He regretted not having said goodbye to her at the airport.

 He regretted not having said goodbye to her at the airport Or He regretted not saying goodbye to her at the airport

 He regretted not having said goodbye to her at the airport.

 He regretted not having said goodbye to her at the airport.

 He regretted not having said goodbye to her at the airport.

=>He regretted not having said goodbye to her at the airport 

giải nghĩa: Anh hối hận vì đã không chào tạm biệt cô ở sân bay

He was sorry he hadn't said goodbye to her at the airport.

=>  He regretted not having said goodbye to her at the airport.

He regretted not having said goodbye to her at the airport.

He regretted not having said goodbye to her at the airport.

 He regretted not having said goodbye to her at the airport.

He regretted because of he hadn't said goodbye to her at the airport