Frequently Asked Questions
Everything you need to know about the Regex Tester.
How does the regex tester work?
Enter a regular expression and a test string. Matches are highlighted live and listed with their positions and capture groups, updating as you type — no button to press.
Is the regex tester free?
Completely free, no sign-up, downloads, or limits.
Which flags are supported?
The standard JavaScript flags: global (g), case-insensitive (i), multiline (m), dotall (s), unicode (u), and sticky (y). Click each flag button to toggle it on or off.
Does it show capture groups?
Yes. Each match in the match list shows its numbered capture groups ($1, $2, …) and any named groups defined with (?<name>…).
What regex flavor does it use?
It uses the JavaScript (ECMAScript) regex engine — the same one that runs in all modern browsers and in Node.js.
What happens if my regex is invalid?
A clear error message appears below the pattern field showing exactly what the JavaScript engine reported, so you can fix the pattern without the tool crashing or freezing.
Can I test replacements?
Yes. Switch to Replace mode using the tabs above the test area, then enter a replacement string. It supports $1, $2 (numbered groups), $<name> (named groups), $& (full match), and $$ (literal dollar sign).
Is my data sent anywhere?
No. Everything runs locally in your browser using the JavaScript regex engine. Your patterns and test text never leave your device.
Is my data stored?
No. Nothing is stored or persisted. Closing or refreshing the page clears everything.
Does it work on mobile?
Yes. The pattern input, flag toggles, test area, and match list are all fully responsive and fit on small screens without horizontal overflow.