mask credit card by regular expressions (regexp) in (Regex.Replace/vb.net)

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim regFind As String = "<AccountNumber>(?'f'\d?)\d\d\d\d\d\d\d\d\d\d(?'e'.*?)</AccountNumber>"
        Dim regReplace As String = "<AccountNumber>${f}**${e}</AccountNumber>"
        TextBox1.Text = Regex.Replace(TextBox1.Text, regFind, regReplace)
End Sub

dot.net 3.5 from visual studio 2008