validate content and length with regexvalidator?

Hi All: I have a two-character field for US State. I add a RegularExpressionValidator control to validate the input using  
ValidationExpression="[a-zA-Z]{2}"which works to validate the content when something's entered, but it doesn't catch if someone just leaves the textbox blank.
Is there any way to validate within the same regex to catch an empty textbox?

 
 

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