Regeneration

[Teysa karlov]

Can you regen a creature that is sacrificed?

[Gorzo]

Nope

Edit: sorry for the gruff answer with no explanation, was in a bit of a hurry and wanted to at least answer you. The reason you cannot regen a sacrifice a creature is two-fold.

1) regen only does the following:
The next time this creature would be destroyed, instead don't destroy it, tap it, remove all damage from it, and remove it from combat.
Sacrifice is not a destroy effect, nor is it damage, so regen won't stop it. Same goes for a creature having 0 toughness, due to effects like -1 counters or things like {dead weight}. and...
2) sacrificing specifically states that a creature sacrificed cannot be saved in any way. Sacrificing doesn't even use the stack, so you can't use stack manipulation to stop it either.

Hope that's more clear than my original 'nope'

RESOLVED

[Keyeto]

A creature that has undying that is sacrificed will come back. The undying trigger will go on the stack, and resolve accordingly.

[Teysa karlov]

I'm so glad, I was play EDH with a mate, he only had his commander on the field and I had mine  {Anowon, the Ruin Sage}, and in my turn's upkeep his commander was ment to be sacrificed by my commander ability, but he would regen it do it didn't die,

I knew he was cheating, even though not intentionally.