After reading this blog post about trolls as unique individuals, I started thinking: what if "troll" isn't a species, but an effect? Sort of how we might talk about mutants, but there is no "mutant" species, or talk about "cursed" creatures, but "cursed creature" isn't a species; it's something that has happened to a creature, not a distinct creature itself.
In this case, "turns to stone in sunlight" and "regenerate" are two supernatural effects out of possibly many that can be applied to some underlying, horrid humanoid. Maybe... the humanoids themselves? I already talk about goblinoids as being essentially unified variety of creature, an insane, murderous, distorted human, dwarf, elf, or halfling that lives in dark places and preys on decent folk. Perhaps "troll" is a more hideous, more enchanted variant of goblin, something bordering on the unholy.
Maybe trolls are goblinoid undead?