The role of the DTD (Document Type Definition) is to define the legal build module of the XML document. In a DTD, the elements are declared by an element declaration.
Declare an element
In DTD, the XML element is declared by an element declaration. Element declaration uses the following syntax:
The empty element is declared by category keyword EMPTY:
Only PCDATA elements
Only PCDATA elements are declared by #pcdata in parentheses:
Elements with any content
By category Keyword Any declared elements, any combination of resolution data can be included:
Elements with children (sequence)
Elements with one or more sub-elements are declared by sub-elements in parentheses:
When the child element declares in the sequence separated by a comma, these sub-elements must appear in the document in the same order. In a complete statement, child elements must also be declared, while the child elements can have sub-elements. The complete statement of the “NOTE” element is:
Declare only one element
Declare minimized elements
The plus sign (+) in the above example declares: The Message child element must appear at least once in the “Note” element.
Declaration of zero or multiple elements
The asterisk (*) in the above example declares: child elements Message can occur zero or multiple times in the “Note” element.
Declaring zero or once elements
The question mark (?) In the above example declares: child element Message can occur zero or once in the “Note” element.
Declare “Non … / ie …” type content
The above example declares: “Note” element must contain “to” elements, “from” elements, “header” elements, and non-“Message” elements, “Body” elements.
Disclaimer mixed content
Example: The above example declared: “Note” element can contain PCDATA, “To”, “from”, “Header”, “Note” element”Or” Message “.