORTED) PERL (?n)recursive call to regexp capturing group n (NOT SUPPORTED) (?+n)recursive call to relative group +n (NOT SUPPORTED) (?-n)recursive call to relative group -n (NOT SUPPORTED) (?C)PCRE callout (NOT SUPPORTED) PCRE (?R)recursive call to entire regexp (≡ (?0)) (NOT SUPPORTED) (?&name)recursive call to named group (NOT SUPPORTED) (?P=name)named backreference (NOT SUPPORTED) (?P>name)recursive call to named group (NOT SUPPORTED) (?(cond)true|false)conditional branch (NOT SUPPORTED) (?(cond)true)conditional branch (NOT SUPPORTED) (*ACCEPT)make regexps more like Prolog (NOT SUPPORTED) (*COMMIT)(NOT SUPPORTED) (*F)(NOT SUPPORTED) (*FAIL)(NOT SUPPORTED) (*MARK)(NOT SUPPORTED) (*PRUNE)(NOT SUPPORTED) (*SKIP)(NOT SUPPORTED) (*THEN)(NOT SUPPORTED) (*ANY)set newline convention (NOT SUPPORTED) (*ANYCRLF)(NOT SUPPORTED) (*CR)(NOT SUPPORTED) (*CRLF)(NOT SUPPORTED) (*LF)(NOT SUPPORTED) (*BSR_ANYCRLF)set \R convention (NOT SUPPORTED) PCRE (*BSR_UNICODE)(NOT SUPPORTED) PCRE

re2 documentation built on March 29, 2022, 5:05 p.m.