It is not about confidence. It is about ownership and being a good engineer. A good engineer would have cared about the code base and ensured that a test would fail, a use case would be linked to that failure, and someone would see that they have now violated a regulation.

Then if the regulation changed, someone would grep through the use cases, and see what behavior is occuring/ how to modify the code base to cope with the change in regulations (this occurs a lot). Of course most finance firms don't do this and actually violate regulations all the time. There is literally a gold mine for the SEC/FINRA to go after, but the government is fairly incompetent at mining through the data and holding companies accountable. The reason being that they don't pay enough to hire the talent that would uncover the violations.