Regeneration is a special form of healing that restores a fixed number of hit points every round. Regeneration doesn’t rely on healing surges.
Heal Each Turn: If a creature has regeneration and at least 1 hit point, it regains a specified number of hit points at the start of its turn. However, if its current hit point total is 0 or lower, it does not regain hit points through regeneration.
Not Cumulative: If a creature gains regeneration more than once, only the largest amount of regeneration applies. For example, a wererat regenerates 5 hit points each round. If another effect grants it regeneration of 2 hit points per round, it still regenerates 5 hit points, not 7.
Published in Player's Handbook, page(s) 293, Monster Manual, page(s) 282, Player's Handbook 3, page(s) 222, Rules Compendium, page(s) 260.