Regression Testing:

The selective retesting of a http://www.webopedia.com/TERM/R/software.html - - bugs have been fixed and that no other previously working functions have failed as a result of the reparations and that newly added features have not created problems with previous versions of the software.  

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Also referred to as verification testing, regression testing is initiated after a http://www.webopedia.com/TERM/R/programmer.html - - source code to a program that may have inadvertently introduced errors. It is a quality control measure to ensure that the newly modified code still complies with its specified requirements and that unmodified code has not been affected by the maintenance activity.

Regression means going back. With testing processes we mean that you need to repeat previously successful tests any time there is a chance that subsequent changes could have affected an aspect of the solution.

The testing process involved building a sequence of tests, many of which stood upon the successful results of earlier tests. If a component of the solution has been changed, all other components which relied upon it might have been affected. Similarly, all tests that relied on an earlier test might no long be valid. Consider carefully which solution and testing components need to be re-validated.

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One consequence of this issue is that changes during the testing process are bad news. There will always be errors and changes during the testing, but it might be better to defer the correction of some minor problems to make better overall progress. A seemingly innocuous example with a big hidden catch is when a software supplier suggests that a problem you are experiencing would be solved by moving to their next software release in which the problem has been fixed. Moving to a new release probably means that every test you did to date is now invalidated and will have to be repeated.

Uses: 

Regression testing can be used not only for testing the correctness of a program, but it is also often used to track the quality of its output. For instance in the design of a http://en.wikipedia.org/wiki/Compiler -