XSD Date or no date validation

Question

The following seems to work for MM-YYYY format but I now have a case where it can either be blank or have a date. Is this possible or should I push to only include the attribute in the XML if there is a date and make the attribute optional?

<xs:attribute name="edition_date" use="required">  
<xs:simpleType>  
   <xs:restriction base="AT_STR">  
     <xs:length value="7"/>   
     <xs:pattern value="(0[1-9]|1[012])[-](19|20)\d\d"/>   
   </xs:restriction>   
</xs:simpleType>   

Answer 1

Only include the attribute in the XML if there is a date and make the attribute optional

Seems like the right option to me

Answer 2

Making the attribute optional, and only including the attribute in the XML if there is a date is a good option, but in some cases you may not have control over the generation of the XML. In this case, you could try the following pattern in the XSD

<xs:simpleType name="edition_date">   
   <xs:restriction base="xs:string">   
      <xs:maxLength value="7"/>
      <xs:pattern value="((0[1-9]|1[012])[-](19|20)\d\d)?"/>    
   </xs:restriction>    
</xs:simpleType>    

This is a similar pattern to yours, but the use of the ? operator indicates the pattern can occur zero or one times, and so will allow an empty string. Also note the use of xs:maxLength instead of xs:length. Not that is bit is needed, because the pattern only allows a fixed length anyway.