Public Function CleanUpZahl(ByVal zahl As String, ByVal chars As String) As _
      String
        Dim l As Integer = zahl.Length
        Dim i As Integer
        Do
            If chars.IndexOf(zahl.Substring(i, 1)) > -1 Then
                zahl = zahl.Remove(i, 1)
                l -= 1
            Else
                i += 1
            End If
        Loop While i < l
        Return zahl
    End Function

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles MyBase.Load
        Dim zahl As String = "<123+45()+78""98"
        Dim chars As String = "+-<>*()/"""
 
        Dim cleanZahl As String = CleanUpZahl(zahl, chars)
        Debug.WriteLine(cleanZahl)
    End Sub