Regression testing is any type of software testing that seeks to uncover new software bugs, or regressions, in existing functional and non-functional areas of a system after changes, such as enhancements, patches or configuration changes, have been made to them.
The intent of regression testing is to ensure that a change such as those mentioned above has not introduced new faults. One of the main reasons for regression testing is to determine whether a change in one part of the software affects other parts of the software.
Common methods of regression testing include rerunning previously-completed tests and checking whether program behavior has changed and whether previously-fixed faults have re-emerged. Regression testing can be used to test a system efficiently by systematically selecting the appropriate minimum set of tests needed to adequately cover a particular change.
Experience has shown that as software is fixed, emergence of new and/or reemergence of old faults is quite common. Sometimes reemergence occurs because a fix gets lost through poor revision control practices (or simple human error in revision control). Often, a fix for a problem will be "fragile" in that it fixes the problem in the narrow case where it was first observed but not in more general cases which may arise over the lifetime of the software. Frequently, a fix for a problem in one area inadvertently causes a software bug in another area. Finally, it may happen that, when some feature is redesigned, some of the same mistakes that were made in the original implementation of the feature are made in the redesign.
Therefore, in most software development situations, it is considered good coding practice, when a bug is located and fixed, to record a test that exposes the bug and re-run that test regularly after subsequent changes to the program. Although this may be done through manual testing procedures using programming techniques, it is often done using automated testing tools. Such a test suite contains software tools that allow the testing environment to execute all the regression test cases automatically; some projects even set up automated systems to automatically re-run all regression tests at specified intervals and report any failures (which could imply a regression or an out-of-date test). Common strategies are to run such a system after every successful compile (for small projects), every night, or once a week. Those strategies can be automated by an external tool, such as BuildBot, Tinderbox, Hudson, Jenkins, TeamCity or Bamboo.
Regression testing is an integral part of the extreme programming software development method. In this method, design documents are replaced by extensive, repeatable, and automated testing of the entire software package throughout each stage of the software development cycle.
In the corporate world, regression testing has traditionally been performed by a software quality assurance team after the development team has completed work. However, defects found at this stage are the most costly to fix. This problem is being addressed by the rise of unit testing. Although developers have always written test cases as part of the development cycle, these test cases have generally been either functional tests or unit tests that verify only intended outcomes. Developer testing compels a developer to focus on unit testing and to include both positive and negative test cases.
Regression testing can be used not only for testing the correctness of a program, but often also for tracking the quality of its output. For instance, in the design of a compiler, regression testing could track the code size, simulation time and compilation time of the test suite cases.
"Also as a consequence of the introduction of new bugs, program maintenance requires far more system testing per statement written than any other programming. Theoretically, after each fix one must run the entire batch of test cases previously run against the system, to ensure that it has not been damaged in an obscure way. In practice, such regression testing must indeed approximate this theoretical idea, and it is very costly."
Regression tests can be broadly categorized as functional tests or unit tests. Functional tests exercise the complete program with various inputs. Unit tests exercise individual functions, subroutines, or object methods. Both functional testing tools and unit testing tools tend to be third-party products that are not part of the compiler suite, and both tend to be automated. A functional test may be a scripted series of program inputs, possibly even involving an automated mechanism for controlling mouse movements and clicks. A unit test may be a set of separate functions within the code itself, or a driver layer that links to the code without altering the code being tested.
See also 
- Characterization test
- Non-regression testing
- Quality control
- Software development process
- Software testing
- Smoke testing
- Myers, Glenford (2004). The Art of Software Testing. Wiley. ISBN 978-0-471-46912-4.
- Savenkov, Roman (2008). How to Become a Software Tester. Roman Savenkov Consulting. p. 386. ISBN 978-0-615-23372-7.
- Kolawa, Adam; Huizinga, Dorota (2007). Automated Defect Prevention: Best Practices in Software Management. Wiley-IEEE Computer Society Press. p. 73. ISBN 0-470-04212-5.
- Automate Regression Tests When Feasible, Automated Testing: Selected Best Practices, Elfriede Dustin, Safari Books Online
- daVeiga, Nada (February 2008). "Change Code Without Fear: Utilize a Regression Safety Net". Dr. Dobb's Journal.
- Dudney, Bill (2004-12-08). "Developer Testing Is 'In': An interview with Alberto Savoia and Kent Beck". Retrieved 2007-11-29.
- Kolawa, Adam. "Regression Testing, Programmer to Programmer". Wrox.