The sample schema defines constraints for the content of two elements: The content of a <state> element must be two characters long, and the content of a <postal-code> element must match the regular expression [0-9]{5}(-[0-9]{4})?. Here's how to do that:

  <xsd:element name="state">
    <xsd:simpleType>
      <xsd:restriction base="xsd:string">
        <xsd:length value="2"/>
      </xsd:restriction>
    </xsd:simpleType>
  </xsd:element>

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

For the <state> and <postal-code> elements, the schema defines new data types with restrictions. The first case uses the <xsd:length> element, and the second uses the <xsd:pattern> element to define a regular expression that this element must match.

This summary only scratches the surface of what XML schemas can do; there are entire books written on the subject. For the purpose of this introduction, suffice to say that XML schemas are a very powerful and flexible way to describe what a valid XML document looks like.