Regulus Black never knew what being loved felt like. He grew up with parents who didn't want him here, a brother he loved with all his heart but who abandoned him in this loveless house.
Maybe he knew what being liked felt like. His friends liked him. But he doesn't think they loved him enough.
One day he loved someone. Someone very dear to him who he thought loved him back. But as soon as something went wrong in Regulus' life, this someone went away and fell in love with someone else immediately.
So no, Regulus Black never knew and would never know what being love felt like.
Because he was going to die without having experienced this feeling. He knew what loving felt like but not mutual love.
In those last moment, when Regulus' lungs were being filled with water, he wondered one last time what that feeling could be.
Being loved by someone.