Regex

Overview

Regex - is a sequence of characters that define a search pattern.

Detailed

Anchors

Regex Component	Meaning	Example
^	Start of string, or start of line in multi-line pattern	^cat cat, cats
\A	Start of string	\Adog dog, dogs
$	End of string, or end of line in multi-line pattern	bird$ lovebird, tweeting bird, bird
\Z	End of string	fish\Z goldfish, jellyfish, fish
\b	Word boundary	\bwolf\b The wolf howls, Lone wolf
\B	Not word boundary	\Bcat\ concatenate, scatter
\<	Start of word	\<day day, days are long
\>	End of word	night\> goodnight, tonight

Character Classes

Regex Component	Meaning	Example
\c	Control character	\c \cC, \cZ
\s	White space	\s \t, \n
\S	Not white space	\S c, a
\d	Digit	\d 0, 1
\D	Not digit	\D c, a
\w	Word (alphanumeric or underscore)	\w a, _
\W	Not word	\W -, .
\x	Hexadecimal digit	\x 0x0, 0xFF
\O	Octal digit	\O 07, 075

POSIX

Regex Component	Meaning	Example
[:upper:]	Upper case letters	[:upper:] A, B
[:lower:]	Lower case letters	[:lower:] a, b
[:alpha:]	All letters	[:alpha:] A, b
[:alnum:]	Digits and letters	[:alnum:] 0, a
[:digit:]	Digits	[:digit:] 0, 1
[:xdigit:]	Hexadecimal digits	[:xdigit:] 0, F
[:punct:]	Punctuation	[:punct:] !, .
[:blank:]	Space and tab	[:blank:] \t, " "
[:space:]	Blank characters	[:space:] \t, \n
[:cntrl:]	Control characters	[:cntrl:] \t, \n
[:graph:]	Printed characters (excluding space)	[:graph:] A, ?
[:print:]	Printed characters and spaces	[:print:] A, ?
[:word:]	Digits, letters and underscore	[:word:] a, _

Assertions

Regex Component	Meaning	Example
?=	Lookahead assertion	cat(?=s) cats, catalog
?!	Negative lookahead	cat(?!s) cat, catch
?<=	Lookbehind assertion	(?<=re)cat recat, precative
?!= or ?<!	Negative lookbehind	(?<!re)cat cat, catch
?>	Once-only Subexpression	(?>foo|fo)bar foobar, fofoobar
?()	Condition (if-then)	(?(?=d)foo|bar) dog, food
?()|	Condition (if-then-else)	(?(?=d)foo|bar) dog, food

Quantifiers

Add ? to a quantifier to make it ungreedy.

Regex Component	Meaning	Example
*	0 or more	go*gle ggle, google
+	1 or more	go+gle google, gogle
?	0 or 1	colou?r color, colour
{3}	Exactly 3	go{3}gle gooogle
{3,}	3 or more	go{3,}gle goooogle, goooogle
{3,5}	3, 4 or 5	go{3,5}gle goooogle, goooooogle

Escape Sequences

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

Regex Component	Meaning	Example
\	Escape following character	\$ The price is $10
\Q	Begin literal sequence	\Q$10\E The price is $10
\E	End literal sequence	\Q$10\E The price is $10

Special Characters

Regex Component	Meaning	Example
\n	New line	Hello\nWorld Hello\nWorld
\r	Carriage return	First line\rSecond line First line\rSecond line
\t	Tab	Name:\tJohn Name: John
\v	Vertical tab	First line\vSecond line First line\vSecond line
\f	Form feed	``Page 1\fPage 2` Page 1\fPage 2
\o	Octal character	\o123 123
\x	Hex character	\x41 41

Groups and Ranges

Regex Component	Meaning	Example
.	Any character except new line (\n)	c.t cAt, c1t
(|)	Or	(cat|dog) cat, dog
()	Group	(ab)+ abab, ababab
(?:)	Passive (non-capturing) group	(?:abc)+ abc, abcabcabc
[]	Range	[ae] a, e
[^]	Not in range	[^ae] b, d
[a-z]	Range of lowercase letters from a to z	[a-z]+ abc, hello
[A-Z]	Range of uppercase letters from A to Z	[A-Z]+ ABC, HELLO
[0-9]	Range of digits from 0 to 9	[0-9]+ 123, 45

Pattern Modifiers

Regex Component	Meaning	Example
g	Global match	/cat/g cat, cats
i	Case-insensitive (PCRE)	/cat/i cAt, Cat
m	Multiple lines (PCRE)	/^cat/m cat\nconcatenat, cat\nsome text\ncat
s	Treat string as single line (PCRE)	/cat.s/m cat\ns, cat\nsome text
x	Allow comments and whitespace in pattern (PCRE)	/c a t/x concatenate
e	Evaluate replacement (PCRE)	preg_replace('/(\d+)/e', '$1 * 2', '123 456') 246 912
U	Ungreedy pattern (PCRE)	/a.*b/U a test b

String Replacement

Regex Component	Example
$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