RE2 Regular Expression Syntax

Original at github

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 Syntax