DTD - Elements
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 /> |
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)> |
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 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.
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.
Declaring Zero or One 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 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.
Declaring Mixed Content
Example: <!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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