Regex tester
Live highlighting, groups, replace preview and errors that tell you what to fix.
How it works
- 01Type a pattern and switch flags like g, i and m on or off, or load a common pattern.
- 02Paste your text. Matches light up as you type, with every capture group and its position listed.
- 03Switch to replace to preview a substitution using $1 or $<name>, then copy the result or the regex as code.
Questions
›Which regex flavor does it use?
JavaScript's, run by your browser's own engine, so results match what your JS or TypeScript code will do. That includes named groups, lookbehind, Unicode property escapes with the u flag and set operations with the v flag. Other flavors such as PCRE or Python differ in places.
›What happens if my pattern is very slow?
Matching runs in a background worker. If it takes more than 2 seconds, usually because of catastrophic backtracking from nested quantifiers like (a+)+, it is stopped and you get an explanation, so the page never freezes.
›How do I use capture groups in a replacement?
Use $1, $2 and so on for numbered groups, $<name> for named groups, $& for the whole match, and $$ for a literal dollar sign. Without the g flag only the first match is replaced.
›Why does my pattern match nothing?
Common causes are case (turn on i), ^ and $ only matching the start and end of the whole text (turn on m to match each line), and . not matching line breaks (turn on s). Special characters like . ? + ( need a backslash to match literally.
›Is my data uploaded anywhere?
No. This tool runs entirely in your browser, and Rounder never sends what you type or drop in to a server, ours or anyone else's. Once the page has loaded, along with any AI model the tool uses, it even works offline.