If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends.
A troll has the Regeneration trait, described as follows:
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.
I know that monsters can't take legendary actions while surprised, as clarified in the Monster Manual errata.
However, traits aren't actions, and the troll's Regeneration trait in particular doesn't require an action, a bonus action, a reaction, or anything; it simply happens at the start of the troll's turn as a passive ability.
And since surprised creatures still take a turn during the first round of combat, even if they can't move or take actions (or bonus actions, as the last sentence on PHB p. 189 clarifies), this would suggest that the troll still regains 10 HP at the start of a turn in which it can't move or take actions.
Is this an accurate interpretation of the rules, or have I missed something?