The Troll stat block includes the following trait:
Regeneration. The troll regains 10 hit points at the start of its turn. If the troll takes acid or fire damage, this trait doesn't
function at the start of the troll's next turn. The troll dies only if
it starts its turn with 0 hit points and doesn't regenerate.
(The same stat block appears for Frost Giant Everlasting Ones in Volo's Guide to Monsters (p. 148), and likely for some other monsters too.)
My reading of the trait is that, unless the monster took acid or fire damage on its last turn, it will be guaranteed to benefit from regeneration at the start of its turn. If that's true, the only way to effectively kill the troll (in this case, or others with this trait) is to use fire or acid damage, making the monster pretty tough to kill.
Am I reading that correctly? Or is there a different intended interpretation that I'm missing?