According to some definitions :
When a bug is fixed, the tester regress the bug to make sure if it is fixed and if it doesn't make an effect on other modules.
Now, if the above definition is right, then it means that against each and every bug, we have to test the whole application. In other words, we are testing the application as well.
So what is it that we say that we are regressing the bug and not testing the application.