DTD - Elements

In a DTD, XML elements are declared with a DTD element declaration.

Declaring an Element

In the DTD, XML elements are declared with an element declaration. An element declaration has the following syntax:

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

Empty elements

Empty elements are declared with the category keyword EMPTY:

<!ELEMENT element-name EMPTY>

example:

<!ELEMENT br EMPTY>

XML example:

<br />

Elements with only character data

Elements with only character data are declared with #PCDATA inside parentheses:

<!ELEMENT element-name (#PCDATA)>

example:

<!ELEMENT from (#PCDATA)>

Elements with any contents

Elements declared with the category keyword ANY, can contain any combination of parsable data:

<!ELEMENT element-name ANY>

example:

<!ELEMENT note ANY>

Elements with children (sequences)

Elements with one or more children are defined with the name of the children elements inside parentheses:

<!ELEMENT element-name 
 (child-element-name)>
or
<!ELEMENT element-name 
 (child-element-name,child-element-name,.....)>

example:

<!ELEMENT note (to,from,heading,body)>

When children are declared in a sequence separated by commas, the children must appear in the same sequence in the document. In a full declaration, the children must also be declared, and the children can also have children. The full declaration of the note document will be:

<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to      (#PCDATA)>
<!ELEMENT from    (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body    (#PCDATA)>

Declaring only one occurrence of the same element 

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

example:

<!ELEMENT note (message)>

The example declaration above declares that the child element message can only occur one time inside the note element.

Declaring minimum one occurrence of the same element

<!ELEMENT element-name (child-name+)>

example:

<!ELEMENT note (message+)>

The + sign in the example above declares that the child element message must occur one or more times inside the note element.

Declaring zero or more occurrences of the same element 

<!ELEMENT element-name (child-name*)>

example:

<!ELEMENT note (message*)>

The * sign in the example above declares that the child element message can occur zero or more times inside the note element.

Declaring zero or one occurrences of the same element 

<!ELEMENT element-name (child-name?)>

example:

<!ELEMENT note (message?)>

The ? sign in the example above declares that the child element message can occur zero or one times inside the note element.

Declaring either/or content

example:

<!ELEMENT note (to,from,header,(message|body))>

The example above declares that the element note must contain a to element, a from element, a header element, and either a message element or a body element.

Declaring mixed content

example:

<!ELEMENT note (#PCDATA|to|from|header|message)*>

The example above declares that the element note can contain any number of elements of parsed character data, to, from, header, and/or message elements.

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