<xs:simpleType name ="tPrefix">
<xs:restriction base ="xs:normalizedString">
<xs:maxLength value ="11"/>
<xs:pattern value ="[A-Z,a-z][0-9,A-Z,a-z,_]*"/>
<xs:pattern value =""/>
</xs:restriction>
</xs:simpleType>
The restriction <xs:pattern value ="[A-Z,a-z][0-9,A-Z,a-z,_]*"/> already allows empty string as value of Prefix, thus the following <xs:pattern value =""/> is redundant.