Will there still bus from airport to TianAn Rega Hotel @ Dongcheng Dist. by 1230MN? What bus number?

Yes, you can take airport shuttle bus line 2 (night line) and get off at Beijingzhan Station. Then walk 50 meters to reach Beijingzhanxizhan Station. There you can take 211 night bus and get off at Wangjinglukoubei Station and then walk 420 meters to reach your hotel.

Peronally, I don't think it's a good idea because you are not familiar with Beijing. For your safety, it's beter to take a taxi.