Regex Tester: Free Online JavaScript Regex Tester — How-To Guide

Regular expressions are powerful, but a small mistake — a missing escape, a greedy quantifier, a wrong flag — can silently break validation, extraction, or filtering. The Regex Tester lets you check a JavaScript pattern against real sample text before it goes into your code, so you can see exactly what matches and what does not.

Try it free: Regex Tester

What the Tool Does

The Regex Tester is a free online tool for testing JavaScript regular expressions. You enter a pattern, add flags, and paste sample text, and it shows you the matches, the capture groups behind each match, and — if you want one — a replacement preview.

In short, it helps you:

• Verify a pattern against real examples before shipping it.
• Understand what each part of a pattern captures, via the capture-group list.
• Preview find-and-replace results with a replace: line.
• Spot risky patterns: it warns about constructs that can slow a page down, and it stops very large match sets so a bad pattern does not freeze your browser.

It is useful for form validation, route matching, text extraction, log inspection, and QA examples — anywhere a pattern has to behave predictably on real input.

How to Use It, Step by Step

1. Open the tool. Go to Regex Tester. Everything runs in your browser; there is nothing to install and no account to create.
2. Enter your pattern on the first line. Type the regular expression you want to test, for example \d{4}-\d{2}-\d{2} to match dates like 2026-11-02. Do not wrap it in slashes — just the pattern itself.
3. Add flags on the second line, if you need them. Put flags such as g, i, or m on the second line. Use i to ignore case, m to make ^ and $ match at line boundaries, and g for a global search. The tool adds g when needed so all matches can be listed.
4. Add a replacement preview (optional). If you want to preview a replacement, add a line starting with replace: before your sample text — for example, replace: [$1] — to see how the replaced output would look.
5. Paste your sample text. Include both examples that should match and examples that should not. A pattern that only gets tested on matching input is only half tested.
6. Read the results. Check the match list, then look at the capture groups for each match to confirm the right parts of the text are being captured. If a match looks wrong, adjust the pattern and re-check — the feedback is immediate.
7. Copy the working pattern into your code. Once the pattern behaves correctly on your samples, including the edge cases, it is ready to use.

Practical Tips

• Test the "should not match" cases too. A validation pattern is only good if it rejects bad input. Always include empty strings, near-misses, and malformed input in your sample text.
• Watch out for greedy matches. A pattern like .* grabs as much as it can, which often surprises people. If a match is longer than expected, try a lazy quantifier (.*?) or a more specific character class.
• Test edge cases before you ship. Try Unicode text, very long strings, and inputs with special characters. Patterns that work on tidy examples can fail on real-world data, and catching that here takes seconds.

Frequently Asked Questions

Q: What regex flavor does this tool use?

A: It uses JavaScript regular expressions, in the browser. That means the pattern you test here behaves the same way it would in JavaScript code — handy if the pattern is headed for a web app, a Node script, or a form validator.

Q: How do I add flags?

A: Put flags such as g, i, or m on the second line. The tool adds g when needed so all matches can be listed, so you do not have to remember to include it yourself.

Q: Can I preview replacements?

A: Yes. Add a line starting with replace: before the sample text to see a replacement preview, including captured groups like $1 in the output.

Q: Is my pattern or sample text sent to a server?

A: No. The tool runs entirely in your browser. Your patterns and sample text stay on your device, which makes it safe to test patterns against realistic — even sensitive-looking — example data.

Privacy Note

The Regex Tester runs 100% in the browser — your patterns and sample text never leave your device. Nothing is uploaded, stored, or sent to a server, so you can test freely without worrying about where your data goes.

Try it free: Regex Tester