8.2 Document Type Definitions

An XML document that obeys the rules of the previous section is described as well-formed.

It is also possible to specify additional rules for the structure and content of an XML document, via a schema for the document. If the document is well-formed and also obeys the rules given in a schema, then the document is described as valid.

The Document Type Definition language (DTD) is a language for describing the schema for an XML document. DTD code consists of element declarations and attribute declarations.

8.2.1 Element declarations

An element declaration should be included for every different type of element that will occur in an XML document. Each declaration describes what content is allowed inside a particular element. An element declaration is of the form:

<!ELEMENT elementName elementContents>

The elementContents can be one of the following:

EMPTY

 
The element is empty.

(#PCDATA)

 
The element may contain plain text.

ANY

 
The element may contain anything (other elements, plain text, or both).

(childName)

 
The element must contain exactly one childName element.

(childName*)

 
The element may contain zero or more childName elements.

(childName+)

 
The element must contain one or more childName elements.

(childName?)

 
The element must contain zero or one childName elements.

(childName?)

 
The element must contain zero or one childName elements.

(childA,childB)

 
The element must contain exactly one childA element and exactly one childB element.

(childA|childB)

 
The element must contain either exactly one childA element or exactly one childB element.

(#PCDATA|childA|childB)*

 
The element may contain zero or more occurences of plain text, childA elements and childB elements.

8.2.2 Attribute declarations

An attribute declaration should be included for every different type of element that can have attributes. The declaration describes which attributes an element may have, what sort of values the attribute may take, and whether the attribute is optional. An attribute declaration is of the form:

<!ATTLIST elementName
    attrName attrType attrDefault
    ...
>

The attrType controls what value the attribute can have. It can have one of the following forms:

CDATA

 
The attribute can take any value.

ID

 
The value of this attribute must be unique for all elements of this type in the document (i.e., a unique identifier). This is similar to a primary key in a database table.

IDREF

 
The value of this attribute must be the value of some other element's ID attribute. This is similar to a foreign key in a database table.

(option1|option2)

 
This provides a list of the possible values for the attribute. This is a good way to limit an attribute to only valid values (e.g., only "male" or "female" for a gender attribute).

The attrDefault either provides a default value for the attribute or states whether the attribute is optional or required (i.e., must be specified). It can have one of the following forms:

value

 
This is the default value for the attribute.

#IMPLIED

 
The attribute is optional. It is valid for elements of this type to contain this attribute, but it is not required.

#REQUIRED

 
The attribute is required so it must appear in all elements of this type.

8.2.3 Including a DTD

A DTD can be included directly within an XML document or the DTD can be located within a separate file and just referred to from the XML document.

The DTD information is included within a DOCTYPE definition following the XML declaration. An inline DTD has the form:

<!DOCTYPE rootElementName [
    ... DTDcode ...
]>

An external DTD stored in a file called file.dtd would be referred to as follows:

<!DOCTYPE rootElementName SYSTEM "file.dtd">

Paul Murrell

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