Regex Tester
Test and debug regular expressions with live match highlighting, capture groups and flag toggles. Runs entirely in your browser.
How to use Regex Tester: Open Regex Tester — it runs entirely in your browser with no setup. Paste or type your input into the text area, or upload a file where supported. Set any options you need. The result appears in the output panel — there is no upload step and no waiting on a server. Everything runs inside your browser, so your data never leaves your device.
Contact us at hello@example.com or sales@test.org.
hello@example.comsales@test.orgTest and debug regular expressions with live match highlighting, capture groups and flag toggles. Runs entirely in your browser. Regex Tester is a free, browser-based tool in the Filevane developer tools collection — there is nothing to install, no account to create and no watermark on your results. It works on desktop, tablet and mobile, and because the work happens on your own device it stays fast even with large inputs.
How to use Regex Tester
- Open Regex Tester — it runs entirely in your browser with no setup.
- Paste or type your input into the text area, or upload a file where supported.
- Set any options you need. The result appears in the output panel — there is no upload step and no waiting on a server.
- Review the output instantly; large inputs are handled locally without lag.
- Copy the result to your clipboard or download it. Your data never leaves the page.
Why use Regex Tester?
- 100% free with no limits, no sign-up and no watermarks.
- Private by design — your data are processed in your browser and never uploaded to a server.
- Fast and lightweight, with instant results even on a phone or a slow connection.
- Works everywhere — any modern browser on Windows, macOS, Linux, Android or iOS.
- Part of a full developer tools suite, so related regex tester tasks are one click away.
Is it private and secure?
Your privacy is built in. Regex Tester runs completely on your device, so whatever you enter stays in your browser and is never transmitted, logged or stored on any server.
Network activity: none. Nothing is sent and nothing is fetched. This tool makes no network request at all once the page has loaded — which is why it keeps working if your connection drops.
You do not have to take that on trust — our privacy proof page publishes the Content-Security-Policy that enforces it, lists every third party it permits, and shows how to verify the whole claim yourself.
Learn more
Is It Safe to Paste Production Data Into Online Tools? — JWTs, API responses and env files get pasted into online formatters every day. Here's what actually happens to that data and how to check before you paste.
Frequently asked questions
How do I test a regular expression safely?
Paste the pattern and sample text and see matches highlighted with each capture group listed as you type. Testing against real data — including the awkward rows — catches far more than reasoning about the pattern in your head.
What is catastrophic backtracking?
A pattern with nested quantifiers such as (a+)+b can take exponential time on input that nearly matches, freezing the thread. It is the mechanism behind ReDoS denial-of-service attacks, and it is why any regex applied to user input deserves a timeout.
Why does my regex behave differently in another language?
Flavours differ in real ways: JavaScript, PCRE, Python and Java disagree on lookbehind support, named-group syntax, and whether . matches a newline. A pattern that works in one engine can fail or match differently in another.
What is the difference between greedy and lazy quantifiers?
Greedy quantifiers such as .* take as much as possible then give back; lazy ones such as .*? take as little as possible. Matching HTML tags with <.*> grabs everything to the last > on the line, which is why <.*?> is usually what you meant.
Related tools
See all free developer tools or browse the full tool catalogue.