Regression Testing:

The purpose of
regression testing is to confirm that a recent program or code changes has not
adversely affected existing features.

Regression
testing is nothing but full or partial selection of already executed test cases
which are re-executed to ensure existing functionalities work fine.

This testing is
done to make sure that new code changes should not have side effects on these
existing functionalities. It ensures that old code still works once the new
code changes are done.

Regression
testing are required when there is a

è   
Changes in requirements and code are modified
according to the requirements.

è   
New feature is added to the software.

è   
Defect fixing

è   
Performance issues fix.

Regressing
testing techniques:

è   
Retest All

è   
Regression Test Selection

è   
Prioritization of Test Cases 

Re-Testing: