My browser tab froze — why?

A pathological pattern (nested quantifiers like (a+)+ against a long non-matching string) can cause catastrophic backtracking. That's inherent to regex engines, not a bug here — simplify the pattern or shorten the test text.

{} Developer

Regex Tester

Build and test JavaScript regular expressions with live highlighting.

Runs in your browserNo account requiredNo uploadFree

Loading tool…

About Regex Tester

Type a pattern, choose flags, and see every match and capture group highlighted in your test text.

Frequently asked questions

JavaScript (ECMAScript). Patterns that work here will work in JS code.

More developer tools

JSON Formatter & Validator

Paste JSON to format, validate, or minify — runs entirely in your browser.

{}Developer

Base64 Encoder / Decoder

Encode to Base64 or decode it back — handles UTF-8 correctly.

{}Developer

JSON ↔ CSV Converter

Convert between JSON and CSV — round-trip safe, runs locally.

{}Developer

UUID / GUID Generator

Generate cryptographically random UUID v4 identifiers.

{}Developer

JSON ↔ YAML Converter

Switch between JSON and YAML — useful for Kubernetes, CI configs, and API specs.

{}Developer

JWT Decoder

Paste a JWT to inspect its header and payload. Decoded locally.

{}Developer