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XML Document Type Declaration (DTD)

The XML Document Type Declaration (DTD) is used to formally describe the grammar of XML documents. In this way, a XML document can be a valid XML document.

What is a DTD and what is it used for?​

DTD stands for Document Type Definition and it defines the structure, the legal elements and attributes of an XML document.

The DTD aims to define the structure of XML documents in order to validate them.

Should I use a DTD?​

It depends on what you want to do:

  • YES, if you want to verify that the data is valid against the DTD. For example, you can verify that the data you received from a server is valid.

  • NO, if you are experimenting with XML or you have small XML files and you don't have time to create DTDs.

Well-formed and Valid XML Documents​

  • An XML document is called well-formed if its syntax is correct.
  • An XML document is called valid if it is well-formed and it is validated against a DTD.

Example of XML document with External Document Type Declaration​

The following example is a well-formed and valid XML document with book.dtd DTD

book.xml
<?xml version="1.0"?>  
<!DOCTYPE book SYSTEM "book.dtd">  
<book>  
	<title>A Great Book</title>  
	<author>Tom Nolan</author>  
	<publisher>Tutorial Reference</publisher>  
</book>
note

The DOCTYPE declaration above contains a reference to a DTD file.

The book.dtd is the following:

book.dtd
<!DOCTYPE book [  
	<!ELEMENT book (title,author,publisher)>
	<!ELEMENT title(#PCDATA)>
	<!ELEMENT author (#PCDATA)>
	<!ELEMENT publisher(#PCDATA)>
]>

and it is interpreted in this way:

  • !DOCTYPE book defines that the root element of the document is book
  • !ELEMENT book defines that the note element must contain the elements: title, author, publisher
  • !ELEMENT title defines the title element to be of type #PCDATA
  • !ELEMENT author defines the author element to be of type #PCDATA
  • !ELEMENT publisher defines the publisher element to be of type #PCDATA
note

#PCDATA means "parseable character data".

Example of XML document with Internal Document Type Declaration​

<?xml version="1.0"?>  
<!DOCTYPE book [  
	<!ELEMENT book (title,author,publisher)>
	<!ELEMENT title(#PCDATA)>
	<!ELEMENT author (#PCDATA)>
	<!ELEMENT publisher(#PCDATA)>
]>

<book>  
	<title>A Great Book</title>  
	<author>Tom Nolan</author>  
	<publisher>Tutorial Reference</publisher>  
</book>

XML DTD with Entity Declaration​

A doctype declaration can also define special strings that can be used in the XML file.

An entity has three parts:

  1. An ampersand (&)
  2. An entity name
  3. A semicolon (;)
<!ENTITY entity-name "entity-value">

Example with ENTITY in DOCTYPE:

<?xml version="1.0" standalone="yes" ?>  
<!DOCTYPE author [  
  <!ELEMENT author (#PCDATA)>  
  <!ENTITY me "Tom">  
]>  
<author>&me;</author>

where me is an entity used inside the author element. In this example, it will print the value of me entity that is "Tom"

note

Learn more about Document Type Declaration in our XML DTD Tutorial.

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