match regex in MaskedTextBox with empty string

Hi,

I'm using a MaskedTextBox to validate an IP Address and would also like it to match when the MaskedTextBox is empty.

Here is my XAML:

<Editors:MaskedTextBox x:Name="ipAddressTextBox"
         Grid.Row="1"
         Width="100"
         Height="24"
         Mask="^(\s*|[0-9]{0,3}[.][0-9]{0,3}[.][0-9]{0,3}[.][0-9]{0,3})$"
         MaskType="Regex"
         Text="{Binding Path=IpAddressComparableValue,
             RelativeSource={RelativeSource TemplatedParent},
             UpdateSourceTrigger=LostFocus,
             ValidatesOnDataErrors=True,
             Converter={StaticResource ipAddressComparableTextConverter}}"
         ToolTip="{x:Static Properties:Resources.IpAddressTextBoxToolTip}" />

The regex that I'm using should work but the behavior in the MaskedTextBox isn't what I hoped for.  Is there a way to match the IP Address regex and allow the text box to be empty also?

Thanks,

Josh