Email validation pattern
Before
Regex worked on 90% of test emails, failed on a few
After
Added `i` flag — emails with capital letters matched
The bug was case sensitivity. Two seconds to fix once I saw the pattern failing live.
All Tools
Regex Tester – Test and Debug Regular Expressions
I built this because I got tired of testing regex one character at a time in the browser console. Paste your pattern and sample text, see matches highlighted live, adjust the flags, and iterate. JavaScript (ECMAScript) syntax — same as what runs in your app.
100% free, no signup
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Highlighted matches (5)
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Your files are processed in your browser and never uploaded to any server.
A note from me
The most common regex bug I run into is missing the global flag. Without `g`, `.match()` and `.replace()` only work on the first match. It looks like the pattern works — until it doesn't. Always test with a string that has multiple matches. Also, if your pattern has a capture group inside a quantifier, you're probably not getting the behavior you expect. Test it here first.
— Mirza Noman Baig
How to use this tool
- 1Enter your regex pattern.
- 2Pick flags — g, i, m, s, u, y.
- 3Paste the text you want to test against.
- 4See highlighted matches, match count, and captured groups.
Real results from testing
These are actual measurements from using this tool, not estimates.
Parsing log lines
Before
Pattern matched the first log entry but not the rest
After
Added `g` flag — all entries matched
Classic missing global flag. This tool makes it obvious because only the first match highlights.
Multiline text extraction
Before
Pattern worked on single lines, failed across newlines
After
Enabled `m` and `s` flags — matched across the whole document
Understanding m vs s is confusing. Testing both here made the difference obvious.
Regex Tester features
Live match highlighting
Matches highlight as you type — patterns and text both update in real time.
Full flag support
g, i, m, s, u, y — all standard JavaScript regex flags.
Match details
Shows match count, positions, and captured groups.
Instant validation
Invalid patterns show clear errors. No guessing.
Frequently asked questions
Which regex flavor does this use?+
JavaScript (ECMAScript). The same syntax your browser and Node.js use.
What are the flags for?+
g = global (all matches), i = case-insensitive, m = multiline (^ and $ match line breaks), s = dotAll (. matches newlines), u = unicode, y = sticky.
Is my test data stored?+
No. Everything runs in your browser. Your patterns and test text never leave your device.
Does it explain errors?+
Yes. Invalid patterns produce a clear error message so you can fix them.
Can I test replace patterns?+
The tool focuses on matching. For replace testing, copy the matched output into your code and test there.