I think I regret basically all photos I've ever taken. I don't take good photos as you might have guessed. Just not very photogenic I guess. That's the thing about photos; you can't really take them back. Especially nowadays where once they get put on the internet they may never disappear.
I never take a nude photo even if it's just for fun. You never know where such photos could end up. We hear about this sort of thing all the time, how someone only did this in fun yet it ended up in the wrong hands and got online where everyone could see it.
Taking nude photos seems to be an artistic act. However, I have never done or will never do this because what if a bad person put my naked photos online and let it go viral for bad purposes, it will be a total disaster and defame myself. It is surely a bad example for the others. Though one has his or her freedom to do that, I won't do it.
We all know that nothing on a computer or phone is ever 100% private. Therefore if you wouldn't potentially want a naked photo of yourself to be made public, then do not take one in the first place. That is my philosophy and therefore I have never taken a naked photo of myself and cannot regret one because of that fact.