Regex Tester

A regex tester is an interactive tool that lets you write a regular expression pattern and immediately see how it matches against a test string. Instead of running your code repeatedly to debug a pattern, you get instant visual feedback with highlighted matches, captured groups, and match positions.

This tool uses JavaScript's native RegExp constructor, so the behavior matches exactly what you'd see in a browser or Node.js environment. All processing happens client-side — your patterns and test strings never leave your device.

Start with a simple pattern and build complexity incrementally. Use the preset patterns as starting points for common tasks like email or URL validation, then refine them for your specific requirements.

Always test edge cases: empty strings, strings with special characters, very long input, and input that almost matches but shouldn't. The match table shows you exactly what each group captures, which is essential for patterns with parenthesized groups used in extraction or replacement.

Email validation is one of the most common regex tasks, though a truly RFC-compliant email regex is extremely complex. The practical approach is to use a reasonable pattern like the one in the presets and combine it with server-side validation. URL matching is similarly nuanced — schemes, ports, paths, query strings, and fragments all add complexity.

Phone number patterns vary by country and format. IP address validation requires range checking (0–255 per octet), which regex handles with alternation groups. Date patterns can validate format but not calendar correctness — "2024-02-30" matches the pattern but isn't a real date.

JavaScript regex uses /pattern/flags literal syntax or the RegExp constructor. Python uses the re module with raw strings (r"pattern"). Java requires double-escaped backslashes in string literals. Go's regexp package uses RE2 syntax, which lacks some features like backreferences and lookahead.

PCRE (Perl Compatible Regular Expressions) is the most feature-rich engine, supporting recursive patterns, conditional groups, and atomic groups. PHP, R, and many text editors use PCRE or PCRE2. When porting patterns between languages, always verify behavior — subtle differences in Unicode handling, newline matching, and group numbering can cause bugs.