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A regular expression (regex) is a pattern that describes text to match. Instead of searching for one exact string, a regex describes a shape: “a word boundary, then some letters, then a boundary”, “any digit repeated three times”, “one of these three words”. Anywhere you’re asked for a regex pattern, it’s evaluated with JavaScript’s regex engine, the same one described here.

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How it works

A pattern is built from a small set of building blocks, combined together.

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Literal characters

Most characters just match themselves. The pattern cat matches the text cat wherever it appears.

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Character classes

A character class matches one character out of a set.

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Quantifiers

A quantifier says how many times the thing before it can repeat. Quantifiers are greedy by default: they match as much as possible before backing off. Adding ? after a quantifier (*?, +?) makes it lazy instead, matching as little as possible.

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Anchors and boundaries

\bcat\b matches cat as its own word, not the cat inside catalog.

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Groups and alternation

Parentheses () group part of a pattern together, so a quantifier can apply to the whole group and so the matched text can be extracted separately. The pipe | inside or around a group means “or”: cat|dog matches either cat or dog.

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Escaping

Characters like . * + ? ( ) [ ] { } ^ $ | are special to regex. To match one of them literally, put a backslash in front of it: \. matches a literal period, \( matches a literal opening parenthesis.

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Flags

Flags go after the pattern and change how the whole match runs.

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Playground

Edit the pattern, flags and test message below. Matches are highlighted live.

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Examples

Some patterns can run very slowly on certain input, particularly nested repetition like (a+)+b. If a pattern needs to run against arbitrary user-supplied text, test it against a long, unusual input in the playground above before relying on it.

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