Regex Tester & Debugger: Test Regular Expressions with Live Highlighting
Regular expressions are powerful but notoriously difficult to write correctly. A regex tester with live highlighting lets you see exactly what your pattern matches in real time — turning regex debugging from trial-and-error guesswork into a visual, interactive process.
Try Regex Tester Free →How the Regex Tester Works
Enter your regex pattern in the pattern field and your test string in the text area. The tester instantly highlights all matches in the text, shows captured groups, and displays match count and positions. Changes to the pattern update results in real time.
Regex Flags Explained
g (global): find all matches, not just the first. i (case-insensitive): match regardless of case. m (multiline): ^ and $ match start/end of each line. s (dotAll): dot matches newlines. Formly's tester supports all standard flags — toggle them with checkbox buttons.
Common Regex Patterns Reference
Email: /^[\w.-]+@[\w.-]+\.\w{2,}$/ — Phone (US): /^\+?1?[-.]?\(?\d{3}\)?[-.]?\d{3}[-.]?\d{4}$/ — URL: /https?:\/\/[\w/:%#$&?()~.=+\-]+/ — IP address: /^(25[0-5]|2[0-4]\d|[01]?\d\d?)\.(...repeat...)$/ — Hex color: /#?([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})/
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Understanding Regex Groups
Capturing groups (parentheses) extract specific parts of a match. Named groups (?<name>pattern) are easier to reference. Non-capturing groups (?:pattern) group without capturing. The tester shows each captured group in a separate panel so you can verify your extractions.
JavaScript vs Python vs PCRE Regex
Most regex syntax is consistent, but flavors differ in edge cases. JavaScript doesn't support lookbehinds in older engines. Python's re module supports different syntax for named groups. PCRE (PHP, etc.) has the most features. Formly's tester defaults to JavaScript flavor (which matches browser usage) with a Python mode toggle.
Regex Performance and Catastrophic Backtracking
Poorly written regex can cause exponential slowdown — "catastrophic backtracking." This happens with patterns like (a+)+ on inputs like "aaaaaaaab". The tester shows execution time and warns about potentially slow patterns.
Frequently Asked Questions
Which regex flavor does the tester use?⌄
JavaScript (ECMAScript) by default. Switch to Python mode for Python-compatible regex.
Can I test multiline patterns?⌄
Yes. Enable the m flag to make ^ and $ match line boundaries rather than string boundaries.
Does it support lookbehind assertions?⌄
Yes, for JavaScript environments that support them (all modern browsers). The tester runs in your browser's JavaScript engine.
Is it free?⌄
Yes. Completely free with no limits.
Can I save and share my regex patterns?⌄
The URL updates with your pattern and test string, so you can share a link to your specific test.
Try Regex Tester — It's Free
No signup needed. 5 free uses daily. Pro plan from $9.99/month.
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