Regression testing tasks of test case prioritization, test suite reduction/minimization, and regression test selection are typically centered around criteria that are based on code coverage, test execution costs, and code modifications. Researchers have developed and evaluated new individual criteria; others have combined existing criteria in different ways to form what we--and some others--call hybrid criteria. In this paper, we formalize the notion of combining multiple criteria into a hybrid. Our goal is to create a uniform representation of such combinations so that they can be described unambiguously and shared among researchers. We envision that such sharing will allow researchers to implement, study, extend, and evaluate the hybrids using a common set of techniques and tools. We precisely formulate three hybrid combinations, Rank, Merge, and Choice, and demonstrate their usefulness in two ways. First, we recast, in terms of our formulations, others' previously reported work on hybrid criteria. Second, we use our previous results on test case prioritization to create and evaluate new hybrid criteria. Our findings suggest that hybrid criteria of others can be described using our Merge and Rank formulations, and that the hybrid criteria we developed most often outperformed their constituent individual criteria.