Regex Tester

Flags

• g find all matches, i ignore case, m make ^/$ match at line breaks, s let . match newlines, u Unicode mode (needed for \p{L} and emoji), y sticky.

Engine differences

This tester uses the JavaScript (ECMAScript) engine. Most syntax is shared with PCRE, Python and Java, but not all of it. JavaScript has named groups (?<name>…) and lookbehind, but no possessive quantifiers, atomic groups or inline modifiers like (?i). Check your target language's documentation before shipping a pattern written here.

Avoid catastrophic backtracking

Nested quantifiers such as (a+)+$ can take exponential time on inputs that almost match, which is a known denial-of-service vector (ReDoS). If the page freezes on a pattern, that pattern will freeze your server too.

FAQ

Why does my pattern match an empty string forever?

Patterns like a* can match zero characters. The tester advances past empty matches, as String.matchAll does, so you see each position once.

Is my test text sent anywhere?

No, matching happens in your browser.