Regex Tester Online — Test & Debug Regular Expressions

A regular expression (regex) is a sequence of characters that defines a search pattern. It's used in programming, text editors, and command-line tools to find, match, validate, and replace text. For example, the regex \d{3}-\d{4} matches patterns like 123-4567. Regex is supported in virtually every programming language including JavaScript, Python, Java, Go, and Rust.

The global flag tells the regex engine to find all matches in the input string, not just the first one. Without the g flag, the regex stops after finding the first match. Other common flags include i (case insensitive), m (multiline — ^ and $ match line boundaries), and s (dotAll — . matches newline characters).

Parentheses () in a regex create capture groups. When the regex matches, each group captures its portion of the matched text separately. You can reference groups by index: group 0 is the full match, group 1 is the first set of parentheses, etc. Named groups (?<name>...) let you reference captures by name instead of index.

Catastrophic backtracking occurs when a regex pattern causes the engine to explore an exponential number of paths on certain inputs. This can freeze your browser or crash a server. Common culprits are nested quantifiers like (a+)+ or (a|a)+. This tool includes a timeout guard to prevent such issues. In production, always test your regex with adversarial inputs.

Yes, this tool uses JavaScript's native RegExp engine directly in your browser. The syntax and behavior match exactly what you'd get in Node.js, Chrome, Firefox, or any JavaScript runtime. The flags g, i, m, s, and u are all supported.