Regex Tester

Pro tips most people learn the hard way

• Pro tip: the most common reason 'my regex doesn't work' is missing flags. The `g` flag finds all matches (without it, you get just the first). The `m` flag makes `^` and `$` match line boundaries instead of string boundaries. The `i` flag is case-insensitive. Stack them freely — `gim` is the common combination.
• JavaScript regex is close to but not identical to Python `re`, Perl, or .NET. Lookbehind has historically been a gotcha — supported in JS since ES2018 but missing or limited in older Python. If your pattern works here but not in another language, check that engine's documentation for the specific feature.
• Catastrophic backtracking is the production-killer most beginners hit. Patterns like `(a+)+b` or `(.*)*` against the wrong input can hang a server for minutes. If a pattern is slow, look for nested quantifiers and rewrite to be linear — `[ab]+` instead of `(a|b)+` is often the fix.
• Don't try to validate emails with regex perfectly — RFC 5322 is borderline impossible to express in a regex you can read. The `[\w.+-]+@[\w-]+\.[\w.-]+` pattern is the 99% pragmatic answer; for the last 1%, send a confirmation email and let SMTP do validation properly.
• Honest take: regex is the right tool for matching patterns in text, but the wrong tool for parsing structured formats. If you're trying to extract data from HTML, JSON, or XML with regex, switch to a proper parser. The result is shorter, more correct, and survives edge cases you'd never write a test for.