DTD

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 />

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE br [
<!ELEMENT br EMPTY>
]>
<br/>

Elements with Parsed Character Data

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

<!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>

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE content [
<!ELEMENT content (any+)>
<!ELEMENT any ANY>
]>
<content>
    <any></any>
    <any>
        <any></any>
    </any>
    <any>&lt;</any>
</content>

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.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note [
<!ELEMENT note (message)>
<!ELEMENT message (#PCDATA)>
]>
<note>
    <message></message>
</note>

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.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note [
<!ELEMENT note (message*)>
<!ELEMENT message (#PCDATA)>
]>
<note>

</note>

Declaring Zero or One Occurrences of an Element

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

Example:

<!ELEMENT note (message?)>

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note [
<!ELEMENT note (message?)>
<!ELEMENT message (#PCDATA)>
]>
<note>
<message></message>
</note>

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.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note [
<!ELEMENT note (to,from,header,(message|body))>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT header (#PCDATA)>
<!ELEMENT message (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>
<note>
    <to></to>
    <from></from>
    <header></header>
    <body></body>
</note>

Declaring Mixed Content

Example:

<!ELEMENT note (#PCDATA|to|from|header|message)*>

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note [
<!ELEMENT note (#PCDATA|to|from|header|message)*>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT header (#PCDATA)>
<!ELEMENT message (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>
<note>
    <to></to>
</note>

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