One slight addition to what dragonstryke58 said, since I’ve seen people tripped up by this before:
Because the replacement effect must be applied at the time of destruction, a spell or ability granting regeneration must be used before the creature would be destroyed; you can’t use regenerate after the creature is dead. So, if there is a Lightning Bolt or Murder aimed at your 3/3, you have to use a spell or ability to give it regeneration when Lightning Bolt / Murder is on the stack, before the kill spell resolves. Once the spell resolves, it is too late.
Also, note regeneration only works when a creature is destroyed. If a creature would die without being destroyed (such as having 0 toughness) regenerate will not be implicated.
dragonstryke58 says... Accepted answer #1
Yes, token creatures can be regenerated.
Regenerate creates a replacement effect that replaces an instance this turn that the permanent would be destroyed due to an effect or lethal damage. For tokens, since they won't leave the battlefield due to destruction event being replaced, they will not cease to exist (they were on the battlefield the whole time).
From the Comprehensive Rules:
If the effect of a resolving spell or ability regenerates a permanent, it creates a replacement effect that protects the permanent the next time it would be destroyed this turn. In this case, “Regenerate [permanent]” means “The next time [permanent] would be destroyed this turn, instead remove all damage marked on it and tap it. If it’s an attacking or blocking creature, remove it from combat.”
August 3, 2019 5:52 a.m. Edited.