Glossary Development

Regular expression

A regular expression (regex) is a pattern written in a compact syntax that describes a set of strings, used to search, validate and replace text.

Last reviewed: 2026-10-03

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For example \d{3}-\d{4} matches three digits, a hyphen and four digits. Regex flavours differ slightly between languages. Very complex patterns become unreadable, and some can run catastrophically slowly on crafted input, so keep them simple on untrusted text.

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Regex Tester

Test JavaScript regular expressions in real time with live match highlighting, capturing group breakdowns, and instant replacement previews.

Find and Replace

Find and replace text online with a live preview and match count.

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Try these tools

Regex Tester

Find and Replace

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