function ReplaceSub(str, sub1, sub2: string): string;
var
  aPos: Integer;
  rslt: string;
begin
  aPos := Pos(sub1, str);
  rslt := '';
  while (aPos <> 0) do
  begin
    rslt := rslt + Copy(str, 1, aPos - 1) + sub2;
    Delete(str, 1, aPos + Length(sub1) - 1);
    aPos := Pos(sub1, str);
  end;
  Result := rslt + str;
end;