Chapter 2: Writing Document Type Definitions

Summary

• Defines the tags and attributes allowed in your XML document.
• Specifies how these tags are to be used (what order, in what context, etc.
• An XML document that conforms to a DTD is a valid document.
• DTDs can be stored within the XML document or in external files.

• Define the tags and what they can contain. Syntax:
  

  <!ELEMENT element-name (content-model)>

• Content-models can use these symbols:

  ,	Strict order
  ?	Optional element
  +	One or more occurrences
  *	Zero or more occurrences
  |	Or
  ( )	Grouping

• Ordinary text is called PCDATA.
• Elements can by EMPTY.
• Elements can contain other elements.
• Elements can contain ANY combination of other characters or PCDATA.

• Declares the attributes for a particular element. Syntax:

  <!ATTLIST element-name attribute-name attribute-type keyword|default>

• Attribute types:

  CDATA	Character data
  ID	Unique identifier
  IDREF	Reference to an existing ID
  NMTOKEN	Named token
  ENTITY	Entity
  enummerated values	List of predefined values

• Optional keywords:

  #REQUIRED	Attribute is required
  #IMPLIED	Attribute is optional
  #FIXED	Attribute has a fixed value
  default	Default value

• General entities expand text in the XML document:

  <!ENTITY entity-name "text to expand">

• System entities declare external entities:

  <!ENTITY entity-name SYSTEM "url">

• Parameter entities are referenced within the DTD itself:

  <!ENTITY % entity-name "content">