To ensure that the code changes have not had an adverse affect to the other modules or on existing functions.It is the process of testing changes to computer programs to make sure that the older programming still works with the new changes. Regression testing is a normal part of the program development process and, in larger companies, is done by code testing specialists.

Regression testing as the name suggests is used to test / check the effect of changes made in the code.

Regression Testing is retesting unchanged segments of application. It involves rerunning tests that have been previously executed to ensure that the same results can be achieved currently as were achieved when the segment was last tested.
The selective retesting of a software system that has been modified to ensure that any 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. Also referred to as verification testing, regression testing is initiated after a programmer has attempted to fix a recognized problem or has added 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.

Purpose of Regression Testing:
* Rerunning of previously conducted tests
* Reviewing previously prepared manual procedures
* Comparing the current test results with the previously executed test results