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Regex Tester
Test regular expressions safely with a timeout.
About this tool
Regex Tester lets you test a regular expression against sample text and inspect matches in the browser. A worker and timeout reduce the chance that an inefficient pattern freezes the page, but they cannot prove that a pattern is safe for every production input. Test cases should include valid matches, expected failures and unusually long strings.
When it is useful
- Prototype a validation or extraction pattern.
- Check capture groups against representative text.
- Reproduce a regex bug without sending sample data elsewhere.
How to use it
- Enter the pattern without surrounding slash characters.
- Choose only the flags your runtime supports.
- Add representative positive and negative sample text.
- Run the test and inspect matches and capture groups.
Quality and privacy notes
Regex behavior differs between programming languages. This tool follows the JavaScript regular-expression engine, so features and escaping may differ from PHP, Python, Java or .NET.
Important limits
- Timeout protection is not a formal ReDoS audit.
- JavaScript syntax may differ from your target language.
- Do not rely on regex alone to validate complex structured data.
Frequently asked questions
Why does a server-side regex behave differently?
Different engines support different flags, escapes and advanced features.
Does timeout mean my pattern is safe?
No. It only limits this browser test. Review performance in the actual runtime.