The 'pattern' element specifes a regular expression for the lexical value.

The book has a whole section on patterns. Read it!!!

If you are familiar with regular expressions, then you'll feel at home.

The syntax is familiar to the java.util.regex package.

A US Zip+4 code:

<xs:simpleType name="ZipPlus4">
   <xs:restriction base="xs:string">
      <xs:pattern value="[0-9]{5}(-[0-9]{4})?"/>
   </xs:restriction>
</xs:simpleType>