What causes catastrophic backtracking in regular expressions, how can a crafted input freeze your app or server (ReDoS), and how do you write linear-time-safe patterns?

Imagine you're trying to find a toy in a messy room. If you keep looking in the same wrong places over and over, it takes forever. Catastrophic backtracking is like that—a regex gets stuck trying many wrong paths, and a bad guy can make it freeze your game.