Regex Tester
Test and debug regular expressions with real-time matching and highlighting.
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Match Results
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Test and debug regular expressions with real-time matching and highlighting.
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Pattern and test text
\b(\w+)\s+\1\b
This is is a testMatched text
is isTest representative valid and invalid inputs before relying on a regular expression in production.
A regular expression tester lets you write a pattern, feed it sample text, and see exactly what matches — before the regex goes anywhere near production code. Built for developers, data analysts, and anyone who has stared at an expression like \b\w+@\w+\.\w+\b wondering why it fails, it updates matches in real time so you can iterate instead of guessing.
A regular expression is a compact notation for describing patterns in text. The idea dates back to mathematician Stephen Kleene’s work in the 1950s, and entered everyday computing through Ken Thompson’s Unix tools like grep. Today every major language ships a regex engine, and while flavors differ, the building blocks — character classes, quantifiers, groups, anchors — are nearly universal. Regex shines at validating structured strings, extracting data from logs, and pattern-based find-and-replace; it is a famously poor fit for parsing nested structures like HTML.
The most common beginner mistake is greediness: .* consumes as much as possible, so ".*" applied to say "hi" and "bye" matches everything between the first and last quote. Use the lazy variant .*? when you want the shortest match. Second, watch out for catastrophic backtracking — nested quantifiers like (a+)+ can freeze the engine on non-matching input. Third, escape special characters: a literal dot is \., which is why \d+\.\d+\.\d+\.\d+ matches IP addresses while the unescaped version matches almost anything.
Developers use this tester to build form validation patterns, craft search expressions for log analysis, and debug capture groups before wiring them into code. It is equally handy for one-off jobs like extracting every URL from a block of text.
JavaScript (ECMAScript) regex, the most widely used flavor in web development.
Add flags after the closing slash, like /pattern/gi for global and case-insensitive matching.
Parentheses in a pattern capture the matched substring, so you can extract parts of a match, like the area code from a phone number.
In JavaScript, the dot matches any character except line terminators by default. Add the s (dotAll) flag to make it match newlines too.