That only works when you control both the pattern and the shape of the input. Once you accept user regexes or start scanning big log dumps without tight boundaries the weird cases stop being weird.

Greedy globs plus unanchored branches can turn a boring batch job into a CPU bonfire, and defensive limits seems a lot less optional when one dumb pattern can pin a core for minutes. A timeout or a restricted regex subset is usually cheaper than pretending everybody writes sane patterns.