RegEx Tester

What is the RegEx Tester?

The RegEx Tester is a live debugging workbench for JavaScript regular expressions. Paste a pattern and some test text, and every match is highlighted instantly — with full capture group inspection, flag toggles, and a token-by-token explanation of what your pattern actually does.

Unlike the Grep tool (which filters whole lines out of a larger text), this tool focuses on a single pattern: it helps you understand, build, and fix that one tricky expression.

Features

• Live matching — results update as you type, no submit button
• Match highlighting — every match is highlighted directly in your test text
• Capture groups — numbered and named groups shown per match, including non-participating groups
• Flag toggles — g, i, m, s, u and y flags, exactly like JavaScript
• Pattern explanation — plain-English breakdown of every token in your expression
• Match list — every match with its position, length, and captured groups
• Safety caps — patterns are limited to 500 characters and text to 100,000 characters to guard against pathological (ReDoS-style) inputs
• Clear errors — invalid patterns produce the exact parser error, immediately

How to use

1. Type or paste your pattern into the expression bar (no surrounding slashes needed).

2. Toggle flags with the chips next to the pattern.

3. Paste your test text into the left panel.

4. Inspect highlighted matches in the preview, and per-match capture groups in the match list below.

Privacy

All matching happens entirely in your browser with the native JavaScript regex engine. Your patterns and test data are never sent to any server.