Regex Tester

A Regex Tester lets you write a regular expression, run it against sample text, and see exactly what it matches in real time. Every match is highlighted in place, each capture group is listed with its value and position, and syntax errors in your pattern are reported with the engine's exact message - so you can debug a pattern in seconds instead of pasting it into your code and re-running your program.

Regular expressions are compact but unforgiving: a missing escape, a greedy quantifier, or a misunderstood character class can make a pattern match nothing, match too much, or throw an error. This tester uses the same JavaScript RegExp engine your browser and Node.js run, so the behavior you see here is the behavior you get in production code. Toggle the g, i, m, s, u, and y flags to see how each one changes the result.

Everything runs 100% in your browser. Your patterns and test data - which often include real log lines, user records, or proprietary formats - are never uploaded or stored. The tool works offline once loaded, so you can test expressions on sensitive data without it ever leaving your device.

The engine here is the browser's own, so behaviour matches what your JavaScript will do rather than approximating it. Two things trip people up. The global flag makes a regular expression stateful: it keeps a lastIndex between calls, so reusing the same object in a loop can skip matches or return null on a string that clearly matches. Creating the expression fresh, or resetting lastIndex, avoids it. The second is performance. Nested quantifiers over overlapping character sets, the (a+)+ shape, can take exponential time on input that fails to match, which is the mechanism behind regular expression denial of service. A pattern that is instant on your examples and hangs on a longer string has this problem, so test the failing cases as deliberately as the passing ones.