The anyURI data type represents a URI reference : anyURI « XML Schema « XML Tutorial

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<!-- schema -->
<xsd:element name="address">
  <xsd:simpleType>
   <xsd:restriction base="xsd:anyURI">
    <xsd:enumeration value=
    "http://www.w3.org/TR/2000/PR-xlink-20001220/"/>
    <xsd:enumeration value=
    "http://www.w3.org/TR/xmlschema-2/"/>
   </xsd:restriction>
  </xsd:simpleType>
</xsd:element>

<!-- instance document -->
<address>http://www.w3.org/TR/xmlschema-2/</address>