XML Reference Guide

Mar 14, 2003

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Because XML doesn't require any particular elements or attributes, developers often need a way to make sure that XML documents contain the content that they expect. XML incorporates the concept of validation, in which a document is compared to a supplied grammar to make sure that its structure is correct. A document that has been successfully validated is known as a valid document. (By contrast, a well-formed document can contain any content, as long as it follows the rules of XML.)

Several validation methods exist, but the most common are Document Type Definitions (DTDs) and XML Schemas.

Document Type Definitions were specified in the XML 1.0 Recommendation itself, and come from XML's ancestor, SGML. DTDs specify the elements that can appear within a document and the attributes and content that they can contain. They can also appear directly within the document, though they often don't. For example, a sample document with an internal DTD subset might look like this:

<?xml version="1.0"?>
<!DOCTYPE order [
  <!ELEMENT order (lineitem+)>
  <!ATTLIST order orderid ID #REQUIRED
          customerNumber CDATA #REQUIRED>
  <!ELEMENT lineitem (quantity, unitprice)>
  <!ATTLIST lineitem itemid CDATA #REQUIRED>
  <!ELEMENT quantity (#PCDATA)>
  <!ELEMENT unitprice (#PCDATA)>
  <!ATTLIST unitprice currency (dollars | pounds) 'dollars'>
]>
<order orderid="THX1138" customerNumber="3263827">
  <lineitem itemid="C33">
    <quantity>36</quantity>
    <unitprice currency="dollars">.35</unitprice>
  </lineitem>
  <lineitem itemid="M48">
    <quantity>1</quantity>
    <unitprice currency="dollars">2200</unitprice>
  </lineitem>
</order>

In this case, the order element has two required attributes, and may contain zero or more lineitem elements as content. The lineitem has one required attribute and quantity and unitprice children, in that order. The quantity and unitprice elements can contain only PCDATA, or parsed character data.

Validation information can also come in the form of XML Schemas, which in many cases is more familiar because it uses XML format rather than the peculiarities of DTDs. For example, a schema that describes the above document might look like:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
 
 <xs:element name="order">
  <xs:complexType>
   <xs:sequence>
     <xs:element ref="lineitem" minOccurs="0"
maxOccurs="unbounded"/>
   </xs:sequence>
   <xs:attribute name="orderid" type="xs:string" use="required"/>
   <xs:attribute name="customerNumber" type="xs:string"
use="required"/>
  </xs:complexType>
 </xs:element>
 
 <xs:element name="lineitem">
  <xs:complexType>
   <xs:sequence>
     <xs:element name="quantity" type="xs:string"/>
     <xs:element name="unitprice" type="priceType"/>
   </xs:sequence>
   <xs:attribute name="itemid" type="xs:string" use="required"/>
  </xs:complexType>
 </xs:element>
 
 <xs:complexType type="priceType">
  <xs:simpleContent>
   <xs:extension base="xs:string">
    <xs:attribute name="currency">
     <xs:simpleType>
      <xs:restriction base = "xs:string">
       <xs:enumeration value="dollars"/>
       <xs:enumeration value="pounds"/>
      </xs:restriction>
     </xs:simpleType>
    </xs:attribute>
   </xs:extension>
  </xs:simpleContent>
 </xs:complexType>
 
</xs:schema>

Not all parsers validate documents. Those that do are called validating parsers.

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