I wouldn't say I regret it, but that almost doesn't feel like an applicable word; it felt inevitable to me, so it's hard to imagine any other path. Like, at the end of undergrad, all I wanted to do was think about math all day, every day, and that's what I got to do for the next five years. There was no convincing 22-year-old me to do anything else. Only near the end did the novelty really start to wear off.

N.b. I'm in a very lucky position career-wise now. If that were not the case, maybe I would feel more regret.