Regex Tester Developer
Test a regular expression against sample text and see every match highlighted.
Regex Tester is a free online tool that test a regular expression against sample text and see every match highlighted. It runs entirely in your web browser using plain JavaScript, so files are processed on your own device and never uploaded to a server. There is no sign-up, no file size limit imposed by the site, no watermark and no paid tier.
- Price
- Free — no account, no quota, no watermark
- Category
- Developer
- Where it runs
- In your browser, on your device
- Files uploaded
- None
- Technology
- plain JavaScript
- Settings
- None — it just works
- Works offline
- Yes, after the first visit
About Regex Tester
Uses your browser’s own JavaScript regex engine, so behavior matches what your code will do — including named groups, lookbehind and unicode escapes where supported.
How to use it
- Everything regex tester needs is on this page.
- Your input is processed locally and discarded when you close the tab.
Under the hood
| Runs on | plain JavaScript — runs the whole thing |
Questions
Which regex flavour is this?
JavaScript's, running in your own browser. Behavior matches what your code will do exactly — including named groups, lookbehind and unicode property escapes where your browser supports them. Patterns written for PCRE, Python or Go may differ.
What do the flags mean?
g finds all matches rather than stopping at the first, i ignores case, m makes ^ and $ match at line breaks, s lets . match newlines, u enables full unicode handling, y anchors matching to the last index.
My pattern hangs the page — why?
Catastrophic backtracking. Nested quantifiers like (a+)+ can take exponential time on non-matching input. Rewrite to avoid ambiguity, or add anchors to cut the search space.