procedure TForm1.Button1Click(Sender: TObject);
var
 i:integer;
 s:string;
begin
 for i:=1 to length(edit1.text) do
  begin
    if pos(edit1.text[i],'1234567890')<>0 then
     begin
       s:=s+edit1.text[i];
     end;
  end;

 edit1.text:=s;
end;