There are many instances of advanced/accelerated healing in the Harry Potter books. For example, in book 2 Harry regrows bones, in book 6 Tonks fixes Harry's broken nose in an instant, and in book 7 Hermione uses essence of dittany to help Ron's splinched arm heal faster.
Then there's this passage from The Deathly Hallows chapter 5: Fallen Warrior:
By the lamplight Harry saw a clean, gaping hole where George's ear had been.
'How is he?'
Mrs Weasley looked round and said, 'I can't make it grow back, not when it's been removed by Dark Magic.'
This pretty clearly implies that ears, at least, can be grown back so long as Dark Magic didn't take the ear in the first place. But is the same true of limbs and other body parts? What if Harry loses a kidney in Quidditch? Would Madam Pomfrey be able to grow him another one?
I realize that Moody has lost an eye, a leg, and much of his nose and doesn't have them back. However, given the above I think that's probably because he lost them to Dark Magic, being an Auror who fought Death Eaters.
So Dark Magic aside, should any part of the body be regenerable? Or are there limits to magical healing regardless of how the damage was incurred? Something that would indicate this would be an instance of "sorry, this is too severe, magic can't fix it" in the books.