DTD - Element

In a DTD, elements are declared through element declarations.

Declaring an Element

In a DTD, XML elements are declared through element declarations. Element declarations use the following syntax:

Empty Element

Empty elements are declared using the keyword EMPTY:

Elements Containing Only PCDATA

Elements containing only PCDATA are declared with #PCDATA in parentheses:

Elements Containing Any Content

Elements declared with the keyword ANY can contain any combination of parseable data:

Elements with Child Elements (Sequence)

Elements with one or more child elements are declared with the child element names in parentheses:

When child elements are declared in a sequence separated by commas, they must appear in the same order in the document. In a complete declaration, child elements must also be declared, and they can have their own child elements. The complete declaration for the "note" element is:

Declaring an Element That Occurs Exactly Once

The above example declares that the message child element must occur once and only within the "note" element.

Declaring an Element That Occurs at Least Once

The plus sign (+) in the above example declares that the message child element must occur at least once within the "note" element.

Declaring an Element That Occurs Zero or More Times

The asterisk (*) in the above example declares that the message child element can occur zero or more times within the "note" element.

Declaring an Element That Occurs Zero or Once

The question mark (?) in the above example declares that the message child element can occur zero or once within the "note" element.

Declaring "Either...or..." Type Content

The above example declares that the "note" element must contain the "to" element, "from" element, "header" element, and either the "message" element or the "body" element.

Declaring Mixed Content

The above example declares that the "note" element can contain zero or more occurrences of PCDATA, "to", "from", "header", or "message".