CanValueBeNull with RegexPattern not working(?)

[XavrasX]

Hi.
I've tried to bind decimal? to DecimalBox, but with any RegexPattern there is validation error. While there's no pattern removig value from decimalbox don't causes errors. I tried different patterns and didn't find solution.
My example:

 xmlns:numeric="http://gu.se/NumericInput"

 <numeric:DecimalBox
        NumberStyles="AllowDecimalPoint"
       CanValueBeNull="True"
       RegexPattern="(^$)|(^\d{4})"
 />