Regex

regex.md

REGEX Cheatsheet

This is a Regex cheatsheet

Summary

A regular expression is a sequence of characters that defines a search pattern for a character combination. Such sequence is composed by preset notations (meta and literal characters) that form conditions to narrow a larger text down to the text of search.

This cheatsheet provides an overview of regex meta characters in the context of Javascript.

Table of Contents

• Examples
• Character Classes
• Assertions
• Grouping Constructs
• Quantifiers
• Bracket Expressions
• The OR Operator
• Flags
• Character Escapes j

Regex

Examples

function	example	Regex
Matching a Hex Value	#FF5733	/^#?([a-f0-9]{6}|[a-f0-9]{3})$/gi
Matching an Email	test.1111@gmail.com	/^([a-z0-9_.-]+)@([\da-z.-]+).([a-z.]{2,6})$/
Matching a URL	https://regex101.com/	/^(https?://)?([\da-z.-]+).([a-z.]{2,6})([/\w .-])/?/
Matching an HTML Tag	<div>this is a div</div>	/^<([a-z]+)([^<]+)(?:>(.)</\1>|\s+/>)$/

• Break down (URL example: https://github.com/heranYang93/REGEX-cheatsheet)

  1°	2°	function	function
  ^		assertion: asserts position at start of the string	https://github.com/heranYang93/REGEX-cheatsheet
  ()?		1st capturing group; the presence of this first group is NOT mandatory	https://github.com/heranYang93/REGEX-cheatsheet
  	^()	Must start with this group of characters	https://github.com/heranYang93/REGEX-cheatsheet
  	http	must contain characters-http	https://github.com/heranYang93/REGEX-cheatsheet
  	s?	there might be an 's'	https://github.com/heranYang93/REGEX-cheatsheet
  	://	':'+'/'+'/'	https://github.com/heranYang93/REGEX-cheatsheet
  ()		2nd capturing group; the presence of this first group is mandatory	https://github.com/heranYang93/REGEX-cheatsheet
  	[]+	search for more than 1 letters that belong to the characters in []	https://github.com/heranYang93/REGEX-cheatsheet
  	\d	numbers	https://github.com/heranYang93/REGEX-cheatsheet
  	a-z	alphabetic	https://github.com/heranYang93/REGEX-cheatsheet
  	\.	.(dot)	https://github.com/heranYang93/REGEX-cheatsheet
  	-	-(dash)	https://github.com/heranYang93/REGEX-cheatsheet
  \.		.(dot)	https://github`.`com/heranYang93/REGEX-cheatsheet
  ()		3rd capturing group; the presence of this first group is mandatory	https://github.`com`/heranYang93/REGEX-cheatsheet
  	[a-z.]{2,6}	search for 2-6 letters that belong to []	https://github.`com`/heranYang93/REGEX-cheatsheet
  ()*		4th. 5th, 6th ... capturing group	https://github.com`/heranYang93`/REGEX-cheatsheet
  ()*		4th. 5th, 6th ... capturing group	https://github.com/heranYang93`/REGEX-cheatsheet`
  	[\/\w /.-]*	0 to many letters that belong to []	https://github.com`/heranYang93/`REGEX-cheatsheet
  	\/	'/'	https://github.com`/`heranYang93/REGEX-cheatsheet
  	/w	alphabetic	https://github.com/`heranYang93`/REGEX-cheatsheet
  		matches the character with index 3210 (2016 or 408) literally (case insensitive)	https://github.com/heranYang93/REGEX-cheatsheet
  	\.	.(dot)	https://github.com/heranYang93/REGEX-cheatsheet``
  	-	-(dash)	https://github.com/heranYang93/REGEX`-`cheatsheet
  \/?		'/', presence NOT mandatory	https://github.com/heranYang93/REGEX-cheatsheet
  $		assertion: asserts position at the end of the string	https://github.com/heranYang93/REGEX-cheatsheet

  -- ^ assertion: asserts position at start of the string -- (https?://) --- ()

Character Classes

Characters, special characters and signs position

topic	function	example	Regex	Alternative
.	any single character	yes make my day	/.y/
\d	any digit	R2-D2	/.\d-.\d/	/[0-9]/
\D	NOT digit	Javascript	/\D/	/[^0-9]/
\w	alphanumeric + _	Javascadfadsf123123ript	/\w/	/[A-Za-z0-9_]/
\W	NOT alphanumeric + _	Javascadfads'"f123123ript	/\W/	/[^a-za-z0-9_]/
\s	white space	foo bar	/\s\w*/
\S	OTHER THAN white space	foo bar	/\S\w*/

Assertions

Anchors mark the position

char	function	example	Regex
^	beginning	Javascript	/^J/
$	end	Javascript	/t$/
x(?=y)	look ahead	Jack Sprat	/Jack(?=\sSprat|\sFrost)/
x(?!y)	NEGATIVE look ahead	Jack Frost	/Jack(?!\sSprat|\sFrost)/
(?<=y)x	look behind	Jack Sprat	(?<=Jack\s|Tom\s)Sprat
(?<!y)x	NEGATIVE look behind	Jack Sprat	(?<!Jack\s|Tom\s)Sprat
\b	word boundary	Bojack's jacket	\bjack
\B	NONE word boundary	Bojack's CK jacket	/\Bck/gi

Grouping Constructs

Group the characters

char	function	example	Regex
x|y	or	red apple	/green|red/
[xyz][a-c]	any enclosed	Javascript	/[abcd]/
[^xyz][^a-c]	NONE enclosed	Javascript	/[^abcd]/
(x)	match groups (matchAll)	foo bar bar bar	/(bar)/
\k	matching the Named capture group	Do you copy? Sir, yes Sir	/(?<title>\w+), yes \k<title>/

Quantifiers

char	function	example	Regex
x*	0 or more repetition	boooooed or bird	/bo*/
x+	1 or more repetition	boooooed but NOT bird	/bo+/
x?	0 or 1	no one cares if is here	/no one cares if( Ben)? is here/
x{n,m}	repeat times	See bracket expressions	-

Bracket Expressions

Bracket	function	example	Regex
[]	a set of character	Elephant	/[a-d]/
{}	match times (2 to 4 times)	Ba-ba-ba-``ba-``ba-nana	/(ba\-){1,3}/
()	remembered matches	'Firsty McLastname''Firsty McLastname'.match(/([A-Za-z]+)\s([A-Za-z]+)/) // -> matches 'First McLastname' with 'Firsty' remembered as $1 and 'McLastname' as $2	/([A-Za-z]+)\s([A-Za-z]+)/

The OR Operator

Bracket	function	example	Regex
(|)	or	One Two three four	/(one|two)/

Flags

Flags are declared after / to allow for special functionalities Common flags are as follow ...

function	Flag
multiline	m
global	g
case-insensitive	i
Generate indices for substring matches	d

Character Escapes

The backslash in a regular expression precedes a literal character. Certain letters that represent common character classes can be escaped, such as \w for a word character or \s for a space.

escaped characters	Flag
\	a single backslash \
/	a single forwardslash \
\A	start of a string
\b	word boundary
\B	not word boundary
\d	[0-9]
\D	[^0-9]
\l	[a-z]
\L	[^a-z]
\u	[^A-Z]
\U	[^A-Z]
\w	[a-Z0-9_]
\W	[^a-Z0-9_]
\r	return
\s	space
\S	NOT space
\S	NOT space
\Q	ignore escaped characters until \E is found
\E	stop processing escaped characters

Author

hy - https://github.com/heranYang93