Validating attribute uniqueness with XSD across XML document

Question

I'm trying to validate the uniquess of an attribute across all elements that exist in an XML document.

Example XML:

<exampleXml>
  <a id="1"/>
  <a id="2">
    <b id="1"/>
  </a>
</exampleXml>

My XSD schema:

<xs:schema elementFormDefault="qualified">
  <xs:element name="exampleXml">
    <xs:complexType>
      <xs:sequence>
        <xs:element minOccurs="0" maxOccurs="unbounded" name="a">
          <xs:complexType>
            <xs:complexContent>
              <xs:extension base="baseRuleType">
                <xs:sequence>
                  <xs:element minOccurs="0" maxOccurs="1" name="b">
                    <xs:complexType>
                      <xs:complexContent>
                        <xs:extension base="baseRuleType"/>
                      </xs:complexContent>
                    </xs:complexType>
                  </xs:element>
                </xs:sequence>
              </xs:extension>
            </xs:complexContent>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
    <xs:unique name="duplicateIdsForbidden">
      <xs:selector xpath="//"/>
      <xs:field xpath="@id"/>
    </xs:unique>
  </xs:element>
  <xs:complexType name="baseRuleType">
    <xs:attribute name="id" use="optional"/>
  </xs:complexType>
</xs:schema>

The xpath is the issue here. I want to match every element under root but the selector xpath above returns:

Element '{http://www.w3.org/2001/XMLSchema}selector', attribute 'xpath': 
The XPath expression '//' could not be compiled

I can change the xpath to "*" but that will only validate the id attribute on the elements that are direct decendants of the root.

I am validating this with lib_xml in PHP using DOMDocument::schemaValidate(). Any help greatly appreciated.

Answer 1

Use <xs:selector xpath=".//*"/>.