The Perils of Catastrophic Backtracking (ReDoS)
Regular Expressions are incredibly powerful, but poorly written patterns containing nested quantifiers (like (a+)+$) can cause Nondeterministic Finite Automata (NFA) engines to explore exponential path combinations. A malicious user sending a 30-character string can lock your server CPU at 100% for minutes (Regular Expression Denial of Service).
The Regex Tester on Tools24Now provides interactive, safe pattern matching with visual match group highlights.
Essential Regex Cheat Sheet
^/$: Matches start / end of string.[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}: Standard email matching.(?=.*[A-Z])(?=.*\d): Positive lookahead for password complexity.
Test your expressions on Regex Tester.