If I play {Altar's Reap} and sac {Thragtusk} can I use {Golgari Charm} to regenerate him?
Regeneration does not interact with sacrificing. It only kicks in if the creature is destroyed.
Here's your ruling:
701.12a 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."
RESOLVED
Even if that did work, you'd have to cast the charm first. Since saccing a creature is part of the cost for {Altar's Reap}, your Tusk would already be gone by the time you could cast another spell.