Regression test selection is a process to select a subset of existing test cases, which are then used with some new test cases for regression testing. Regression testing ensures that the changes made to the system have not affected the existing functionality. To date, there is no adequate technique which can do regression test selection by considering changes in semantics of operations (along with the other syntax and semantics changes) using UML diagrams. Change in semantics of an operation refers to the change in conditional statements, change in independent paths/unique paths, change in control flow and addition or deletion of any content from the existing functionality. In this study, a novel approach is presented which can do this using class, sequence and activity diagrams. The tool compared old and new versions of UML diagrams to categorise test cases into reusable, retestable, obsolete and newly generated category. Activity diagrams are specifically used to test the semantics of operations. The changed operations corresponding to these activity diagrams are also searched in class and sequence diagrams for regression test selection. This study has been validated by comparison with previous study. It is found that the authors’ work provides significant increase in accuracy.