Stuff

procedure TForm1.btnWriteClick(Sender: TObject);
const
    verif: set of char =
    ['0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F','-'];
var
    i: integer;
    flag: boolean;
begin
try
    flag:= true;
    myReg := TRegistry.Create;
    myReg.RootKey := HKEY_LOCAL_MACHINE;
    if myReg.OpenKey('SYSTEM\ControlSet001\Control\Class\{4D36E972-E325-11CE-BFC1-08002bE10318}\0001', TRUE) then
    for i:=1 to length(txtName.Text) do
       if not (txtName.Text[i] in verif) then flag:=false;
    if not flag then
        ShowMessage('Error: invalid MAC!')
    else
    begin
    myReg.WriteString('NetworkAddress', txtEdit.Text);
end;
    finally
    myReg.Free;
    end;
end;