Are there any regional airlines in US that fly airbus?
No there aren’t and I doubt you ever will (hopefully). In ALL Major airline contracts there is language called “scope”. While scope doesn’t restrict the particular make or model of aircraft it does restrict seating capacity. Basically scope states that Regional partners can’t fly any aircraft with more than X amount of seats (usually in the area of 75 give or take). The idea is to prevent the Major carriers from taking flying away from their own pilots and giving it to a Regional partner to save money.
Adam