If you don't understand regexpr, like many of us, look at the new Match Pattern functions on this page. They start about halfway down.
I would look at the extractafter function for your task.
folder = "\skurk.IntUser\ABCD_DeutS_1v1.IntPrj\Netzmodell.IntPrjfolder\Netzdaten.IntPrjfolder\320_HERTZA.ElmNet\HERTZA NETZ\Österreich\D8D81 GRD00.ElmSite\Trafo101.ElmTr3";
str = extractAfter(folder,"Netzdaten.IntPrjfolder\")