Regex Tester

Web & HTTP

Test regular expressions with live match highlighting and capture groups.

Regex Pattern

Flags

Test String

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About Regex Tester

Test regular expressions with live match highlighting and capture groups. Regex Tester runs entirely in your browser — no installation, no uploads, and no data ever leaves your device.

Common use cases: debug and refine regex patterns before adding to code; test email, url, or phone number validation patterns; explore regex capture groups interactively. Whether you are debugging, prototyping, or building production code, having a fast and private tool for this task saves time compared to writing one-off scripts or using an online tool that logs your data.

Like every tool in Catalyst, this one is free forever, requires no login, and is safe to use with confidential data. All processing happens client-side via native browser APIs.

How to use Regex Tester

1
Open the tool
Open Regex Tester directly — no signup or installation required. The interactive interface loads instantly.
2
Paste or enter your input
Drop your data into the input area. The tool validates and processes your input live as you type or paste.
3
Review the result
The output updates instantly. If the tool has configuration options, adjust them and watch the result update.
4
Copy or download
Use the copy button to send the result to your clipboard. Nothing is saved between page loads, so closing the tab discards everything.

Frequently asked questions

Which regex flavor does the tester use?

JavaScript (ECMAScript) regex syntax. This is very close to PCRE but with some differences around lookbehind, named groups, and Unicode properties. Patterns that work here usually work in Python, Java, and Go with minor adjustments.

Can I use named capture groups?

Yes. JavaScript supports named capture groups with the syntax (?<name>...) and named backreferences with \k<name>. The tester displays captured group names alongside their values.

Does it support lookbehind assertions?

Yes. Modern JavaScript (ES2018+) supports both positive (?<=...) and negative (?<!...) lookbehind. The tester uses your browser's native regex engine, so support matches your browser version.