Which regex flavour does this use?

JavaScript regex (ECMAScript). It supports lookbehind, named groups, Unicode property escapes (\p{...}), and the y (sticky) flag. Patterns from Python, PCRE, or .NET may not transfer 1:1 — most do, but features like atomic groups and possessive quantifiers do not exist in JS.

How do I match across newlines?

By default, "." does not match newlines. Add the s flag (dotAll) and "." will match \n, \r, and other line terminators. Use the m flag if you need ^ and $ to match at line boundaries inside a multiline string.

My regex hangs the page — why?

Catastrophic backtracking. Patterns like (a+)+b on a long string of "a" characters can take exponential time. Avoid nested quantifiers; if you need them, use atomic-style patterns or anchor your alternations.

Are my pattern and text sent anywhere?

No. The tester runs the regex in your browser via the JavaScript engine. Patterns and text never leave the page.

Can I see capture groups?

Yes — every match shows its numbered groups and any named groups (? ...). The tool displays them in a table beneath each match.

Files never leave your device

Regex Tester

Test JavaScript regular expressions with live match highlighting, group capture display, and replacement preview.

Pattern

Flags

Test text3 matches

Highlighted result

Send mail to alice@example.com or bob@test.org. Try also: charlie@foo.io.

Matches & groups

#1 @13 alice@example.com

#2 @34 bob@test.org

#3 @58 charlie@foo.io

How to use Regex Tester

Enter your regular expression in the pattern box.
Add flags (g, i, m, s, u, y) — the global flag is on by default for match-all behaviour.
Paste the text you want to test against in the larger area below.
Matches highlight in real time. Capture groups and named groups appear underneath.

Regex flag cheatsheet

g — global, return all matches instead of just the first
i — case-insensitive
m — multiline; ^ and $ match at line boundaries
s — dotAll; "." also matches newline characters
u — Unicode; enables \p{...} property escapes
y — sticky; matches only at lastIndex (advanced)

Patterns worth memorizing

Email-ish: /^[\w.+-]+@[\w-]+\.[\w.-]+$/. URL-ish: /https?:\/\/\S+/g. Whitespace squash: /\s+/g. None are perfect — real-world inputs always contain edge cases. Use this playground to confirm before shipping a pattern to production.