Those were all very helpfull hints thank you. I'm going to mark my answer as the correct as this is what led to the solution;
=subStringCount(concat(Expr_ID_m,'|'),'A') and (GetSelectedCount(_expr_m)> 0)
So 2 things:
getselectedcount was for the actual selection, which was done on the field _expr_m
and it has to pass the test >0.
Sean has made a good point: You don't need to test against > 0.
Sorry I can't mark it usefull after the question is marked as answered but that is a very good point.