no subject spam

I have been getting a lot of messages where there is nothing in the subject and the only thing in the body is something like

http://wwww.somewhere.com

so I came up with this rule

ifnot subject has "a,e,i,o,u,1,2,3,4" and body matches "*http://*" weight 51 tag "empty subject link"

but it is catching a lot of messages maybe all that obviously have something in the subject. Does anyone know where I am going wrong.

Thanks,

Jim