James Farrell: Voldemort never told anyone about his horcruxes, so how on earth did regulus black discover his secret
J.K. Rowling: Horcrux magic was not Voldemort's own invention; as is established in the story, other wizards had done it, though never gone as far as to make six.
J.K. Rowling: Voldemort dropped oblique hints; in his arrogance, he did not believe anybody would be clever enough to understand them.
J.K. Rowling: (He does so in the graveyard of Little Hangleton, in front of Harry). He did this before Regulus and Regulus guessed, correctly, what it was that made Voldemort so convinced he could not die.