Regex Sandbox

Live pattern testing · match details · replace mode · built-in cheat sheet

⚠️ Your browser blocked the background safety worker, so pattern runs are no longer protected by the timeout described below. Avoid testing untrusted or very complex patterns.

Regular Expression

/ /

Flags

Test String

Replace With (use $1 $2 for groups)

Without the g flag, only the first match is replaced — this matches standard JavaScript behavior.

Result

Enter a pattern and test string to see matches highlighted here.

JavaScript regex only. This sandbox runs your browser's built-in RegExp engine. It does not support PCRE/Python-only syntax such as possessive quantifiers, atomic groups, recursion, or \K — patterns using these will show a syntax error here even if valid elsewhere.
Runs are capped at 1.5 seconds. Every test or replace runs in a background worker with a hard-coded timeout. If a pattern hasn't finished in that time (usually a sign of catastrophic backtracking), the run is aborted and you'll see a "took too long" warning instead of a frozen tab.
Matches are capped at 1,000 per run. If your pattern matches more than that, only the first 1,000 are highlighted and listed, for performance.
Replace Mode follows standard JS rules: without the g flag, only the first match is replaced — that's normal String.replace() behavior, not a bug.
Recent patterns are stored in your browser's local storage only. They aren't synced across devices and will disappear if you clear your browsing data or use a different browser.
Everything runs on your device, not a server. Nothing you type is uploaded anywhere, but very large test strings (we'd suggest staying under ~50,000 characters) will be slower on lower-powered devices.
The 1.5s timeout needs Web Workers to work. If your browser or a strict site policy blocks them, you'll see a warning banner above and patterns will run unprotected — avoid pasting untrusted patterns in that case.