need regex that validates 4-digits and empty value

Hi,
I can validate textbox field by RegularExpressionValidator with regex "^([0-9]{3,3})$".  For the required field validation I put requiredfieldvalidator. Thus, I have two validation controls. 
Can I combine them into a sigle RegularExpressionValidator? Then what does regex look like? How can I validate empty fields with RegularExpressionValidator?
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