Long ago I once had a Wrym of mine get beat up, and run away. It only retreated halfway across the battlefield before it got tied up trying to fight its way home. Then something happened, maybe it regen'ed enough HP, and it turned around and started fighting again, no longer in retreat mode. Does that count as regrouping? I guess it's pretty hard for a single unit to regroup, even if it does have two heads.
Coincidentally, it was in a battle against thejeff.
Also, this is pretty much just a useless anecdote.
The battle is long gone, and when it occured, I didn't know how to look at the debug output. It's the only time I've ever seen it happen.