I think I read a comic where he is basically burnt alive in some kind of explosion and he goes through the regeneration process mentioning, I think, nerves are the worst to regenerate because it's really painful
It probably depends on the writer who is currently writing him, but chat gpt thinks he would eventually recover from decapitation or attacks that destroy his brain. I can't remember where I got this from, but my understanding is that as long as some of his cells survive, he will regenerate. Which could rule out the surviving the sun depending on whether he'd burn up faster than he can regenerate (and assuming no new powers affect this).