Let S be a semigroup with
zero. The extended centralizer Q(Ms) of a right S-set Ms is defined. Necessary and
sufficient conditions are given for Q(Ms) to be a regular semigroup. In particular,
Q(Ss) is shown to be a regular semigroup when S is regular. We also show that
whenever the singular congruence on S is the identity, then Q(Ss) is the injective hull
of |Ss and is right self injective.