Regex Lab — Offline Regex Tester

Write. Match. Understand.

Regex Lab is a free, lightweight, offline workbench for JavaScript regular expressions. See what your pattern matches, understand its capture groups, safely experiment with replacements, and check your expectations with test cases — directly in the browser.

Highlights

• Live regex matching: highlight every match with positions and detailed match cards.
• Capture groups: inspect numbered and named groups, including start/end positions when supported.
• Replacement preview: use standard JavaScript replacement expressions ($&, $1, $<name>, $$).
• Test cases: write positive and negative tests, see pass/fail results, and export JSON.
• Built-in examples: log lines, email-like text, date formats, named groups and whitespace.
• Regex cheat sheet: handy JavaScript syntax examples while you work.
• Private by design: no account, no analytics, no tracking, no upload and no external network calls.
• Safer experimentation: regex evaluation runs inside a disposable Web Worker with a 900 ms timeout.
• Easy exporting: JSON match reports and plain-text replacement results.

Getting started

Click Run tool, edit the regex pattern and test string, then explore the Matches, Replace, Test Cases and Cheat Sheet tabs. You can also open the included HTML file locally.

Compatibility

Uses the JavaScript/ECMAScript regex engine provided by your browser. It does not emulate PCRE2, Python or .NET regex syntax. Newer flags and constructs depend on browser support. Input and match counts are capped to protect responsiveness.

Privacy note: All computation happens locally in your browser. If you export match reports, those files may include parts of your test data.

License: MIT. Free to use and redistribute under the included terms.

Version: 1.0.0

Regex Lab is independently developed and is not affiliated with regex101 or any other regex testing service.