Regex Tester

A regular expression (regex) is a sequence of characters that defines a search pattern. It's used for pattern matching within strings, enabling powerful text search, validation, and manipulation. Regex is supported in virtually all programming languages including JavaScript, Python, PHP, Java, and more. Common uses include validating email addresses, parsing log files, and search-and-replace operations.

This tool uses JavaScript's built-in RegExp engine, which follows the ECMAScript specification. It's compatible with most PCRE (Perl Compatible Regular Expressions) patterns and works identically to regex in browsers and Node.js environments. Some advanced features like lookbehind assertions are supported in modern browsers but may not work in older ones.

g (global) finds all matches instead of stopping at the first match.
i (case-insensitive) makes the pattern match regardless of letter case (A matches a).
m (multiline) makes ^ and $ match the start/end of each line, not just the whole string.
s (dotall) makes the dot (.) match newline characters as well, allowing patterns to span multiple lines.

Common issues include: forgetting to escape special characters (like . or * which have special meanings), not enabling the correct flags (e.g., 'i' for case-insensitive matching), using syntax not supported in JavaScript (like some lookbehind patterns in older browsers), or having unexpected whitespace in your pattern or test string. Check that your pattern syntax is valid - the tool will show an error message for invalid regex.

Yes, this regex tester is completely free with no signup required. All pattern matching happens entirely in your browser using JavaScript - no data is sent to any server. This makes it completely safe and private for testing patterns against sensitive data like log files, personal information, or proprietary code.