I was five. Very windy rain-thunder storm outside. I opened the door to the yard, clutch the doorframe, heavy gust blows the door shut jamming my right hand, and the "edge" of the door coming down on my right thumbnail. I pulled my hand out of the jam, and was trying to examine it in the murky gloom (power was out), when the lightning flashed and I saw how horrific it looked. The nail had cracked horizontally across, flattening the middle and prying the upper half loose so that the nail fragment was dangling, only attached to the lower nail-base by a flap of skin and the corner of the crack. Dark red blood was also running down my thumb. At first it didn't even hurt, I was in such a shock, but once I did feel it, it was excruciating.
But yeah, it does grow back in a cool way when the skin where the nail itself should be hardens to form the nail. I'd almost forgotten it, but now I remember with this subject being brought up. At first it'll look kinda disgusting, with a very thin bumpy, pink, semi-hard surface, but it should grow from there and for me at least there is no sign of anything ever having happened to my thumbnail.
Here's hoping for a speedy painless regrowth.
If it breaks, you get to keep both pieces.