We'll know we have trolls for sure when they say that they can re-grow any severed limb, but cannot heal burns from fire or acid.
Actually, that is the case already.
(They can
replace burnt tissue by removing it so there is a 'normal' lack of skin which can be regrown. Burnt skin itself is useless and cannot be healed. So, had the fingertip been burnt shut, they wouldn't have been able to regenerate it without first cutting another bit off.)