RE2 regular expression syntax reference
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Single characters:
.	any character, including newline (s=true)
[xyz]	character class
[^xyz]	negated character class
\d	Perl character class
\D	negated Perl character class
[:alpha:]	ASCII character class
[:^alpha:]	negated ASCII character class
\pN	Unicode character class (one-letter name)
\p{Greek}	Unicode character class
\PN	negated Unicode character class (one-letter name)
\P{Greek}	negated Unicode character class

Composites:
xy	«x» followed by «y»
x|y	«x» 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

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
(?P<name>re)	named & numbered capturing group
(?<name>re)	named & numbered capturing group NOT SUPPORTED
(?'name're)	named & numbered capturing group 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:
i	case-insensitive (default false)
m	multi-line mode: ^ and $ match begin/end line in addition to begin/end text (default false)
s	let «.» match «\n» (default false)
U	ungreedy: 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)
\A	at beginning of text
\b	at word boundary («\w» on one side and «\W», «\A», or «\z» on the other)
\B	not a word boundary
\G	at beginning of subtext being searched NOT SUPPORTED pcre
\G	at end of last match NOT SUPPORTED perl
\Z	at end of text, or before newline at end of text NOT SUPPORTED
\z	at 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
\zs	sets start of match (= \K) NOT SUPPORTED vim
\ze	sets end of match NOT SUPPORTED vim
\%^	beginning of file NOT SUPPORTED vim
\%$	end of file NOT SUPPORTED vim
\%V	on screen NOT SUPPORTED vim
\%#	cursor position NOT SUPPORTED vim
\%'m	mark «m» position NOT SUPPORTED vim
\%23l	in line 23 NOT SUPPORTED vim
\%23c	in column 23 NOT SUPPORTED vim
\%23v	in virtual column 23 NOT SUPPORTED vim

Escape sequences:
\a	bell (== \007)
\f	form feed (== \014)
\t	horizontal tab (== \011)
\n	newline (== \012)
\r	carriage return (== \015)
\v	vertical tab character (== \013)
\*	literal «*», for any punctuation character «*»
\123	octal character code (up to three digits)
\x7F	hex character code (exactly two digits)
\x{10FFFF}	hex character code
\C	match a single byte even in UTF-8 mode
\Q...\E	literal text «...» even if «...» has punctuation

\1	backreference NOT SUPPORTED
\b	backspace NOT SUPPORTED (use «\010»)
\cK	control char ^K NOT SUPPORTED (use «\001» etc)
\e	escape NOT SUPPORTED (use «\033»)
\g1	backreference 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
\lX	lowercase «X» NOT SUPPORTED
\ux	uppercase «x» NOT SUPPORTED
\L...\E	lowercase text «...» NOT SUPPORTED
\K	reset beginning of «$0» NOT SUPPORTED
\N{name}	named Unicode character NOT SUPPORTED
\R	line break NOT SUPPORTED
\U...\E	upper case text «...» NOT SUPPORTED
\X	extended Unicode sequence NOT SUPPORTED

\%d123	decimal character 123 NOT SUPPORTED vim
\%xFF	hex character FF NOT SUPPORTED vim
\%o123	octal character 123 NOT SUPPORTED vim
\%u1234	Unicode character 0x1234 NOT SUPPORTED vim
\%U12345678	Unicode character 0x12345678 NOT SUPPORTED vim

Character class elements:
x	single character
A-Z	character range (inclusive)
\d	Perl character class
[:foo:]	ASCII character class «foo»
\p{Foo}	Unicode character class «Foo»
\pF	Unicode 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:
\d	digits (== [0-9])
\D	not digits (== [^0-9])
\s	whitespace (== [\t\n\f\r ])
\S	not whitespace (== [^\t\n\f\r ])
\w	word characters (== [0-9A-Za-z_])
\W	not word characters (== [^0-9A-Za-z_])

\h	horizontal space NOT SUPPORTED
\H	not horizontal space NOT SUPPORTED
\v	vertical space NOT SUPPORTED
\V	not 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:
C	other
Cc	control
Cf	format
Cn	unassigned code points NOT SUPPORTED
Co	private use
Cs	surrogate
L	letter
LC	cased letter NOT SUPPORTED
L&	cased letter NOT SUPPORTED
Ll	lowercase letter
Lm	modifier letter
Lo	other letter
Lt	titlecase letter
Lu	uppercase 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	math symbol
So	other symbol
Z	separator
Zl	line separator
Zp	paragraph separator
Zs	space separator

Unicode character class names--scripts:
Arabic	Arabic
Armenian	Armenian
Balinese	Balinese
Bengali	Bengali
Bopomofo	Bopomofo
Braille	Braille
Buginese	Buginese
Buhid	Buhid
Canadian_Aboriginal	Canadian Aboriginal
Carian	Carian
Cham	Cham
Cherokee	Cherokee
Common	characters not specific to one script
Coptic	Coptic
Cuneiform	Cuneiform
Cypriot	Cypriot
Cyrillic	Cyrillic
Deseret	Deseret
Devanagari	Devanagari
Ethiopic	Ethiopic
Georgian	Georgian
Glagolitic	Glagolitic
Gothic	Gothic
Greek	Greek
Gujarati	Gujarati
Gurmukhi	Gurmukhi
Han	Han
Hangul	Hangul
Hanunoo	Hanunoo
Hebrew	Hebrew
Hiragana	Hiragana
Inherited	inherit script from previous character
Kannada	Kannada
Katakana	Katakana
Kayah_Li	Kayah Li
Kharoshthi	Kharoshthi
Khmer	Khmer
Lao	Lao
Latin	Latin
Lepcha	Lepcha
Limbu	Limbu
Linear_B	Linear B
Lycian	Lycian
Lydian	Lydian
Malayalam	Malayalam
Mongolian	Mongolian
Myanmar	Myanmar
New_Tai_Lue	New Tai Lue (aka Simplified Tai Lue)
Nko	Nko
Ogham	Ogham
Ol_Chiki	Ol Chiki
Old_Italic	Old Italic
Old_Persian	Old Persian
Oriya	Oriya
Osmanya	Osmanya
Phags_Pa	'Phags Pa
Phoenician	Phoenician
Rejang	Rejang
Runic	Runic
Saurashtra	Saurashtra
Shavian	Shavian
Sinhala	Sinhala
Sundanese	Sundanese
Syloti_Nagri	Syloti Nagri
Syriac	Syriac
Tagalog	Tagalog
Tagbanwa	Tagbanwa
Tai_Le	Tai Le
Tamil	Tamil
Telugu	Telugu
Thaana	Thaana
Thai	Thai
Tibetan	Tibetan
Tifinagh	Tifinagh
Ugaritic	Ugaritic
Vai	Vai
Yi	Yi

Vim character classes:
\i	identifier character NOT SUPPORTED vim
\I	«\i» except digits NOT SUPPORTED vim
\k	keyword character NOT SUPPORTED vim
\K	«\k» except digits NOT SUPPORTED vim
\f	file name character NOT SUPPORTED vim
\F	«\f» except digits NOT SUPPORTED vim
\p	printable character NOT SUPPORTED vim
\P	«\p» except digits NOT SUPPORTED vim
\s	whitespace character (== [ \t]) NOT SUPPORTED vim
\S	non-white space character (== [^ \t]) NOT SUPPORTED vim
\d	digits (== [0-9]) vim
\D	not «\d» vim
\x	hex digits (== [0-9A-Fa-f]) NOT SUPPORTED vim
\X	not «\x» NOT SUPPORTED vim
\o	octal digits (== [0-7]) NOT SUPPORTED vim
\O	not «\o» NOT SUPPORTED vim
\w	word character vim
\W	not «\w» vim
\h	head of word character NOT SUPPORTED vim
\H	not «\h» NOT SUPPORTED vim
\a	alphabetic NOT SUPPORTED vim
\A	not «\a» NOT SUPPORTED vim
\l	lowercase NOT SUPPORTED vim
\L	not lowercase NOT SUPPORTED vim
\u	uppercase NOT SUPPORTED vim
\U	not uppercase NOT SUPPORTED vim
\_x	«\x» plus newline, for any «x» NOT SUPPORTED vim

Vim flags:
\c	ignore case NOT SUPPORTED vim
\C	match case NOT SUPPORTED vim
\m	magic NOT SUPPORTED vim
\M	nomagic NOT SUPPORTED vim
\v	verymagic NOT SUPPORTED vim
\V	verynomagic NOT SUPPORTED vim
\Z	ignore 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