In the DTD, XML elements are declared with an element
declaration. An element declaration has the following syntax:
<!ELEMENT element-name (element-content)>
Empty elements
Empty elements are declared with the keyword EMPTY inside the parentheses:
<!ELEMENT element-name (EMPTY)>
example:
<!ELEMENT img (EMPTY)>
Elements with data
Elements with data are declared with the data type inside parentheses:
<!ELEMENT element-name (#CDATA)>
or
<!ELEMENT element-name (#PCDATA)>
or
<!ELEMENT element-name (ANY)>
example:
<!ELEMENT note (#PCDATA)>
#CDATA means the element contains character data that is not supposed to be
parsed by a parser.
#PCDATA means that the element contains data that IS going to be parsed by a
parser.
The keyword ANY declares an element with any content.
If a #PCDATA section contains elements, these elements must also be declared.
Elements with children (sequences)
Elements with one or more children are defined with the name of the children elements inside
the 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 (#CDATA)>
<!ELEMENT from (#CDATA)>
<!ELEMENT heading (#CDATA)>
<!ELEMENT body (#CDATA)>
Wrapping
If the DTD is to be included in your XML source file, it should be wrapped in a DOCTYPE
definition with the
following syntax:
<!DOCTYPE root-element [element-declarations]>
example:
<?xml version="1.0"?>
<!DOCTYPE note [
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#CDATA)>
<!ELEMENT from (#CDATA)>
<!ELEMENT heading (#CDATA)>
<!ELEMENT body (#CDATA)>
]>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend</body>
</note>
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.
The example above declares that the element note must contain at least
one to child element, exactly one from child element, exactly one header,
zero or more message, and some other parsed character data as well.
Puh!
