Regex Tester

What Is a Regex Tester?

A regex tester is an interactive tool that lets developers write, test, and debug regular expressions in real time. Regular expressions (regex) are powerful patterns used to search, match, and manipulate text in virtually every programming language. This tool highlights matches instantly, displays captured groups, and includes a comprehensive cheat sheet so you can build the perfect pattern without switching between tabs.

How Does This Regex Tester Work?

Enter your regular expression pattern in the pattern field, select the flags you need (global, case-insensitive, multiline, or dotAll), then type or paste your test string below. The tool will highlight every match in real time and list each match with its position index and any captured groups. If your pattern has a syntax error, a clear error message will appear instantly.

Common Use Cases for Regular Expressions

Regular expressions are used for email validation, URL parsing, log file analysis, data extraction from HTML or CSV, password strength validation, search-and-replace operations in code editors, and much more. This tester lets you experiment with patterns safely before deploying them in production code.

Frequently Asked Questions

What do the regex flags mean?

The g (global) flag finds all matches instead of stopping at the first one. The i flag makes the pattern case-insensitive. The m (multiline) flag makes ^ and $ match the start and end of each line instead of the entire string. The s(dotAll) flag makes . match newline characters as well.

Why is my regex not matching anything?

Common issues include forgetting the global flag when you expect multiple matches, not escaping special characters like dots or parentheses, and case sensitivity. Use the i flag for case-insensitive matching and double-check your character escaping.

Is my data processed on a server?

No. All regex matching happens entirely in your browser using JavaScript's native RegExp engine. Your text and patterns never leave your device.

Can I use lookahead and lookbehind assertions?

Yes. This tool supports all regex features available in modern JavaScript engines, including positive and negative lookaheads ((?=...), (?!...)) and lookbehinds ((?<=...), (?<!...)), named capture groups, and Unicode property escapes.