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In a DTD, elements are declared with an ELEMENT declaration.
Declaring Elements
In a DTD, XML elements are declared with an element declaration with the following syntax:
<!ELEMENT element-name category>
or
<!ELEMENT element-name (element-content)>
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 />
Example:
<!ELEMENT br EMPTY>
XML example:
<br />
Elements with Parsed Character Data
Elements with only parsed character data are declared with #PCDATA inside parentheses:
<!ELEMENT element-name (#PCDATA)>
Example:
<!ELEMENT from (#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>
Example:
<!ELEMENT note ANY>
Elements with Children (sequences)
Elements with one or more children are declared with the name of the children elements inside parentheses:
<!ELEMENT element-name (child1)>
or
<!ELEMENT element-name (child1,child2,...)>
Example:
<!ELEMENT note (to,from,heading,body)>
or
<!ELEMENT element-name (child1,child2,...)>
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" element is:
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
Declaring Only One Occurrence of an Element
<!ELEMENT element-name (child-name)>
Example:
<!ELEMENT note (message)>
Example:
<!ELEMENT note (message)>
The example above declares that the child element "message" must occur once, and only once inside the "note" element.
Declaring Minimum One Occurrence of an Element
<!ELEMENT element-name (child-name+)>
Example:
<!ELEMENT note (message+)>
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 an Element
<!ELEMENT element-name (child-name*)>
Example:
<!ELEMENT note (message*)>
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 an Element
<!ELEMENT element-name (child-name?)>
Example:
<!ELEMENT note (message?)>
Example:
<!ELEMENT note (message?)>
The ? sign in the example above declares that the child element "message" can occur zero or one time inside the "note" element.
Declaring either/or Content
Example:
<!ELEMENT note (to,from,header,(message|body))>
<!ELEMENT note (to,from,header,(message|body))>
The example above declares that the "note" element must contain a "to" element, a "from" element, a "header" element, and either a "message" or a "body" element.
Declaring Mixed Content
Example:
<!ELEMENT note (#PCDATA|to|from|header|message)*>
<!ELEMENT note (#PCDATA|to|from|header|message)*>
The example above declares that the "note" element can contain zero or more occurrences of parsed character data, "to", "from", "header", or "message" elements.
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