How to Test a Regular Expression: Step-by-Step Guide

Test and debug a regex in a few steps — enter a pattern, add flags, watch matches, and try replace mode.

Published August 28, 20264 min read

How to test a regex in a few steps

Testing a pattern takes seconds.

Go to Regex Tester. It runs entirely in your browser — nothing to install, no account to create.

Type the regular expression you want to check, for example `\d{3}-\d{4}` for a phone fragment.

Paste text you want to match against. Matches highlight in real time, so you can adjust as you go.

Add flags (`g`, `i`, `m`) to change how matches are found, and switch to replace mode to test substitutions.

Start with small patterns and add complexity gradually
Use the case-insensitive (`i`) flag to test both letter cases
Use replace mode to verify your substitution string before committing it to code

A modern browser (Chrome, Firefox, Safari, or Edge). The tool runs entirely on your device.

Skip installs and uploads: open Regex Tester and debug your patterns in seconds.

Matches highlight in real time in the sample text as you type your pattern.

Add flags like g (global), i (case-insensitive), and m (multiline) to change matching behavior.

Yes. Switch to replace mode to test find-and-replace logic.