Function zeige_inputbox(Optional ByVal text As String = "") As String
 
start_inputbox:
 
        Dim sonderzeichen As Boolean
 
        text = InputBox("Texteingabe", , text)
 
        For i As Integer = 0 To text.Length - 1
 
            If _
            (Asc(text.Substring(i, 1)) >= Asc("0") And Asc(text.Substring(i, _
            1)) <= Asc("9")) Or _
            (Asc(text.Substring(i, 1)) >= Asc("A") And Asc(text.Substring(i, _
            1)) <= Asc("Z")) Or _
            (Asc(text.Substring(i, 1)) >= Asc("a") And Asc(text.Substring(i, _
            1)) <= Asc("z")) Then
                sonderzeichen = False
            Else
                sonderzeichen = True
                Exit For
            End If
        Next
 
        If sonderzeichen Then
            MsgBox("In der Texteingabe sind keine Sonderzeichen zugelassen!")
            GoTo start_inputbox
        End If
 
        Return text
 
    End Function