Regex Tester

Q: Which regex flavor does this use?

JavaScript ECMAScript. Differences from PCRE/Python: no atomic groups, no possessive quantifiers, no recursive patterns. Lookbehind requires fixed width in Safari (Chromium and Firefox accept variable-width lookbehind).

Q: Does it support named capture groups?

Yes. (?<year>\d{4}) defines a named group; matches show both numeric indices and names. Use $<name> in the replacement field.

Q: Can I test against multi-line input?

Yes. Add the m flag to make ^ and $ match line boundaries. Add s to make . match newlines.

Test regular expressions live. JavaScript ECMAScript flavor — the same engine your browser, Node.js backend, and Bun runtime use. See matches highlighted, capture groups extracted, and replacements previewed as you type. Catastrophic backtracking is caught with a 2-second timeout.

Pattern

/ /

Test string

Replacement (optional — uses $1, $<name> etc.)

How to use the regex tester

Enter your pattern in the top field (without surrounding slashes — the tester adds them). Set flags on the right: g for global match, i for case-insensitive, m for multiline, s for dotall, u for full Unicode, y for sticky. Paste your test string in the larger field. The tester runs on every change with a short debounce so you can iterate quickly.

For replacement, fill the replace field — you can use $1, $2 for numbered groups or $<name> for named groups. The result panel shows both the match list and the replaced output.

JavaScript regex features you'll actually use

Character classes: \d, \w, \s, \b and inverses (\D, \W, \S, \B).
Quantifiers: ?, +, *, {n}, {n,}, {n,m}. Add ? after for lazy matching.
Capture groups: (...) numbered, (?:...) non-capturing, (?<name>...) named.
Lookarounds: (?=X) positive lookahead, (?!X) negative, (?<=X) lookbehind, (?<!X) negative lookbehind.
Unicode property escapes (with u flag): \p{L} any letter, \p{Emoji} emoji, \p{Script=Cyrillic} etc.

Common patterns to copy

Pattern	Matches
`^\d{4}-\d{2}-\d{2}$`	ISO 8601 date (YYYY-MM-DD)
`\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}\b`	Email (case-insensitive with `i` flag)
`https?://[\w.-]+(?:/[\w./?=&%-]*)?`	URL (loose)
`\b(?:[0-9a-f]{8}-){3}[0-9a-f]{12}\b`	UUID v4 (loose)
`(?:^\|\s)#([A-Za-z0-9_]+)`	Hashtags
`\b\w{6,}\b`	Words 6+ characters

Frequently asked questions

Which regex flavor does this use?

JavaScript ECMAScript. Differences from PCRE/Python: no atomic groups, no possessive quantifiers, no recursive patterns. Lookbehind requires fixed width in Safari (Chromium and Firefox accept variable-width lookbehind).

Does it support named capture groups?

Yes. (?<year>\d{4}) defines a named group; matches show both numeric indices and names. Use $<name> in the replacement field.

Why is my regex slow?

Catastrophic backtracking. Patterns with nested quantifiers like (a+)+ or alternations that overlap can take exponential time on long strings. The tester will time out at 2 seconds and report which input character it was stuck on — that's usually enough to spot the runaway.

Can I test against multi-line input?

Yes. Add the m flag to make ^ and $ match line boundaries. Add s to make . match newlines.