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XML DTD Elements

Elements in Document Type Definition (DTD) are defined with an ELEMENT declaration.

Different types of elements are shown below.

How to declare DTD Elements​

DTD Elements are declared in this way:

<!ELEMENT element-name category>

or

<!ELEMENT element-name (element-content)>

Empty DTD Elements​

Empty elements are declared with the category keyword EMPTY:

<!ELEMENT element-name EMPTY>

DTD Elements with Parsed Character Data​

Elements with only parsed character data are declared with #PCDATA inside parentheses:

<!ELEMENT element-name (#PCDATA)>

DTD Elements with any Contents​

Elements declared with the category keyword ANY, can contain any combination of parsable data:

<!ELEMENT element-name ANY>

DTD Elements with Children​

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,...)>
note

When children are declared in a sequence separated by commas, the children must appear in the same sequence in the document.

For example:

<!ELEMENT book (title,author,publisher)>
<!ELEMENT title(#PCDATA)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT publisher(#PCDATA)>

Declaring Only One Occurrence of a DTD Element​

In this way we declare a child-name element that occurs once and inside the element-name element

<!ELEMENT element-name (child-name)>

Declaring Minimum One Occurrence of a DTD Element​

The + sign declares that the child-name element must occur once or more times inside the element-name element.

<!ELEMENT element-name (child-name+)>

Declaring Zero or More Occurrences of a DTD Element​

The * sign declares that the child-name element can occur zero or more times inside the element-name element.

<!ELEMENT element-name (child-name*)>

Declaring Zero or One Occurrences of a DTD Element​

The ? sign declares that the child-name element can occur zero or one time inside the element-name element.

<!ELEMENT element-name (child-name?)>

Declaring either/or Content​

In this example, the | sign declares that book element can contain a title element, either an author or a #PCDATA element and a publisher element.

<!ELEMENT book (title,(author|#PCDATA),publisher)>

Declaring Mixed Content​

In this example, the * sign declares that book element can contain zero or more occurrences of #PCDATA, title, author or publisher elements. 

<!ELEMENT book (#PCDATA|title|author|publisher)*>

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