Regex Tester

Test .NET Regular Expressions.

A client-side Regex tester using the .NET regex engine via WebAssembly.

🔍 The Power (and Pain) of Regex

Regular expressions were invented by mathematician Stephen Cole Kleene in the 1950s as part of formal language theory. Ken Thompson brought them to computing in 1968 when he implemented them in the QED text editor. The famous quote? "Some people, when confronted with a problem, think 'I know, I'll use regular expressions.' Now they have two problems."

🤯 Wacky Facts

• The regex to validate an email address according to RFC 5322 is over 6,500 characters long
• A poorly written regex once crashed Cloudflare's entire network for 27 minutes
• Perl Compatible Regular Expressions (PCRE) aren't actually Perl-compatible anymore
• The .* pattern is called "greedy" because it matches as much as possible
• ReDoS (Regular Expression Denial of Service) is a real security vulnerability!

💡 Useful Tips

• Start simple: Build your pattern incrementally, testing each addition
• Use non-greedy: Add ? after quantifiers (.*?) to match minimally
• Named groups: (?<name>pattern) makes captures readable
• Use comments: The (?x) flag allows whitespace and comments in your regex
• Anchor your patterns: Use ^ and $ to match whole strings
• Test edge cases: Empty strings, special characters, very long inputs

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