Regex Tester & Explainer
Write patterns, test against real strings, and get a plain-English breakdown of every token.
Learn more Browser-first developer tools
Build and debug regex patterns without guessing what each token or match is doing.
What to expectEnter a pattern and test string to see live matches, execution feedback, and a plain-English explanation of each regex token.
Good to knowGreat for validation, parsing, and search patterns, especially when a regex is correct but hard to reason about.
How it works
- Enter or paste a regular expression pattern and optional flags in the pattern bar.
- Paste the test string to see every match highlighted in real time.
- Use the explainer panel to understand each token before copying your pattern into code or config.
All processing happens in your browser. Test strings and patterns are never sent to any server or logged.
No matches yet.
Pattern Explained
Tokens appear here as you type
Frequently Asked Questions
How is this different from regex101?
DataSmith's Regex Tester focuses on JavaScript regex and includes a bidirectional explainer: hover any token in the pattern to see what it matches, or hover a match in the test string to highlight the responsible part of the pattern. It is designed to help you reason about patterns, not just see matches.
What regex engine does this use?
The tool uses JavaScript's native RegExp engine, the same engine used in Node.js and modern browsers. That means features and quirks like lookbehind support, Unicode handling, and backtracking behavior match what you'll see in production JavaScript code.
Will this tool freeze if my regex causes catastrophic backtracking?
No. Long-running patterns are executed in a Web Worker with a short timeout. If execution takes too long, the worker is terminated and you see a timeout warning instead of a frozen tab, so you can safely explore problematic patterns.
Can I use this tester for languages other than JavaScript?
The behavior shown is specific to JavaScript's regex engine, not PCRE or .NET. You can still use the tool to design and explain patterns conceptually, but always validate language-specific nuances in your target runtime, especially around lookbehinds and Unicode.
Which flags are supported in this regex tester?
You can use standard JavaScript flags like g (global),i (case-insensitive),m (multiline),s (dotAll),u (Unicode), andy (sticky). The tester applies them directly when it runs your pattern.
Is my test data or regex pattern logged or sent anywhere?
No. Patterns and test strings live only in your browser while the page is open. DataSmith.io does not send this content to a backend, store it, or use it for model training, which makes it safe for debugging real-world input samples.
How to use this tool
Paste a regex pattern between the / delimiters. Add flags (g=global, i=case insensitive, m=multiline). Matches highlight in real time. The explainer panel breaks down each token into plain English.