Regex Tester
Build and test JavaScript regular expressions with live highlighting.
About Regex Tester
Regex Tester is a live sandbox for JavaScript regular expressions. You type a pattern, set flags (g, i, m, s, u, y), paste sample text, and the tool highlights every match in the text and lists each match with its index and capture-group contents below. It is the fast feedback loop you want when you are building a pattern character by character.
Worked example: with the pattern (\w+)@(\w+\.\w+) and the g flag, against 'Email me at hello@example.com or test@mello.tools.', the tool highlights both addresses and shows two matches with their captured groups (local part and domain) underneath. Named groups using (?<name>...), backreferences, and lookarounds (?= ... ?<= ...) all work because the engine is the browser's own RegExp.
Two limits are worth knowing. First, the flavor is JavaScript / ECMAScript — patterns built here will run unchanged in JS code, but PCRE, .NET, and POSIX dialects support features (recursion, possessive quantifiers) that ECMAScript does not. Second, regex engines are vulnerable to catastrophic backtracking: a pattern like (a+)+b against a long non-matching string can hang the tab. If that happens, simplify the pattern or shorten the test text — it is inherent to backtracking engines, not a bug here.
Everything runs in your browser using the native RegExp engine, so nothing you paste is uploaded. That matters when your test text is a fragment of real logs, customer data, or other content you would rather not send to a third-party server.