regex-cheatsheet.md

Title	Description	Created	Updated
Regex Cheatsheet	All necessary regex commands for programming	2022-10-23	2022-10-23

Author: Soham Gupta

Regular Expressions Cheatsheet

Regular expressions (also referred to as rational expressions) are sequences of characters that specify a search pattern in the text. Such patterns are often used in string-searching algorithms to perform "find" and "find and replace" operations on strings, or to validate inputs.

Anchors

Operators	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

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Quanti­fiers

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

Add a ? to a quantifier to make it ungreedy.

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Groups and Ranges

Operator	Description
.	Any character except new line (\n)
(a|b)	a or 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.

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Escape Sequences

Operator	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 regex literally, rather than as special charac­ters.

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Common Metacharacters

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

The escape character is usually \

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Special Characters

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

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Assertions

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

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String Replacement

Operator	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 $.

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Pattern Modifiers

Operator	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