Topic is a bit old, but I stumbled upon this query while looking for regex query example for my own usecase.
I think the following would work (without the primitive.Regex):
games, e = h.service.GameRepo().Find(bson.M{
"slug": bson.M{"$regex": "^[0-9]", "$options": "i"}
}, &options.FindOptions{
Sort: bson.D{{"name", 1}},
})