Regex Tester
Test patterns with live highlighting and an explanation.
Regex Tester matches your pattern against sample text with live highlighting, lists every match with its capture groups and positions, and explains what each part of the pattern does. Execution is time-limited, so a catastrophically backtracking pattern reports a timeout instead of freezing the tab. The pattern and your test text never leave the page.
Loading Regex Tester…
Runs entirely on your device. No file or text you enter here is uploaded. How to verify it.
How to use Regex Tester
- Type a pattern and choose flags.
- Paste test text and watch matches highlight.
- Inspect capture groups, or read the pattern explanation.
Questions about Regex Tester
What is ReDoS and why does the timeout exist?
Some patterns take exponential time on inputs that nearly match —
(a+)+b against a long string of a's is the classic example. Without a guard the tab locks up. Matching here runs in a worker with a time limit and reports the problem instead.Which regex flavour is it?
JavaScript's, which is what your browser runs. It differs from PCRE, Python and Go in places — no atomic groups or possessive quantifiers, and lookbehind support varies by engine — so a pattern that works here may need adjusting elsewhere.
Can it explain a pattern someone else wrote?
Yes. The explanation panel breaks the pattern into its tokens and describes each one, which is usually faster than deciphering a wall of brackets by hand.
Can I test a Python or PCRE pattern here?
You can paste one, but it runs under the JavaScript engine. Most patterns behave identically; named groups, lookbehind and Unicode property escapes are where the flavours part company, so verify anything unusual in the target language.
Does it work on a phone?
Yes, though a long pattern is awkward on a touch keyboard. The highlighting and the match list reflow for a narrow screen, and matching runs on the device the same way it does on a desktop.
Is Regex Tester really free, with no limits?
Yes. There is no account, no daily cap, no file-size tier and no watermark, because there is no server cost to recover. The site is funded by clearly disclosed partner recommendations on the Partners page, never by metering the tools.
Are my files or text uploaded when I use Regex Tester?
No. Everything is processed by JavaScript running in your browser, on your device. You can verify it: open your browser's developer tools, switch to the Network tab, and use the tool. You will see no request carrying your data — because there is nowhere for it to go.