How to Test and Debug a Regular Expression
Published September 12, 2026 · 3 min read
Writing a regular expression that actually matches what you intend — and nothing else — usually takes some trial and error. This tool runs your pattern against real text safely in a background worker, so even a badly-written catastrophic-backtracking pattern can't freeze your browser tab.
Try Regex TesterSteps
- 1
Open the Regex Tester tool
Go to the regex tester.
- 2
Enter your test text
Paste the sample text you want to match against.
- 3
Enter your pattern and flags
Type your regular expression and any flags like global or case-insensitive.
- 4
Review matches, or switch to replace mode
See every match highlighted, including named capture groups, or test a replacement pattern.
Frequently asked questions
The pattern runs inside a separate worker with a timeout, so a pattern prone to catastrophic backtracking is stopped safely rather than freezing your whole browser tab.
Yes — switch to replace mode and enter a replacement pattern to preview exactly what the result would look like.
Yes — if your pattern uses named groups, their captured values are shown individually alongside the full match.
Because testing happens safely in an isolated worker, you can experiment freely with a tricky pattern without any risk of hanging your browser.