Test and debug regular expressions

Frequently asked questions

Which regex flavor does this use?

JavaScript's built-in RegExp engine (ECMAScript), including the d and v flags. Most syntax is shared with Python, Go and PHP, but not all of it, so the Code view writes the pattern out for each of them and warns where they behave differently (Go has no lookaround or backreferences, Python's \d also matches non-ASCII digits, and so on).

Why does my pattern only find one match?

Turn on the "g" (global) flag. Without it, the regex engine stops after the first match, and this tool shows exactly that, as JavaScript does. The same goes for "y" (sticky) on its own: one match, and only at the very start.

Is my pattern or sample text sent to a server?

No. Matching runs entirely in your browser using the native RegExp engine. Nothing is uploaded, logged, or stored anywhere. A share link carries everything inside the link itself (after the #, which browsers never send to a server).

What do the highlighted matches mean?

Every match your pattern finds is highlighted in the text, with each capture group in its own colour at its exact position. Click a highlighted match, or its card in the list, to pick it out and see its numbered and named groups with their positions.

What if my pattern is slow?

Matching runs in the background with a 1.5-second limit. A pattern with nested repeats like (a+)+ can take practically forever on text that almost matches (catastrophic backtracking); instead of freezing the page, it's stopped and you're told why.

Can I test a find-and-replace?

Yes. Switch to Replace and type the replacement, using $1, $<name> and $& as in JavaScript. The result updates live and can be copied, downloaded or opened side by side with the original in Code Compare.

What happens if my pattern is invalid?

The error straight from the regex engine's parser is shown immediately — things like an unterminated group or a bad escape sequence — so you can fix the syntax without guessing.

Is the Regex Tester really free?

Completely free. No login, no credit card, no time-limited trial, and no premium tier gating anything. Live highlighting, capture groups, replace, tests, code export and every flag are included. The list shows the first 1,000 matches (the count says "1,000+" when there are more), and a pattern gets 1.5 seconds per run.

Do I need an account to use it?

No — nothing to register for, nothing to download, and no email address asked for at any point. Type a pattern and some sample text, and matches highlight immediately.