Regular expressions are a notation for describing sets of character strings. When a string is in the set described by a regular expression, we often say that the regular expression matches the string.
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 altered 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.
Syntax tables
Kinds of single-character expressions
Examples
any character, possibly including newline (s=true)
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
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 ASCII word boundary (\w on one side and \W, \A, or \z on the other)
\B
not at ASCII 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})
