Regression tests are deterministic white box tests written to ensure that the software doesn't exhibit unwanted behavior in the future.
By that definition, regression test can be realized as acceptance, confirmation, sanity or as an A/B test.
While some can be written as unit tests, most regression tests will be system tests.
Given that you find a bug during manual testing of your game.
You should write a confirmation test first to ensure that you can reproduce the observed bug.
Once you can reliably reproduce it, you can move on to fix the bug.
Now the confirmation tests that you have written prior becomes a regression test that prevents you from reintroducing the given bug.