Private Sub Text1_KeyPress(KeyAscii As Integer)
  Select Case KeyAscii
    Case 48 To 57, 44   ' 44 ist der ASCII-Code für Komma 
       If Instr(Text1.Text,Chr(44)) > 0 And InstrRev(Text1.Text,Chr(44)) <> 0 _
         Then
           If Instr(Text1.Text,Chr(44))<> InstrRev(Text1.Text, Chr(44)) Then _
           KeyAscii=0 
       End If  
    Case Else
      KeyAscii = 0
  End Select
End Sub