Regex Tester

Test JavaScript regular expressions live, inspect every capture group, and preview replacements.

Text ToolsRuns in your browserFree, no sign-up

Options

Pattern

Without delimiters. Named groups (?<name>…) are supported.

Replacement (optional)

Previews String.replace. Use $1 or $<name> for groups.

Ignore case (i)

Multiline ^ $ (m)

Dot matches newline (s)

Unicode (u)

Test text

Matches

Output appears here as you type.

Ctrl + Enter to run

About the Regex Tester

Matches as you type and shows capture groups, which turns pattern debugging from guesswork into observation. Most of the time a pattern that fails is matching more or less than you think at one specific point, and seeing the highlight makes that immediate.

One limit is worth stating plainly: regular expressions cannot parse nested structures. HTML, JSON and source code all nest arbitrarily deep, so a pattern that works on your sample will fail on the first nested tag or escaped quote. Use a parser for those and keep regex for flat, well-defined patterns.

Frequently asked questions

Which regex flavour is this?

JavaScript (ECMAScript), because it runs in your browser's own engine — so what you see here is exactly what your JavaScript will do. Be aware it differs from PCRE, Python and Go in real ways: no atomic groups, no possessive quantifiers, no recursion, and lookbehind is supported in JS but not in every other flavour.

What is the ReDoS warning about?

Catastrophic backtracking. A pattern with nested quantifiers such as (a+)+$ can take exponential time on input that nearly matches — 30 characters can hang for minutes. Because JavaScript regexes run synchronously and cannot be interrupted, that would freeze the page with no way to recover. The tool flags the dangerous shapes before you run them.

How do I use named capture groups?

Write (?<name>pattern) in your regex, and reference it as $<name> in the replacement. The sample above uses (?<user>…)@(?<domain>…), which is far more readable than counting $1 and $2 — and it does not break when you add a group in the middle.

Why is the g flag always on?

So you see every match rather than only the first, which is what you want from a tester. Bear in mind that a regex object with g is stateful in JavaScript — it carries lastIndex between calls — which is a classic source of bugs when you reuse one across invocations.