Ah, I see now.... /x??:/ will behave exactly like /x{0}/
In english /x??:/ would read something like this:

Match 'x' zero or one times, prefer 0 over 1, lock in the first value that allows this atom to match (i.e. don't backtrack).

Since the 0 will always cause the atom to match, the engine will never be allowed to try the 1.

I know thats not the best description.... hope it helps.

-Blake