Only test the happy path, but make sure to add a test case for any regression that couldn’t have been caught at lower levels with better tests (for example, if a regression is found, regression tests should be added at the lowest level possible).

Why separate out red tests from green tests? Because my green tests serve a fundamentally different purpose. They are there to act as a living specification, validating that the behaviors work as expected. Regardless of whether they are implemented in a unit testing framework or an acceptance testing framework, they are in essence acceptance tests because they’re based upon validating behaviors or acceptance criteria rather than implementation details.

Sometimes a change impact analysis is performed to determine an appropriate subset of tests

non-regression testing

Regression testing (rarely non-regression testing[1]) is re-running functional and non-functional tests to ensure that previously developed and tested software still performs after a change.[2] If not, that would be called a regression.