Regular expressions

A regular expression is a tiny pattern language for describing text: "two digits, a dot, then four digits". Type a pattern below and watch it match, capture and replace live.

Live tester

Matches update as you type. Invalid patterns are explained instead of crashing.

Pattern

One or more name characters, an @, a domain, then a dot and the top-level domain.

Flags

Test text

Result — 2 matches

Contact ada@example.com or grace.hopper+navy@navy.mil, not 'nope@'.

Matches & capture groups

#	Index	Match	Groups
1	8	ada@example.com	—
2	27	grace.hopper+navy@navy.mil	—

Replace with

Use $1, $2 for numbered groups, $<name> for named ones and $& for the whole match.

Contact [email] or [email], not 'nope@'.

Live examples

Load one into the tester and tweak it.

Cheat sheet

The pieces you will use ninety percent of the time.

Characters

.: Any character except newline
\d \D: Digit / not a digit
\w \W: Word character [A-Za-z0-9_] / not
\s \S: Whitespace / not whitespace
[abc]: Any one of a, b, c
[^abc]: Anything except a, b, c
[a-z0-9]: Ranges inside a character class
\. \+ \\: Escape a special character

Quantifiers

*: 0 or more
+: 1 or more
?: 0 or 1 (optional)
{3}: Exactly 3
{2,5}: Between 2 and 5
{2,}: 2 or more
+? *?: Lazy — match as little as possible

Anchors & groups

^ $: Start / end of string (or line with m)
\b \B: Word boundary / not a boundary
(abc): Capturing group
(?:abc): Group without capturing
(?<name>abc): Named capturing group
a|b: Either a or b
\1: Backreference to group 1
(?=x) (?!x): Lookahead: followed / not followed by x
(?<=x) (?<!x): Lookbehind: preceded / not preceded by x