JS 

bool K_BACK = false;
...
void __fastcall TForm1::Edit1Change(TObject *Sender)
{
  int l = Edit1->Text.Length();
  if (l ==  0  || K_BACK ) return;

  if (l ==  2  || l ==  5  )
  {
    Edit1->Text = Edit1->Text+".";
    Edit1->SelStart = l+ 1 ;
  }
  if (l !=  3  && l !=  6  )
  {
    if (!TryStrToInt(Edit1->Text[l],NULL))
    {
      ShowMessage("Err");
    }
  }
}
// --------------------------------------------------------------------------- 
void __fastcall TForm1::Edit1KeyDown(TObject *Sender, WORD &Key,
      TShiftState Shift)
{
 if (Key ==VK_BACK){K_BACK = true; } else { K_BACK= false; }

}

 JS