Difference between functional and regression test cases

Question

I am aware of the definition of functional and regression test case. But I am not able to distinguish which test case should fall in functional and which one should fall in regression test. For example, in sprint1 say I have a requirement to reconcile two datasets. So I have added 4 different test scenarios in functional test case. So at this point what should be added in regression test cases for this feature of reconciliation as all scenarios are covered in functional test case itself. Also by definition regression test case ensures existing code should not break by adding new feature. So after sprint2 if I add new test scenarios for new feature of sprint2 in functional test case itself. Then why I need regression test case(to ensure existing code works as expected)? Also am I right in saying functional test case of sprint1 will become regression test case for sprint2 to ensure existing functionality is not breaking in sprint2? Anyone can explain with example.