what is it you want to do, and how are you trying to do it?
is the format of the invoice numbers consistent, ie 3 letters and 3 numbers?
this quick formula will give you a flag
Note that it doesn't properly test the alpha segment, ie WHS and WH2 both evaluate as text in my quick test
Unless you have a consistent naming convention you may need to test each character
and string length may be another test...