Much work has been done on automating regression testing for Web applications, but most of them focus on test data generation or test execution. Little work has been done on automatically determining if a test passed or failed; testers would need to visually confirm the result which can be a tedious task. The difficulty is compounded by the fact that parts of a Web page (such as advertisements) may change each time the Web application is executed even though it has no bearing on the Web application function itself. We thus propose a test oracle for automatically determining the result of regression testing a Web application. The key point of our approach is the identification of parts that may change, which we call variable region. We first generate the expected result, by executing the original (premodification) Web application multiple times so that variable regions can be identified. Then, after the Web application is modified, regression testing is conducted by comparing the output of the modified Web application against the expected output. An evaluation confirmed the usefulness of our approach.