How to Restrict XML Attributes with Enumeration Facets

In XML Schema Definition (XSD), enumeration facets limit the acceptable values of an XML attribute by defining a strict, finite whitelist of valid strings. By encapsulating xs:enumeration elements within a xs:restriction base type, schema designers can prevent arbitrary text entry and enforce strict data consistency across XML documents. When an XML parser validates a document against the schema, any attribute value not explicitly declared in the enumeration list will trigger a validation error.

The Mechanism: Simple Types and Restrictions

XML attributes cannot contain child elements, so their values are always simple types. To constrain an attribute to a fixed list of strings:

1. A <xs:simpleType> element is defined (either anonymously within the attribute declaration or as a named global type).
2. Inside the simple type, a <xs:restriction> element sets the base data type to xs:string.
3. Multiple <xs:enumeration> elements are placed inside the restriction, each specifying one permitted value via its value attribute.

Example Schema Definition

The following schema defines a <task> element with a status attribute restricted to three values: pending, in-progress, and completed.

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="task">
    <xs:complexType>
      <xs:attribute name="status" use="required">
        <xs:simpleType>
          <xs:restriction base="xs:string">
            <xs:enumeration value="pending"/>
            <xs:enumeration value="in-progress"/>
            <xs:enumeration value="completed"/>
          </xs:restriction>
        </xs:simpleType>
      </xs:attribute>
    </xs:complexType>
  </xs:element>
</xs:schema>

Validation Behavior

During validation, the XML parser compares the string value inside the XML instance document against the defined enumeration list:

• Valid XML:

  <task status="in-progress"/>

  The value in-progress matches one of the defined xs:enumeration entries, so validation succeeds.

• Invalid XML:

  <task status="archived"/>

  The value archived is not present in the enumeration list, causing the validating parser to reject the document with a schema violation error.

Key Considerations

• Case Sensitivity: String enumerations in XSD are strictly case-sensitive. If an enumeration specifies pending, providing Pending or PENDING will cause a validation failure unless those variations are also explicitly declared as separate enumerations.
• Whitespace Handling: By default, base type xs:string preserves whitespace. When combined with enumerations, leading or trailing whitespace that does not match the exact enumeration value will cause validation to fail.
• Reusability: Defining the enumeration as a named global <xs:simpleType> allows the same fixed list to be reused across multiple attributes and elements throughout the schema.