Regex Cheat Sheet for JavaScript

A regular expression describes a pattern of text. This page is a quick reference for the syntax JavaScript supports, with the mistakes that cause most bugs called out. Paste any example into the tester to see what it matches.

Regex TesterWrite a pattern and see every match highlighted as you type, with capture groups and positions. Matching runs in a background thread, so even a catastrophically backtracking pattern will not freeze the page.

Characters and classes

Pattern	Matches
.	Any character except a line break
\d	A digit, 0 to 9
\w	A letter, digit or underscore
\s	Whitespace: space, tab, line break
\D \W \S	The opposite of \d, \w, \s
[abc]	One of a, b or c
[a-z]	One character in the range
[^abc]	Any character except a, b, c
\.	A literal dot; escape . * + ? ^ $ ( ) [ ] { } \ to match them literally

Quantifiers

Pattern	Meaning
*	Zero or more
+	One or more
?	Zero or one
{3}	Exactly three
{2,5}	Two to five
{2,}	Two or more

Quantifiers are greedy by default: they take as much as they can. Add ? to make one lazy. Against <a><b>, the pattern <.+> matches the whole string, while <.+?> matches only <a>.

Anchors and boundaries

Pattern	Matches
^	Start of the input, or of a line with the m flag
$	End of the input, or of a line with the m flag
\b	A word boundary
\B	Anywhere that is not a word boundary

Groups and alternatives

Pattern	Meaning
(abc)	Capturing group, numbered from 1
(?<year>\d{4})	Named capturing group
(?:abc)	Non-capturing group
a|b	Either a or b
\1	Back-reference to group 1

Replacement strings refer to groups as $1 or $<year>.

Lookarounds

Lookarounds assert what comes before or after without including it in the match.

Pattern	Meaning
a(?=b)	a followed by b
a(?!b)	a not followed by b
(?<=b)a	a preceded by b
(?<!b)a	a not preceded by b

Flags

Flag	Effect
g	Find every match, not just the first
i	Ignore case
m	^ and $ match at each line
s	. also matches line breaks
u	Full Unicode support, including \p{...} classes
y	Sticky: match only at the current position

Common mistakes

• Forgetting to escape the dot. 1.5 also matches 1x5; write 1\.5.
• Unanchored validation. \d{4} accepts abc12345. Use ^\d{4}$ to require the whole string.
• Reusing a global regex. A regex with the g or y flag remembers lastIndex, so calling .test() twice on the same object can give different answers.
• Catastrophic backtracking. Nested quantifiers such as (a+)+$ can take exponential time on a failing input. Prefer simpler patterns for untrusted text.