[qntm]

This was actually going to be my solution as well. Regex engines are optimised for exactly this sort of problem, why duplicate the work manually?

[saucetenuto]

rfumani gave one reason: it won't produce the right answer. Here's another: your regex engine will probably fall over dead if you ask it apply a million-word alternation.

[eru]

grep could be fine. I wouldn't trust backtracking regex engines, though.