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Document Type Definition (DTD) Tutorial

This tutorial covers the basics of DTDs. Before reading this tutorial you should already be familiar with XML. You may want to read my XML tutorial. Click on the above XML link to do so.

DTD stands for Document Type Definition.

XML lets applications share data easily. However, XML lets you make up your own set of tags. A DTD defines the legal elements and their structure in an XML document.

Independent developers can agree to use a common DTD for exchanging XML data. Your application can use this agreed upon DTD to verify the data it receives. The DTD can also be used to verify your own data.

DTD standards are defined by the World Wide Web Consortium (W3C). The W3C site provides a comprehensive reference of DTDs.

However, this tutorial focuses on Microsoft's implementation of the XML and DTDs. All examples contained herein require Internet Explorer 5.0 or later.

DTDs can be declared inline in your XML code or they can reference an external file.

This is an example of an internal DTD. You can open it in Explorer then select View | Source to view the complete XML document with the DTD included.

Line 2 defines the <message> element as having the four child elements: <to>, <from>, <subject> and <text>.

This line defines the <to> element to be of type PCDATA.

Here is the same document with an external DTD reference:

And here is the referenced DTD file. You can open it in Explorer and select View | Source as above.

Before talking further about DTDs, we should review some of the basic XML components.

• XML elements
  An XML element is made up of a start and end tag with data in between. The tags describe the data. The data is called the value of the element. Elements can have a value, child elements or they can be empty (have no value).

  For example, this XML element is a <director> element with the value "Bill Smith".

  <director>Bill Smith</director>

• Attributes
  An element can optionally contain one or more attributes in its start tag. An attribute is a name-value pair separated by an equal sign (=). Attribute values must always be quoted.

  <CITY ZIP="01085">Westfield</CITY>

  ZIP="01085" is an attribute of the <CITY> element.
  
  

• Parsed character data - PCDATA
  Like we said, elements have values. If a value has tags representing child elements, these tags need to be expanded or parsed and handled as separate elements.
  
  
• Character data - CDATA
  The value of an element is treated as a single item and is not expanded.
  
  
• Entities
  Certain characters, like "<", have special meaning in XML. If you want to use these special characters in your data you need a way to tell the XML parser not to interpret them as having their normally meaning.

  Entities are sets of characters that can be used to represent these special characters or other text.

  Entity	Special Character
  &lt;	<
  &gt;	>
  &amp;	&
  &quot;	"
  &apos;	'

  To use "<" in your data, you would use the entity "&lt;" instead.

In DTD, elements are declared using an element declaration with the following syntax:

• Empty elements
  Empty elements use the empty keyword. Move the mouse over the text for additional information.

  <!ELEMENT img (EMPTY)>

  
  
  
• Elements with data
  If an element has data, you must specify the type of its data.

  <!ELEMENT element-name (#CDATA)> <!ELEMENT element-name (#PCDATA)> <!ELEMENT element-name (ANY)> Example: <!ELEMENT message (#PCDATA)>

  ANY can contain any type of data.
  #CDATA is character type data.
  #PCDATA is character data that must be parsed and expanded. If a #PCDATA section contains elements, those elements must also be declared.

  
  
  
• Elements with children - Sequences
  If an element has child elements, the child elements must be enumerated in the same order that they appear in the document.

  Using the message example:

  <!ELEMENT element-name (child-element,child-element,...)> Example: <!ELEMENT message (to,from,subject,text)>

  The child elements must also be declared. Here are their declarations:

  <!ELEMENT message (to,from,subject,text)> <!ELEMENT to (#CDATA)> <!ELEMENT from (#CDATA)> <!ELEMENT subject (#CDATA)> <!ELEMENT text (#CDATA)>

  
  
  
• DOCTYPE definition
  With an internal DTD, you need a DOCTYPE definition to indicate the DTD code.

  <!DOCTYPE root-element [element declarations]> Example: <?xml version="1.0"?> <!DOCTYPE message [   <!ELEMENT message (to,from,subject,text)>   <!ELEMENT to         (#CDATA)>   <!ELEMENT from      (#CDATA)>   <!ELEMENT subject (#CDATA)>   <!ELEMENT text     (#CDATA)> ]> <message>   <to>Dave</to>   <from>Susan</from>   <subject>Reminder</subject>   <text>Don't forget to buy milk on the way home.</text> </message>

  
  
  
• Element instances
  You can specify how many times an element can occur in a document.

  Symbol	Instances
  none	must occur exactly 1 time
  *	0 or more times
  +	1 or more times
  ?	Exactly 0 or 1 time

  For example:

  <!Element message (to+,from,subject?,text,#PCDATA)>

In DTD, attributes are declared using an ATTLIST declaration. An attribute declaration specifies the associated element, the attribute, its type, and possibly its default value. Here are the syntax variations:

• Attribute type
  Attributes can have these types:

  Type	Description
  CDATA	Character data
  ENTITY	An entity
  ENTITIES	List of entities data
  ID	Unique ID data
  IDREF	ID of another element
  IDREFS	List of IDs of other elements
  NMTOKEN	XML name data
  NMTOKENS	List of XML names
  NOTATION	Name of a notation
  (val1|val2|...)	List of values
  xml:	Predefined value

  
  
  
• Default values
  Attribute default values can be:

  Default Value	Description
  #DEFAULT value	If no value exists in the XML data, the value specified in the DTD will be used
  #FIXED value	If another value exists in the XML data, an error will occur
  #IMPLIED	The value doesn't have to be supplied in the XML data.
  #REQUIRED	If the XML data doesn't have a value, an error will occur.

  
  
  

• DTD Examples
  Here are some sample DTD statements. Move the mouse over the text for more information.

  <!ATTLIST person gender CDATA #DEFAULT "male"> <!ATTLIST person gender CDATA #FIXED "male"> <!ATTLIST person gender CDATA #REQUIRED> <!ATTLIST person gender CDATA #IMPLIED> <!ATTLIST person gender (male|female) "male">

  Here is an XML statement that will satisfy all of the above DTD statements.

  <person gender="male">

  This XML statement does not satisfy DTD rule 2 which requires a value of "male".

  <person gender="female">

  This XML statement fails DTD rule 5 because "unknown" is not an acceptable value.

  <person gender="unknown">

Recall, entities are variables that represent other values. The value of the entity is substituted for the entity when the XML document is parsed.

Entities can be defined internally or externally to your DTD.

The above entity make this line of XML valid.

Click this link to view a complete DTD example.