Regex Cheat Sheet

Regular expression syntax and flags, with examples.

Regular expressions describe text patterns with a compact syntax: classes say which characters can appear, quantifiers say how many times, anchors say where, and groups capture parts of a match or combine alternatives. The same core syntax works in JavaScript, Python, Java, Go, PCRE (PHP, many editors) and grep with -E, with differences in advanced features.

This sheet follows JavaScript (ECMAScript), the flavor the regex tester uses. Notable differences elsewhere: Python spells named groups (?P<name>...), Go's RE2 has no lookaround or backreferences by design, and POSIX tools use classes like [[:digit:]] instead of \d. Try any pattern on your own text in the regex tester before you rely on it.

Filter syntax

54 entries

Characters and classes

Syntax	Meaning	Example
.	Any character except a line break (any character with the s flag)	a.c matches abc, a-c
\d	A digit, 0-9	\d\d matches 42
\D	Not a digit
\w	A word character: A-Z, a-z, 0-9 or _	\w+ matches user_1
\W	Not a word character
\s	Whitespace: space, tab, line breaks and other Unicode spaces
\S	Not whitespace
[abc]	One of a, b or c	gr[ae]y matches gray and grey
[^abc]	Any character except a, b or c
[a-z]	A character in the range a to z	[A-Fa-f0-9] matches a hex digit
\t \n \r	Tab, line feed, carriage return
\uFFFF	A character by its UTF-16 code unit	\u00e9 matches é
\u{1F600}	A code point (needs the u flag)
\p{L}	A Unicode property: any letter (needs the u flag)	\p{Lu} uppercase, \p{N} number
\.	A literal dot: escape . * + ? ^ $ ( ) [ ] { } \| \ / with a backslash	\.com matches .com

Anchors and boundaries

Syntax	Meaning	Example
^	Start of the string (start of each line with the m flag)	^Hello
$	End of the string (end of each line with the m flag)	world$
\b	A word boundary	\bcat\b matches cat but not concat
\B	Not a word boundary

Quantifiers

Syntax	Meaning	Example
*	0 or more	ab*c matches ac, abc, abbc
+	1 or more	ab+c matches abc, abbc
?	0 or 1 (optional)	colou?r matches color and colour
{3}	Exactly 3	\d{3} matches 123
{2,}	2 or more
{2,5}	Between 2 and 5
*? +? ??	Lazy: as few as possible	<.+?> matches one tag at a time

Groups, alternation and lookaround

Syntax	Meaning	Example
(abc)	Capturing group	(\d+)-(\d+) captures both numbers
(?<name>abc)	Named capturing group	(?<year>\d{4})
(?:abc)	Group without capturing	(?:https?://)?
a\|b	a or b	cat\|dog
\1	Backreference to group 1	(\w)\1 matches a doubled letter
\k<name>	Backreference to a named group
(?=abc)	Lookahead: followed by abc	\d+(?=px) matches 12 in 12px
(?!abc)	Negative lookahead: not followed by abc
(?<=abc)	Lookbehind: preceded by abc	(?<=\$)\d+ matches 5 in $5
(?<!abc)	Negative lookbehind: not preceded by abc

JavaScript flags

Syntax	Meaning	Example
g	Global: find all matches, not just the first	/a/g
i	Ignore case	/hello/i matches HELLO
m	Multiline: ^ and $ match at line breaks
s	Dot-all: . also matches line breaks
u	Unicode: code points and \p{...} escapes
v	Unicode sets: u plus set operations in classes (ES2024)
y	Sticky: match only at lastIndex
d	Indices: report start and end of each group

Handy patterns

Starting points, not complete validators: real-world formats have edge cases, so test with your own data.

Syntax	Meaning	Example
^\d+$	Whole string is digits	12345
^-?\d+(\.\d+)?$	Integer or decimal number	-3.14
^[^\s@]+@[^\s@]+\.[^\s@]+$	Rough email shape (something@something.tld)	ada@example.com
^https?://\S+$	A URL starting with http:// or https://
^\d{4}-\d{2}-\d{2}$	Date shaped like YYYY-MM-DD (doesn't check the date exists)	2026-10-02
^#?([0-9a-f]{3}\|[0-9a-f]{6})$	Hex color, with the i flag	#33aabb
^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$	UUID, with the i flag
^\s+\|\s+$	Leading or trailing whitespace (use with g to trim)
\s{2,}	Runs of two or more whitespace characters
^(?=.\d)(?=.[a-z])(?=.*[A-Z]).{8,}$	At least 8 characters with a digit, a lowercase and an uppercase letter

Good to know

Avoid catastrophic backtracking

Nested quantifiers such as (a+)+ or (.*)*, or alternatives that can match the same text, can take exponential time on input that almost matches. On a server that's a denial-of-service risk (ReDoS). Make each part of the pattern match as specifically as possible.

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