Regression testing is the process of running all existing test cases and verifying that all test cases pass. The purpose of regression testing is to detect unexpected faults—especially those faults that occur because a developer did not fully understand the internal code correlations when he or she modified or extended code that previously functioned correctly. Regression testing is the only reliable way to ensure that modifications did not introduce new errors into code or to check whether modifications successfully eliminated existing errors. Every time code is modified or used in a new environment, regression testing should be used to check the code's integrity. Ideally, you perform regression testing nightly (during automated nightly builds) to ensure that errors are detected and fixed as soon as possible.
