100% free · Runs 100% in your browser · No signup needed
The Regex Tester lets you build and debug regular expressions against your own test text, with matches highlighted live as you type. Regular expressions are the Swiss-army knife of text processing - used for validating emails, extracting data, reformatting logs and searching code - but writing them blind is painful. This tool shows you instantly what your pattern actually matches.
How to use: 1) Type your regular expression in the pattern box (without the surrounding slashes). 2) Toggle the g (global), i (case-insensitive) and m (multiline) flags. 3) Paste your test string below. 4) Matches are highlighted in yellow in real time, with a total count and a list showing each match plus its position in the text.
Why use it: A regex that looks right can still fail on edge cases - a phone validator that rejects valid numbers, or an email pattern that accepts garbage. Testing live against realistic sample data catches these problems before the pattern ships into production code. The position list also helps you understand exactly which part of the text matched, which is invaluable when debugging complex patterns with groups and alternations.
Key benefits: Real-time highlighting as you type - no submit button needed. Full flag support including global, case-insensitive and multiline modes. Match index positions shown for precise debugging. Clear, specific error messages when your pattern has invalid syntax. Completely private - patterns and test data never leave your browser, so proprietary log formats and customer data stay on your machine.
FAQs: Which regex flavor is this? JavaScript regular expressions - the same engine used in browsers and Node.js. Why do I get zero matches? Check your flags (add g for all matches) and remember that special characters like dots and brackets need escaping. Can I test multiline text? Yes - enable the m flag so ^ and $ match at line boundaries. Is there a match limit? The display caps at 500 matches to keep the page responsive, but the count reflects all matches.
Pro tips: Build complex patterns incrementally - test each piece before combining them, and the live highlighting will show you exactly when a piece stops matching. Use the match position list to verify capture groups land where you expect. For learning, try classic patterns first: email validation, URL extraction and date matching are great exercises. FistHost keeps this tester free and unlimited, so you can iterate on tricky patterns without signing up anywhere.
Related tools: Combine the Regex Tester with the JSON Formatter when mining structured data from logs, or the Diff Checker to verify a find-and-replace across files. All FistHost developer tools run locally in your browser at no cost.
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