Web-based applications are one of the most widely used types of software, and have become the backbone of many e-commerce and communications businesses. These applications are often mission-critical for many organizations, motivating their precise validation. Although regression testing has been widely used to gain confidence in the reliability of software by providing information about the quality of an application, it has suffered limited use in this domain due to the frequent nature of updates to websites and the difficulty of automatically comparing test case output. We present techniques to address these challenges in regression testing web-based applications. Without precise comparators, test cases that fail due to benign program evolutions must be manually inspected. Our approach harnesses the inherent similarities between unrelated web-based applications to provide fully automated solutions to reduce the number of such false positives, while simultaneously returning true faul...