Regex Tester
Test regular expressions with live match highlighting, flag controls, and instant error feedback.
What is a Regex Tester?
A regex tester is an interactive tool for building, testing, and debugging regular expressions against sample text. Regular expressions (regex or regexp) are patterns that describe sets of strings — they power search, validation, and text transformation in virtually every programming language. A good tester provides real-time match highlighting, named capture group extraction, and flag controls (case-insensitive, multiline, global) so you can iterate on a pattern without touching your codebase.
Common Use Cases
- Validate email addresses, phone numbers, URLs, or postal codes in form inputs
- Extract structured data (dates, IDs, prices) from unstructured text or log files
- Find and replace patterns in code editors or CI scripts
- Parse log entries to detect error patterns or security events
- Write input sanitisation rules for user-supplied strings
How It Works
This tester uses the JavaScript RegExp engine, so results are accurate for JavaScript applications. Enter your pattern, select flags (g for global, i for case-insensitive, m for multiline), and paste your test string. Matches are highlighted inline and capture groups are listed below. Patterns are evaluated as you type with a short debounce to avoid performance issues on large inputs.
Frequently Asked Questions
Why does my regex work here but not in Python?
Different languages implement slightly different regex flavours. JavaScript lacks lookbehind assertions in older engines, uses different escape sequences, and does not support POSIX classes. Test your pattern in the language you will actually deploy it in if exact matching matters.
What is catastrophic backtracking?
Certain regex patterns (especially nested quantifiers like (a+)+) can cause the engine to explore an exponential number of paths before determining there is no match — this can freeze your application. Test with long, non-matching inputs to check for backtracking issues.
What is the difference between a greedy and a lazy quantifier?
Greedy quantifiers (*, +, ?) match as much as possible. Lazy quantifiers (*?, +?, ??) match as little as possible. Use lazy quantifiers when you want to stop at the first match rather than the last.
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JavaScript RegExp: This tester uses the browser's native RegExp engine — the same one used in Node.js and all modern browsers.
All matching happens locally in your browser. No text or patterns are sent anywhere.