The difference between re-testing and regression testing is:

Re-testing and regression testing are essential concepts in software testing, each serving a different purpose. According to the ISTQB CTFL Syllabus, version 4.0, the difference between these two types of testing is highlighted as follows:

Re-testing(also known as confirmation testing) involves executing the same tests again on the same version of the software after defects have been fixed. The primary goal is to confirm that the original defects have been successfully removed and that the software behaves as expected.

Regression testing, on the other hand, is performed to ensure that recent code changes have not adversely affected the existing functionalities of the software. This testing aims to detect any unexpected side effects introduced by the new changes.

The syllabus states: "Re-testing is running a test again; regression testing looks for unexpected side effects"​​. This explanation clarifies that while re-testing focuses on verifying specific defect fixes, regression testing ensures the overall stability and integrity of the software after changes.