RAB can't be Regulus unless he had a horcrux as he was killed by Voldemort. Which means he is well and truly dead:

Regulus Black: Regulus, who shared his parents' pureblood prejudices, joined the Death Eaters willingly because he agreed with their general philosophy, but tried to back out when he realised how far they were prepared to go. He was consequently killed on Voldemort's orders not long before Voldemorts fall.