How to Test a Regular Expression Online (with Live Highlighting)

Writing a regular expression from scratch and getting it exactly right on the first try is rare for almost anyone. The usual workflow is trial and error — tweak the pattern, run it against some real text, see what matched and what didn't, adjust, repeat. That loop is a lot faster with live feedback than with print statements in your actual code.

See Every Match as You Type

QR Ivify's Regex Tester highlights every match in your sample text live, as you edit the pattern — no separate "run" step. Capture groups, named groups, and match positions are all broken out below the highlighted text, so you can see exactly what your pattern is actually capturing, not just where it matches.

Step by Step

1. Open Regex Tester.
2. Type or paste a regular expression pattern — no need to add the surrounding slashes.
3. Toggle flags like case-insensitive or multiline as needed.
4. Type or paste sample text — matches highlight instantly as you type.
5. Check the match details list below for each match's position and captured groups.

What Regex Flavor Does This Use?

This uses your browser's native JavaScript regex engine, so patterns follow JavaScript regex syntax — which is close to, but not identical to, PCRE, Python's re module, or other flavors. If a pattern behaves differently here than in another language, that's usually a flavor difference (like lookbehind support or named group syntax) rather than a bug.

One thing worth knowing: a pathological pattern with catastrophic backtracking can make matching slow or freeze the tab, the same as it would in any JavaScript environment — there's no execution timeout here, so keep an eye on patterns with nested quantifiers against long input.

Frequently Asked Questions

Does this upload my text anywhere? No. Matching runs entirely in your browser using JavaScript's native RegExp — nothing is sent to a server.

Do I need to add slashes around my pattern? No — just type the pattern itself. The slashes shown around the input are just a visual reminder of the flags applied.

What flags are supported? Case insensitive (i), multiline (m), dot-matches-newline (s), and unicode (u). Matches are always found globally so every occurrence is shown, not just the first.

Why does my pattern show an error? The pattern isn't valid JavaScript regex syntax — the error message shown is the browser's own explanation of what's wrong (e.g. an unterminated group or invalid flag combination).

Try It

Debugging a regex the slow way, one code run at a time? QR Ivify's free Regex Tester shows every match live as you type. Need to find and replace text using a pattern instead? See Find & Replace.