Match every word except those listed

My problem is that I need to match if the string contains any words other than the list (s) I am listing.

For example, I might have this approved list:

User1
User2

      

        
        
        
      

    

Here are two examples of what should and shouldn't.

Must match (because User3 is not approved):

User1
User2
User3

      

        
        
        
      

    

Should not be matched (because every string on the approved list):

User1

      

        
        
        
      

    

I've tried search assertions, but they don't actually consume letters as they are trying to match, so with a string like "User1\r\nUser2"

I get matches like "ser1\r\n"

. I want to know if there are other words besides what I think are valid.

I cannot use a programming language for this; I am allowed to pass a regular expression to the program. The language will be Perl.