Element not containing 'blank' text value

How do I specify in the schema that an Element must have a text value which is not blankL or Empty?

I have looked at the Content attribute, but it won't help me much.


      textOnly
     The element can contain text but no sub elements.
     
      eltOnly
     Element can contain sub elements only.
     
      empty
     Text and sub-elements are not allowed.
     
      mixed
     Both text and sub-elements are allowed.
     



Regards, -stig henning