Regex Tester
Test regular expressions against sample text with live match highlighting.
Regular expressions are notoriously easy to get almost right — a pattern that matches your first three test cases can silently fail on the fourth, and debugging why usually means adding print statements to a script just to see what's actually matching. A dedicated regex tester short-circuits that loop entirely: change the pattern, see every match highlighted instantly, and understand exactly what's being captured.
This tool uses JavaScript's native RegExp engine — the same one that runs in Node.js and every modern browser — so patterns you test here behave identically when used in real JavaScript or TypeScript code. Matches are highlighted directly in your test string, capture groups are broken out individually, and invalid regex syntax is caught immediately with the exact error message the engine produced, rather than a generic failure.
Flags (g, i, m, s, u, y) control matching behavior: i for case-insensitive matching, m for multi-line anchors, s for allowing . to match newlines, and so on. The global flag is applied automatically so you always see every match in your test string rather than just the first one.
How to use Regex Tester
- 1
Write your pattern
Enter your regular expression in the Pattern field, without the surrounding slashes.
- 2
Set flags
Add flags like i (case-insensitive) or m (multi-line) as needed.
- 3
Paste test text
Enter the text you want to test the pattern against.
- 4
Review highlighted matches
Every match is highlighted inline, with a running count below.
Features
- Uses JavaScript's native RegExp engine for accurate real-world behavior
- Live match highlighting directly in your test text
- Capture group breakdown for each match
- Clear, engine-native error messages for invalid patterns
Frequently asked questions
Common mistakes to avoid
- Forgetting to escape special regex characters like . or ( when you mean them literally.
- Using greedy quantifiers (.*) when a non-greedy version (.*?) is what actually matches the intended text.
- Assuming a pattern that works in one language's regex flavor (e.g. Python) will behave identically in JavaScript.
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