Regex Tester
Test JavaScript regular expressions in real time with match highlighting and capture groups.
Test JavaScript regular expressions in real time with match highlighting and capture groups.
This free Regex Tester from KX Toolkit is part of our all-in-one online toolkit. It runs entirely in your browser, so your data never leaves your device for client-side operations. 100% free, forever - no paywall, no credit card, no trial.
How to use the Regex Tester
- Paste your input - JSON, regex pattern, JWT, URL etc.
- Pick any flags or options the tool supports.
- Click the action button (Format, Test, Decode).
- Copy the result or download it as a file.
What you can do with the Regex Tester
- Format and validate API responses while debugging.
- Test regex patterns against real input before deploying.
- Decode JWTs to inspect claims and expiry.
- Generate UUIDs for migrations, tests and seeders.
Why use KX Toolkit's Regex Tester
- Browser-based: Works on Windows, macOS, Linux, iOS and Android - no install, no extension.
- Privacy-first: Client-side tools never upload your data; server-side tools delete files right after processing.
- Mobile-friendly: Full feature parity on phones and tablets - not a stripped-down view.
- Fast: Optimised for instant feedback. No artificial waiting screens, no email-gated downloads.
- One hub for everything: 300+ tools across SEO, text, image, PDF, code, color, calculators and more - skip switching between sites.
Tips for the best results
Bookmark the most-used tools - your browser bookmark bar is faster than retyping the URL every time.
Related Developer Tools
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What flavor of regex does this tester use?
It uses the JavaScript regex engine because the tool runs in your browser. That means lookbehinds need a modern browser, named capture groups use the (?<name>...) syntax, and Unicode property escapes require the u flag. If you are testing patterns destined for PCRE, .NET, or POSIX, behavior differs around features like atomic groups and recursion - verify in a flavor-specific tool.
Why does my pattern not match what I expect?
The most common causes are forgetting to escape special characters such as dot, plus, and parentheses; missing the global flag when iterating with matchAll; greedy versus lazy quantifiers consuming too much or too little; and anchors like ^ and $ behaving per-line only when the m flag is set. Step through with a tester that highlights each match incrementally.
How do capture groups differ from non-capturing groups?
Both group expressions for quantification and alternation, but capture groups (parentheses) save the matched text into numbered slots while non-capturing groups (?:...) do not. Use non-capturing groups when you only need grouping for syntax, because they are slightly faster and keep your numbered captures aligned. Named groups (?<name>...) are the most readable choice in modern code.
What are common pitfalls with the global flag?
A regex with /g remembers its last index between exec calls via lastIndex. Calling test or exec on the same pattern with different strings can return unexpected misses because lastIndex was advanced. Either reset lastIndex to 0 between calls, create a fresh regex object each time, or use string.match and matchAll which manage state for you.
When is a regex the wrong tool?
Anything with nested or recursive structure - HTML, JSON, code - outgrows regex quickly. Use a real parser instead. Regex is great for line-level extraction, validation of simple formats, and search-and-replace. For email validation it works for most formats but the full RFC grammar is too complex to encode reliably; pair a simple regex with a confirmation email instead.
How do I avoid catastrophic backtracking?
Backtracking blows up when nested quantifiers can match the same characters in many ways, like (a+)+. Rewrite to be unambiguous: use a possessive form via the trick (?=(a+))\1 in JavaScript, or restructure so each quantified subexpression has a unique anchor. Always test patterns against worst-case input - a pathological string can hang the browser tab for seconds.
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