Regex Tester and Debugger

Try a pattern and flags against sample text, then inspect every match and capture with its character range.

How this tool works

JavaScript regular expressions can use named groups, lookarounds, Unicode flags, and zero-length matches. Seeing match boundaries helps distinguish an unexpected capture from an unexpected search position.

A passing sample does not prove a pattern safe or correct for every input. Catastrophic backtracking remains possible with some expressions.

Examples

Find issue IDs in a synthetic log line.
Compare numbered and named captures.
Check an optional group and a zero-length boundary.

Common use cases

Check a log parser before adding it to a build pipeline.

Worked synthetic example

Valid example

Pattern: (?<id>BUG-\d+) | Text: fixed BUG-42

Expected: Match BUG-42; id = BUG-42. The named capture identifies the issue token within the line.

Failure or warning to recognize

Invalid/failure example

Pattern [abc against sample abc (unterminated character class)

Expected: An unterminated [ character class reports a regular-expression syntax error; no matches are computed.

Official references

ECMAScript RegExp