Regex Tester

How It Works

Regular expressions (regex) are patterns used to find, match, and extract text. This tester uses the browser's built-in JavaScript regex engine — the same one used in String.prototype.match(), replace(), and matchAll().

Flags change how matching works: g finds all matches (without it, only the first is returned); i ignores case; m makes ^ and $ match line boundaries instead of the whole string; s (dotAll) makes . match newlines; u enables full Unicode support.

Capture groups let you extract parts of a match. Use () for numbered groups and (?<name>) for named groups. Non-capturing groups (?:) group without capturing, which is faster when you don't need the captured value.

Greedy vs. lazy: quantifiers like * and + are greedy by default — they match as much as possible. Adding ? makes them lazy: .*? matches as little as possible. Be careful with patterns like (a+)+ on long strings — catastrophic backtracking can cause the browser to hang.