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JavaScript Regex Tester Online

Test regular expressions against your own text with live match highlighting and capture group details.

Runs entirely in your browser — your files are never uploaded to a server.

Last updated

This tool processes your input entirely in your browser. Nothing is sent to our servers.

How It Works

1. Choose Your File

Drag & drop or select the file you want to work with.

2. Processed in Your Browser

Everything happens locally on your device — your file is never uploaded anywhere.

3. Download Your Result

Get your finished file instantly, ready to save or share.

About JavaScript Regex Tester Online

Regular expressions are powerful but genuinely hard to get right on the first try — a pattern that looks correct can subtly fail to match what you intended, or worse, match things you didn’t intend at all. Testing against real sample text before deploying a pattern in actual code catches these issues immediately rather than in production.

This tool highlights every match live as you type your pattern and test string, so you can see immediately whether your regex is behaving as expected, rather than writing a pattern blind and only discovering problems later when running it against real data.

All standard JavaScript regex flags are supported — g (global, find all matches not just the first), i (case-insensitive), m (multiline, changes how ^ and $ behave), s (dotall), and u (Unicode) — letting you test a pattern exactly as it will actually behave in real code.

Frequently Asked Questions

Which regex flavor does this tester use?

It uses your browser's JavaScript RegExp engine, so patterns behave as they would in JavaScript code. Python, PCRE, Java and C# syntax can differ, for example Python's named-group syntax is invalid here. Treat results for other languages as a rough guide.

Which regex flags are supported?

Four flags are available as checkboxes: g for global, i for ignore case, m for multiline and s for dotAll. The g flag is on by default. Other JavaScript flags, such as unicode or sticky, cannot be switched on.

What does the regex tester show for each match?

Matches are highlighted in your test string, and a list shows each match with its starting index. Capture groups appear numbered beneath it as Group 1, Group 2 and so on. There is no replace mode or pattern explanation panel.

Why does it say invalid regular expression?

Your pattern failed to compile in JavaScript, and the message shown comes from the browser's regex engine. Typical causes are unbalanced parentheses, an unescaped special character or syntax from another flavor. Fix the pattern and the results refresh automatically.

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