DTD

A DTD (Document Type Definition) defines the elements or record structure of a XML document. A DTD allows your XML files to carry a description of its format with it.

Example DTD for a FAQ database:

<?xml version="1.0"?>

<!ELEMENT faq-list (question+)>
   <!ELEMENT question (query, response*)>
      <!ELEMENT query    (#PCDATA)>
      <!ELEMENT response (#PCDATA)>

Notes:

• #PCDATA (parsed character data) means that the element contains data that can be parsed by a parser like HTML
• The + sign in the example above declares that the "QUESTION" element must occur one or more times inside the "FAQ-LIST" element.
• The * sign in the example above declares that the "QUERY" element can occur zero or more times inside the "QUESTION" element.

The W3C formulated a new standard, called XML Schemas that superceded DTD's. Schemas allow for more complex data types within your tags and better ways to constrain (validate) data within these tags.

Also see

• XML
• XSL

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