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)>
Empty Elements
Empty elements are declared with the category keyword EMPTY:
<!ELEMENT element-name EMPTY> Example: <!ELEMENT br EMPTY> XML example: <br />
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE br [ <!ELEMENT br EMPTY> ]> <br/>
Elements with Parsed Character Data
Elements with only parsed character data are declared with #PCDATA inside
<!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>
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE content [
<!ELEMENT content (any+)>
<!ELEMENT any ANY>
]>
<content>
<any></any>
<any>
<any></any>
</any>
<any><</any>
</content>
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)>
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)>
Declaring Only One Occurrence of an Element
<!ELEMENT element-name (child-name)> Example: <!ELEMENT note (message)>
The example above declares that the child element "message" must occur once, and only once inside the "note" element.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note [
<!ELEMENT note (message)>
<!ELEMENT message (#PCDATA)>
]>
<note>
<message></message>
</note>
Declaring Minimum One Occurrence of an 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 an 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.
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE note [ <!ELEMENT note (message*)> <!ELEMENT message (#PCDATA)> ]> <note> </note>
Declaring Zero or One Occurrences of an Element
<!ELEMENT element-name (child-name?)> Example: <!ELEMENT note (message?)>
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE note [ <!ELEMENT note (message?)> <!ELEMENT message (#PCDATA)> ]> <note> <message></message> </note>
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))>
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.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note [
<!ELEMENT note (to,from,header,(message|body))>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT header (#PCDATA)>
<!ELEMENT message (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>
<note>
<to></to>
<from></from>
<header></header>
<body></body>
</note>
Declaring Mixed Content
Example: <!ELEMENT note (#PCDATA|to|from|header|message)*>
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note [
<!ELEMENT note (#PCDATA|to|from|header|message)*>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT header (#PCDATA)>
<!ELEMENT message (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>
<note>
<to></to>
</note>
The example above declares that the "note" element can contain zero or more occurrences of parsed character data, "to", "from", "header", or "message" elements.