Free Online Regex Tester

Regular expressions are powerful but easy to get wrong. QodeCat Regex Tester shows you exactly what your pattern matches while you type: every match is highlighted in the text, and a match list shows the position, the matched value and each capture group. It uses the same regex engine as JavaScript, so what you see here is what your code will do.

• Live highlighting: See every match in your test string instantly.
• Capture groups: Inspect numbered ($1, $2) and named (?<name>) groups for each match.
• All flags: Toggle g, i, m, s, u and y and see the effect right away.
• Replace preview: Try replacement strings with $1, $<name> and $& references.

Why use this tool?

Safe against runaway patterns

Some patterns, like nested quantifiers, can take exponential time on certain inputs (catastrophic backtracking) and freeze a browser tab. This tester runs matching in a background worker and stops it after one second, so a bad pattern never locks up your page.

Private by design

Your patterns and test data never leave your device. Everything is evaluated locally in your browser, which makes it safe to test against logs, customer data or configuration files.

Frequently Asked Questions