Function ColumnNumToColumnLetter(ColumnNum As Long) As String
'returns an excel column letter from a column number
'if the column letter cannot be determine returns vbNullString
    Dim regex As Object
    Dim matches As Object
    Dim addr As String
    Set regex = CreateObject("VBScript.RegExp")
    regex.Pattern = "[A-Z]+"
    addr = Cells(1, ColumnNum).Address(False, False)
    If regex.test(addr) Then
        Set matches = regex.Execute(addr)
        ColumnNumToColumnLetter = matches(0)
    Else
        ColumnNumToColumnLetter = vbNullString
    End If
    Set regex = Nothing
    Set matches = Nothing
End Function