regular-expression.md

Achors

Expression	Description
^	Start of string, or start of line in multi-line pattern
\A	Start of string
$	End of string, or end of line in multi-line pattern
\Z	End of string
\b	Word boundary
\B	Not word boundary
<	Start of word
>	End of word

Character Classes

Expression	Description
\c	Control character
\s	White space
\S	Not white space
\d	Digit
\D	Not digit
\w	Word
\W	Not word
\x	Hexade­cimal digit
\O	Octal digit

Special Characters

Expression	Description
\n	New line
\r	Carriage return
\t	Tab
\v	Vertical tab
\f	Form feed
\xxx	Octal character xxx
\xhh	Hex character hh

Quantifiers

Expression	Description
*	0 or more
+	1 or more
?	0 or 1
{3}	Exactly 3
{3,}	3 or more
{3,5}	3, 4 or 5

Add a ? to a quantifier to make it ungreedy.

Escape Sequences

Expression	Description
\	Escape following character
\Q	Begin literal sequence
\E	End literal sequence

"­Escapi­ng" is a way of treating characters which have a special meaning in regular expressions literally, rather than as special characters.

POSIX

Expression	Description
[:upper:]	Upper case letters
[:lower:]	Lower case letters
[:alpha:]	All letters
[:alnum:]	Digits and letters
[:digit:]	Digits
[:xdigit:]	Hexade­cimal digits
[:punct:]	Punctu­ation
[:blank:]	Space and tab
[:space:]	Blank characters
[:cntrl:]	Control characters
[:graph:]	Printed characters
[:print:]	Printed characters and spaces
[:word:]	Digits, letters and underscore

Assertions

Expression	Description
?=	Lookahead assertion
?!	Negative lookahead
?<=	Lookbehind assertion
?!= or ?<!	Negative lookbehind
?>	Once-only Subexp­ression
?()	Condition [if then]
?()|	Condition [if then else]
?#	Comment

Groups and Ranges

Expression	Description
.	Any character except new line (\n)
(a	b)
(...)	Group
(?:...)	Passive (non-c­apt­uring) group
[abc]	Range (a or b or c)
[^abc]	Not (a or b or c)
[a-q]	Lower case letter from a to q
[A-Q]	Upper case letter from A to Q
[0-7]	Digit from 0 to 7
\x	Group/­sub­pattern number "­x"

Ranges are inclusive.

Pattern Modifiers

Expression	Description
g	Global match
i *	Case-i­nse­nsitive
m *	Multiple lines
s *	Treat string as single line
x *	Allow comments and whitespace in pattern
e *	Evaluate replac­ement
U *	Ungreedy pattern

* PCRE modifier

String Replacement

Expression	Description
$n	nth non-pa­ssive group
$2	"­xyz­" in /^(abc­(xy­z))$/
$1	"­xyz­" in /^(?:a­bc)­(xyz)$/
$`	Before matched string
$'	After matched string
$+	Last matched string
$&	Entire matched string

Some regex implementations use \ instead of $.

Common Meta characters

^ [ . $ { * (
+ ) | ? < >

The escape character is usually \

Expression	Description
<word>	match the boundary of a word