Regex Tester, private by default

What regular expressions are good for

A regular expression describes a text pattern. Developers use regex for validation, extraction, cleanup, search, routing rules, log parsing, and quick checks inside editors or scripts.

Regex is powerful for compact pattern matching, but it can become hard to read when it tries to parse a full language or document format. Use it for focused text patterns rather than every kind of parsing problem.

What this tester shows

The tester runs JavaScript RegExp in your browser. It reports whether the pattern is valid, which flags are active, how many matches were found, the index of each match, and any capture groups.

It adds the global flag during testing so multiple matches can be listed. If a pattern is invalid, the tool shows the browser's error message rather than sending the pattern to a server.

• Pattern validity and JavaScript regex errors
• Match count and starting indexes
• Captured groups for each match
• Flags such as g, i, m, s, u, v, y, and d when supported
• Up to the first 100 matches for readable output

Safe regex habits

Avoid testing private production data when a small redacted sample will prove the pattern. Browser-only testing keeps text local, but it does not decide whether the sample is appropriate to share.

For application code, watch out for overly broad validation, patterns that miss unicode or locale cases, and expressions that can become slow on crafted input. Test with realistic examples and failure cases before relying on a regex in production.