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DTD Elements

Introduction to DTD

A Document Type Definition (DTD) is a document that specifies the legal building blocks for an XML file. The DTD defines the structure of an XML file and contains a list of elements and attributes that can make up the XML document.

Since XML allows users to structure, store, and transport data from all sorts of applications, it is important to ensure that the shared data is in a format that can be used and properly handled by all parties. The use of a DTD is important because it enables independent groups of people to agree to use a standard DTD for interchanging data. Also, applications can use a standard DTD to verify that data received from the outside world is valid.

The use of a DTD specifies, in effect, the syntax of an XML file. XML files that follow the syntax rules specified by a DTD are said to be valid.

DTD Elements

Recall, an XML document can contain any number of elements or tags. An XML element is everything from the element's start tag (<element>) to the element's end tag (</element>). An element can contain other elements, simple text or a mixture of both. Elements can also have attributes. In order to define the legal elements that an XML file can contain, the DTD ELEMENT declaration is used.

In a DTD, elements are declared using the following syntax:

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

Listing 2-1. DTD element declaration.

element-name refers to the name of the XML element and element-content refers to the contents of the element. The content can include text or additional tag elements. An example is shown below:

<?xml version="1.0"?>


Listing 2-2. DTD elements.

The XML file above includes an Employee element. The Employee element contains five child elements. Each child element contains text data (a specific first name, last name, salary, etc...). The DTD below is used to validate this document:

<!ELEMENT Employee (SSN,FirstName,LastName,Salary,Department)>
<!ELEMENT FirstName (#PCDATA)>
<!ELEMENT Department(#PCDATA)>

Listing 2-3. Employee child elements.

With root or parent elements (those elements that contain other elements or tags), element-content is specified by listing each of the child elements in the order in which they are nested inside of the parent element. element-content is defined as #PCDATA (parsed character data) when only text is found between the element start tag and element closing tag. #PCDATA text will be parsed by the parser.

When declaring child elements, it is also possible to specify the number of times the element can occur within the parent or root element. The "*" sign is used to declare that a child element can occur zero or more times inside of a parent element. The "?" sign declares that the child element can occur zero or one time inside of the parent element. The "+" sign states that the child element must occur one or more times inside of the parent element.

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