Document Type Definition

Document Type Definition
Filename extension	.dtd
File formats category -   edit

A Document Type Definition (DTD) provides a grammar for a class of documents. A document may contain an external DTD, an internal DTD, or both. A DTD is a type of XML schema, such as XSD.

Syntax

Elements

Elements in a document are considered valid by declaring the !ELEMENT syntax. A line is started with "<!ELEMENT ", then the name of the element, then how its content is read, and ends with a ">".

How its content is read can be either a listing of its possible content in parentheses, or an element type. Example:

An example of a valid XML using only this DTD:

<book>
    <author />
    <content />
</book>

<!-- A book element is valid, and it must have 
     the elements author and content as children -->
<!ELEMENT book (author,content)>

Parsed character data

Elements with only text as content are specified by placing the keyword #PCDATA in the parentheses. Example:

PCDATA: An example of a valid XML using only this DTD:

<content>Lorem ipsum dolor sit amet &amp; consectetuer</content>

<!ELEMENT author (#PCDATA)>

Empty elements

Elements that cannot have any content are specified using the keyword EMPTY. Being a keyword for an element type, EMPTY shouldn't be in parentheses. Example:

<!ELEMENT br EMPTY>
<!-- Means that a br element is valid, and must be empty,
     e.g. <br /> or <br style="..." /> -->

Any content

Elements that can contain anything are specified using the keyword ANY. Example:

<!ELEMENT p ANY>

Elements with children

Elements that has children must declare each child's name in parentheses, comma-separated. Refer to first example.

Children - Exactly one instance of a child

A child may be declared as occurring in the document exactly once as a child of another as-is.

Children - Optional child

A child may be declared as occurring in the document zero or one time by appending a question mark ("?") to its name. Example:

<!-- Here, glossary and index are optional elements -->
<!ELEMENT book (author, content, glossary?, index?)>
<!-- ... declarations of other elements ... -->
<!ELEMENT index (#PCDATA)>

Children - At least one is required

A child may be declared as occurring in the document one or more times by appending a plus sign ("+") to its name. Example:

<!ELEMENT tbody (tr+)>

Children - Any number of occurrences

A child may be declared as occurring in the document zero or more times by appending an asterisk ("*") to its name. Example:

<!ELEMENT tr (td*)>

Options and grouping

Children may be grouped using parentheses. Options may be declared using a vertical bar ("|"), which separates the options that the content of the element may take. Example:

<!-- means that the key element may either have one n1 element or one n2 element. -->
<!ELEMENT key (n1|n2)>
<!-- means that the gpoint element must have
     a title element and either point elements or an xref element. -->
<!ELEMENT gpoint (title,(point*|xref))>

Attributes

Attributes are defined using the #!ATTLIST syntax. The syntax is: <!ATTLIST <element name> <attribute name> <attribute type> <default value>>

The element name is the element to which the ATTLIST syntax is applied. The attribute name is a specific attribute to which the syntax is applied. (Note: undeclared attributes found in an element do not cause an error)

The attribute type may be one of the following:

• CDATA (character data) - the attribute is composed of just text.
• list of possible values for the attribute, vertical bar separated, in parentheses - specifies legal values for the attribute.
• ID - specifies that the value of the attribute must be unique for each element.
• IDREF - the attribute is a reference to the ID of another element.
• IDREFS - the attribute is a list of references to other IDs.
• xml:someidentifier - the attribute is a predefined XML command to be used by the XML parser.
• ENTITY - the attribute contains an entity.
• ENTITIES - the attribute contains entities.
• NMTOKEN - the attribute is a valid XML name.
• NMTOKENS - the attribute contains valid XML names.
• NOTATION - the attribute contains a notation.

The default value may be one of the following:

• value - specifies a default value to be used when the attribute is not set in the element.
• #REQUIRED - specifies that the attribute must be set.
• #IMPLIED - specifies that the attribute is optional; it may either be set or not.
• #FIXED value - specifies that the attribute must be set to the specified value.

Entities

An entity is a shortcut to a string to be replaced in the document upon presentation or other purposes. It is composed of an ampersand, a character combination (the name of the entity) and a semicolon. For example, the entity &lt; is a shortcut to the less than sign, "<".

Entities are declared by the !ENTITY syntax. The syntax is <!ENTITY <entity name> "<entity value>">. Example:

<!ENTITY excl "!">

For example, an XML document that has the text

Hello World!

would be rendered as: Hello World!

Entities can be referenced from an external DTD by using the SYSTEM keyword: <!ENTITY <entity name> SYSTEM "<URI to external DTD>">

Document type

A DTD can be declared internally in a document using the !DOCTYPE syntax. The syntax for internal DOCTYPE is: <!DOCTYPE <root element name> [ ...DTD syntax... ]>. Example:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE book [
    <!ELEMENT book (isbn, chapter+, glossary?, index?)>
    <!ELEMENT isbn (#PCDATA)>
    <!ELEMENT chapter (title, content+)>
    <!ELEMENT content (#PCDATA)>
    <!ELEMENT glossary (item+)>
    <!ELEMENT index (item+)>
    <!ELEMENT item (#PCDATA)>
    
    <!ATTLIST item ref ID #IMPLIED>
]>
<book>
    <isbn>1001234</isbn>
    <chapter>
        <title>
            1
        </title>
        <content>
            Lorem ipsum
    </chapter>
    <index>
        <item ref="65">abcd</item>
    </index>
</book>

A DTD in an external file (usually with a .dtd extension) can be used by using public identifiers, system identifiers, or both. Public identifiers take the form of a URI-like string, intended to be unique and to be used across many applications. System identifies take the form of a URI, which refers to a document type definition intended to be used in a single application.

An external DTD may be referenced using the following DOCTYPE syntax: <!DOCTYPE <root element name> PUBLIC <public identifier> SYSTEM <system identifier>>. At least one identifier is required.

For example, the XML above may also be:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE book SYSTEM "http://example.com/chml.dtd">
<book>
    <isbn>1001234</isbn>
    <chapter>
        <title>
            1
        </title>
        <content>
            Lorem ipsum
    </chapter>
    <index>
        <item ref="65">abcd</item>
    </index>
</book>

Notes

• DTD examples are not necessarily copied from actual standard DTDs that may exist.