Check an Input with jQuery. Contains()

I want to validate a form with jQuery. So I need to check if an input field ("#validatename") does not contain few words, like 'United States of America', 'United Kingdom' or 'France' for instance.

To forbidden 'United States of America" (I'll see later for an array with all forbidden strings!), I have...

$("#validatename").keyup(function()
{
 var name = $("#validatename").val();
 if(name != '')
 {
   if ($('#validatename:contains("United States of America")').length > 0) // 
   {
    $("#validatename").css({
    "background-image": "url('no.png')"
    });
   }
   else
   {
    $("#validlastname").css({
    "background-image": "url('yes.png')"
    });
   }
 }
}

... But it doesn't work: yes.png is always displaying!

Any help or idea will be greatly appreciated!

Thanks a lot.