TextBox should only allow numbers and one DOT (.) in Silverlight

It has nothing to do with Regular Expression. You need to handle the event System.Windows.UIElement.TextInput. This event can be canceled, which is done by assigned true to the property System.Windows.RoutedEventArgs.Handled (runtime type will be different, see below) passed to your event handler. You will need to check up the value of the property System.Windows.TextCompositionEventArgs.Text, also passed in the same event arguments object and see if it is good for input or not. If not, assign true to Handled, to ignore the change. This text is supposed to be just one character the user tries to type.

Please see:
http://msdn.microsoft.com/en-us/library/system.windows.uielement.textinput%28v=vs.110%29.aspx[^],
http://msdn.microsoft.com/en-us/library/system.windows.input.textcompositioneventhandler.aspx[^],
http://msdn.microsoft.com/en-us/library/system.windows.input.textcompositioneventargs.aspx[^],
http://msdn.microsoft.com/en-us/library/system.windows.routedeventargs.handled.aspx[^],
http://msdn.microsoft.com/en-us/library/system.windows.input.textcompositioneventargs.text.aspx[^].

Something like:

myTextBox.TextInput += (sender, eventArgs) => {
   bool isGoodInput = CheckIfYouWantThisInput(eventArgs.Text); // don't forget to allow #8 (backspace) which is also considered as text, by some historical reasons
   eventArgs.Handled = !isGoodInput; // ignore then
};

—SA