Test regular expressions live

A free online JavaScript regex tester that runs in your browser. Type a pattern, optionally tweak the flags (g/i/m/s/u/y), and the right pane highlights every match in real time with alternating colours so adjacent hits stay distinct. The match list shows numbered captures and named groups; the replacement field gives a live preview of String.prototype.replace() with $1 $2 backreferences. Built-in safety guard against catastrophic-backtracking infinite loops on zero-width matches.

How to use

1
Enter your pattern

Without slashes — they're drawn around the field for you.

2
Adjust flags

Default is g (global). Add i for case-insensitive, m for multiline ^$.

3
Paste a test string

The right pane highlights matches live.

4
Inspect groups

Numbered and named captures appear under each match.

5
Optionally preview Replace

Type a replacement to see the substituted output.

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Live JavaScript regex testing with highlighted matches and replacement preview

/ /
g global · i case-insensitive · m multiline ^$ · s . matches \n · u unicode · y sticky
Test string
Highlighted matches 0
No matches yet.

Cheat sheet

\ddigit 0-9
\wword char
\swhitespace
.any char
^ $start, end
* + ?0+, 1+, 0/1
{n,m}n to m times
[abc]character class
(...)capture group
(?:..)non-capturing
(?<n>..)named group
a|ba or b
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Features

Live highlighting Group inspection Replace preview All JavaScript flags Cheat sheet built in Safety net

Common uses

  • Debug an "Unexpected token" regex pattern by seeing the exact error position.
  • Verify a tricky email or phone-number regex against a list of test inputs.
  • Preview a sed-style search/replace before applying it to a 10MB log file.
  • Inspect a captured group to ensure the right substring goes into the right slot.

Why test here

Most browser regex testers either swallow zero-width match infinite loops (crashing your tab) or hide named groups behind a paywall. This one explicitly guards the iteration cap, lists named and numbered groups by default, and supports every JavaScript flag including the sticky y.

Frequently asked questions

Free?

Yes — no account, no limits.

Does it support PCRE / Python regex?

No, this is the JavaScript regex flavour (ECMAScript). Differences from PCRE: no \K, no possessive quantifiers, no recursive patterns; lookbehinds and named groups are supported.

What if my pattern hangs the browser?

There's a 50,000-iteration safety cap on the match loop, and zero-width matches advance the cursor by one. Catastrophic-backtracking on a single match (e.g. /(a+)+b/) is still possible — those are inherent to the regex, not the tester.

Multiline?

Enable the m flag — then ^ and $ match line starts and ends, not just string start and end.

Unicode chars in classes?

Enable the u flag. This also unlocks \p{...} property escapes (e.g. \p{L} for any letter).

Mobile?

Yes. The cheat-sheet wraps to one column on narrow screens.

💡 Want us to improve this tool just for you?

We can — and it's free! Just send us a quick message with your idea. If you'd like to discuss it in detail, leave your email and we'll get back to you. You can stay anonymous.

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Who is this tool for?
💻 Developers & IT Pros
Published