During the life time of software or even within the development stage, it is inevitable that the software needs to be modified. Test cases used before for the previous version could be reused in order to reduce the cost of regression testing. This paper proposes a regression testing methodology, called Partitioning of Domain Testing (PDT), to analyze the input domains of the previous version and the modified version so that test cases can be maximally reused. As a result, software test engineers can spend less effort in generating test cases to test the modified software. This methodology is different from traditional domain-based testing strategies in the sense that the partitioning is achieved by two supplementary perspectives, namely the specifications of the program and its testing criterion. First, the input domain is partitioned according to the specification and testing criterion of the program under test. Then, further refining methods are introduced to obtain 100\% reusable test cases. We also illustrate the idea of PDT using a case study with 3 different testing criteria.