If I were you I would not eat that after the expiration date. Yogurt has mold as an ingredient and I would be afraid if it activated some other ingredient that it will make you sick. Think of milk when you get it to the expiration date, it smells horrible, yogurt is no different.
It's safe to eat up for 2 to 3 weeks after "sell by" date...it rarely has an expiry date on the yogurt container (there's a difference). Yogurt doesnt have mold in it by the way, it has good bacteria (active cultures), and has been treated to remove all bad bacteria. So once it was properly sealed- it's good for the least 2 to 3 weeks after sell by date.