Regen happens when you drive (correct me if i'm wrong), typically at a steady speed on the highway. If the fan stays on after you shut off the car, the ECU has determined that something is too hot and so the electric water pump and fans will run until its cool, most likely the turbo. It can run up to 15 minutes (I think) after the car has been turned off.
Current: BMW Welt Delivery 4/14/16:
European Delivery - 2016 340i EBII/Saddle - ZMP | ZCW | ZDA | ZLP | ZTP | M Adaptive | Rear Shades | MPPSK Before and After Dyno
2011 335D Space Grey/Black - ZSP | ZPP | ZCW | 6FL | Sat Radio | Nav | $600 Base Audio Upgrade