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Free Regex Tester Online

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What is Free Regex Tester Online?

Regex Tester is a live regular expression playground that highlights all matches in your test string as you type. Regular expressions (regex) are a powerful pattern-matching language used across programming to validate input, extract data, and transform text. Writing regex by hand can be error-prone - a small mistake can cause unexpected matches or catastrophic backtracking. This tool shows matches in real time, making it easy to iterate and debug patterns quickly.

How it works

The tester uses JavaScript's RegExp engine. As you type, the pattern is compiled and executed against your test string. Each match is highlighted with its index position and captured groups displayed. Named groups (?<name>...) are also extracted and shown. A 1000-match cap prevents runaway matches from freezing the browser.

Features & Benefits

  • Live highlighting updates as you type - no button needed
  • Runs in your browser, so your test data stays private
  • Shows match details including index, captured groups, and named groups
  • Limits output to 1000 matches to prevent browser freeze on pathological patterns

Frequently Asked Questions

What regex flavour does this support?

JavaScript regex (ECMAScript). This covers the vast majority of common regex patterns and is compatible with JS, TypeScript, and many other languages.

What flags are supported?

g (global), i (case-insensitive), m (multiline), s (dotAll), u (unicode), and d (indices).

Why does it stop at 1000 matches?

Catastrophic backtracking with certain patterns against large inputs can freeze a browser tab. The 1000-match limit prevents this.

Does it support named capture groups?

Yes. Named groups (?<name>...) are shown in the match details.

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