DTD

Introduction
• Stands for Document Type Definition
• Allows to create rules for the elements within
your XML documents
• So, for an XML document to be well-formed, it
needs to use correct XML syntax, and it needs to
conform to its DTD or schema
• Is declared at the top of your XML document
• The actual contents of the DTD can be included
within your XML document or included in
another .dtd document

DTD <!DOCTYPE>
• DTD can either be
– internal (written into the same document that it's
being used in)
– external (located in another document).
• DTD is declared at the top of your XML
document using the !DOCTYPE declaration.
The basic syntax is:

DTD Variations
• <!DOCTYPE rootname [DTD]>

• <!DOCTYPE rootname SYSTEM URL>
• The keyword SYSTEM indicates that it's a
private DTD (not for public distribution)
• DTD is defined in a document located at the
URL

• <!DOCTYPE rootname SYSTEM URL [DTD]>
• The keyword SYSTEM indicates that it's a
private DTD (not for public distribution)
• The presence of URL and [DTD] together
indicates that this is both an external and
internal DTD (part of the DTD is defined in a
document located at the URL, the other part is
defined within the XML document)

Internal DTD
• Whether we use an external or internal DTD,
the actual syntax for the DTD is the same
• Same code could just as easily be part of an
internal DTD or an external one
• The only difference between internal and
external is in the way it's declared with
DOCTYPE

External DTD
• Is one that resides in a separate document
• To use the external DTD, we need to link to it
from our XML document by providing the URI
of the DTD file
• This URI is in the form of a URL
• The URL can point to a local file using a
relative reference, or a remote one (eg, using
HTTP)

Combined DTD
• Can use both an internal DTD and an external
one at the same time
• This could be useful if you need to adhere to a
common DTD, but also need to define your
own definitions locally

DTD Elements
• Creating a DTD is quite straight forward
• To define an element in your DTD, we use the
<!ELEMENT> declaration
• The actual contents of your <!ELEMENT>
declaration will depend on the syntax rules
you need to apply to your element

Basic Syntax
• <!ELEMENT element_name content_model>
– element_name is the name of the element
– content_model could indicate a specific rule, data
or another element
• If it specifies a rule, it will be set to either ANY or
EMPTY
• If specifies data or another element, the data
type/element name needs to be surrounded by
brackets (i.e. (tutorial) or (#PCDATA))

Plain Text
• If an element should contain plain text, you define the
element using #PCDATA.
• PCDATA stands for Parsed Character Data
• Syntax:
– <!ELEMENT element_name (#PCDATA)>
• Example:
– <!ELEMENT name (#PCDATA)>
• The above line in your DTD allows the name element to
contain non-markup data in your XML document:
– <name>XML Tutorial</name>

Unrestricted Elements
• If it doesn't matter what element contains, we
can create an element using the
content_model of ANY
• Doing this removes all syntax checking, so we
should avoid using this if possible
• Syntax:
– <!ELEMENT element_name ANY>
• Example:
– <!ELEMENT tutorials ANY>

Empty Elements
• Empty element is one without a closing tag.
• For example, in HTML, the <br /> and <img /> tags are
empty elements.
• Here's how you define an empty element:
• Syntax:
– <!ELEMENT element_name EMPTY>
• Example:
– <!ELEMENT header EMPTY>
• The above line in your DTD defines the following empty
element for your XML document:
– <header />

Child Elements
• Can specify that an element must contain
another element, by providing the name of
the element it must contain.
• Here's how you do that:
• Syntax:
– <!ELEMENT element_name
(child_element_name)>
• Example:
– <!ELEMENT tutorials (tutorial)>

• The above line in DTD allows the tutorials
element to contain one instance of the
tutorial element in XML document:

Multiple Child Elements (Sequences)
• You can also provide a comma separated list of
elements if it needs to contain more than one element.
This is referred to as a sequence. The XML document
must contain the tags in the same order that they're
specified in the sequence.
• Syntax:
<!ELEMENT element_name (child_element_name,
child_element_name,...)>
• Example:
<!ELEMENT tutorial (name, author)>

• The above line in DTD allows the tutorial
element to contain one instance of the name
element and one instance of the author
element in XML document:

DTD Element Operators
• Are used to specify the number the times the
child elements can be used inside parents
elements

Zero or More
• To allow zero or more of the same child
element, use an asterisk (*)
• Syntax:
<!ELEMENT element_name
(child_element_name*)>
• Example:
<!ELEMENT tutorials (tutorial*)>

One or More
• To allow one or more of the same child
element, use a plus sign (+):
• Syntax:
(child_element_name+)>
• Example:
<!ELEMENT tutorials (tutorial+)>

Zero or One
• To allow either zero or one of the same child
element, use a question mark (?):
• Syntax:
(child_element_name?)>
• Example:
<!ELEMENT tutorials (tutorial?)>

Choices
• Can define a choice between one or another
element by using the pipe (|) operator.
• Syntax:
<!ELEMENT element_name (choice_1 | choice_2
| choice_3)>
• For example, if the tutorial element requires a
child called either name, title, or subject (but only
one of these)
<!ELEMENT tutorial (name | title | subject)>

DTD Operators with Sequences
• Can apply any of the DTD operators to a
sequence:
• Syntax:
(child_element_name dtd_operator,
child_element_name dtd_operator,...)>
• Example:
<!ELEMENT tutorial (name+, author?)>

DTD Attributes
• Just as we need to define all elements in your
DTD, we also need to define any attributes
they use.
• Use the <!ATTLIST> declaration to define
attributes
• Single <!ATTLIST> declaration to declare all
attributes for a given element

DTD

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DTD