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Document Type Definition (DTD) Tutorial

This tutorial covers the basics of DTDs. Before reading this tutorial you should already be familiar with XML. You may want to read my XML tutorial. Click on the above XML link to do so.

What is a DTD?

DTD stands for Document Type Definition.

XML lets applications share data easily. However, XML lets you make up your own set of tags. A DTD defines the legal elements and their structure in an XML document.

Independent developers can agree to use a common DTD for exchanging XML data. Your application can use this agreed upon DTD to verify the data it receives. The DTD can also be used to verify your own data.

DTD standards are defined by the World Wide Web Consortium (W3C). The W3C site provides a comprehensive reference of DTDs.

However, this tutorial focuses on Microsoft's implementation of the XML and DTDs. All examples contained herein require Internet Explorer 5.0 or later.

Using a DTD in an XML Document

DTDs can be declared inline in your XML code or they can reference an external file.

This is an example of an internal DTD. You can open it in Explorer then select View | Source to view the complete XML document with the DTD included.

 

<?xml version="1.0"?>
<!DOCTYPE message [
   <!ELEMENT message (to,from,subject,text)>
   <!ELEMENT to (#PCDATA)>
   <!ELEMENT from (#PCDATA)>
   <!ELEMENT subject (#PCDATA)>
   <!ELEMENT text (#PCDATA)>
]>
<message>
  <to>Dave</to>
  <from>Susan</from>
  <subject>Reminder</subject>
  <text>Don't forget to buy milk on the way home.</text>
</message>

Line 2 defines the <message> element as having the four child elements: <to>, <from>, <subject> and <text>.

 

<!ELEMENT message (to,from,subject,text)>

This line defines the <to> element to be of type PCDATA.

 

<!ELEMENT to (#PCDATA)>

Here is the same document with an external DTD reference:

 

<?xml version="1.0"?>
<!DOCTYPE message SYSTEM "message.dtd">
<message>
  <to>Dave</to>
  <from>Susan</from>
  <subject>Reminder</subject>
  <text>Don't forget to buy milk on the way home.</text>
</message>

And here is the referenced DTD file. You can open it in Explorer and select View | Source as above.

 


<?xml version="1.0"?>
<!ELEMENT message (to,from,subject,text)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT subject (#PCDATA)>
<!ELEMENT text (#PCDATA)>

 

XML Components

Before talking further about DTDs, we should review some of the basic XML components.

  • XML elements
    An XML element is made up of a start and end tag with data in between. The tags describe the data. The data is called the value of the element. Elements can have a value, child elements or they can be empty (have no value).

    For example, this XML element is a <director> element with the value "Bill Smith".

    <director>Bill Smith</director>

  • Attributes
    An element can optionally contain one or more attributes in its start tag. An attribute is a name-value pair separated by an equal sign (=). Attribute values must always be quoted.

    <CITY ZIP="01085">Westfield</CITY>

    ZIP="01085" is an attribute of the <CITY> element.

  • Parsed character data - PCDATA
    Like we said, elements have values. If a value has tags representing child elements, these tags need to be expanded or parsed and handled as separate elements.

  • Character data - CDATA
    The value of an element is treated as a single item and is not expanded.

  • Entities
    Certain characters, like "<", have special meaning in XML. If you want to use these special characters in your data you need a way to tell the XML parser not to interpret them as having their normally meaning.

    Entities are sets of characters that can be used to represent these special characters or other text.

    Entity

    Special Character

    &lt;

    <

    &gt;

    >

    &amp;

    &

    &quot;

    "

    &apos;

    '

    To use "<" in your data, you would use the entity "&lt;" instead.

DTD Elements

In DTD, elements are declared using an element declaration with the following syntax:

 

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

  • Empty elements
    Empty elements use the empty keyword. Move the mouse over the text for additional information.

    <!ELEMENT img (EMPTY)>



  • Elements with data
    If an element has data, you must specify the type of its data.

    <!ELEMENT element-name (#CDATA)>
    <!ELEMENT element-name (#PCDATA)>
    <!ELEMENT element-name (ANY)>

    Example:
    <!ELEMENT message (#PCDATA)>

    ANY can contain any type of data.
    #CDATA is character type data.
    #PCDATA is character data that must be parsed and expanded. If a #PCDATA section contains elements, those elements must also be declared.



  • Elements with children - Sequences
    If an element has child elements, the child elements must be enumerated in the same order that they appear in the document.

    Using the message example:

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

    Example:
    <!ELEMENT message (to,from,subject,text)>

    The child elements must also be declared. Here are their declarations:

    <!ELEMENT message (to,from,subject,text)>
    <!ELEMENT to (#CDATA)>
    <!ELEMENT from (#CDATA)>
    <!ELEMENT subject (#CDATA)>
    <!ELEMENT text (#CDATA)>



  • DOCTYPE definition
    With an internal DTD, you need a DOCTYPE definition to indicate the DTD code.

    <!DOCTYPE root-element [element declarations]>

    Example:

    <?xml version="1.0"?>
    <!DOCTYPE message [
      <!ELEMENT message (to,from,subject,text)>
      <!ELEMENT to         (#CDATA)>
      <!ELEMENT from      (#CDATA)>
      <!ELEMENT subject (#CDATA)>
      <!ELEMENT text     (#CDATA)>
    ]>
    <message>
      <to>Dave</to>
      <from>Susan</from>
      <subject>Reminder</subject>
      <text>Don't forget to buy milk on the way home.</text>
    </message>



  • Element instances
    You can specify how many times an element can occur in a document.

    Symbol

    Instances

    none

    must occur exactly 1 time

    *

    0 or more times

    +

    1 or more times

    ?

    Exactly 0 or 1 time

    For example:

    <!Element message (to+,from,subject?,text,#PCDATA)>

DTD Attribute Declaration

In DTD, attributes are declared using an ATTLIST declaration. An attribute declaration specifies the associated element, the attribute, its type, and possibly its default value. Here are the syntax variations:

 

<!ATTLIST element-name attribute-name attribute-type #DEFAULT  default-value>

<!ATTLIST element-name  attribute-name attribute-type #FIXED   fixed_value>

<!ATTLIST element-name attribute-name attribute-type (Val1|Val2|..) default_val>

<!ATTLIST element-name attribute-name attribute-type  #IMPLIED>  

<!ATTLIST element-name attribute-name attribute-type  #REQUIRED>

  • Attribute type
    Attributes can have these types:

    Type

    Description

    CDATA

    Character data

    ENTITY

    An entity

    ENTITIES

    List of entities data

    ID

    Unique ID data

    IDREF

    ID of another element

    IDREFS

    List of IDs of other elements

    NMTOKEN

    XML name data

    NMTOKENS

    List of XML names

    NOTATION

    Name of a notation

    (val1|val2|...)

    List of values

    xml:

    Predefined value



  • Default values
    Attribute default values can be:

    Default Value

    Description

    #DEFAULT value

    If no value exists in the XML data, the value specified in the DTD will be used

    #FIXED value

    If another value exists in the XML data, an error will occur

    #IMPLIED

    The value doesn't have to be supplied in the XML data.

    #REQUIRED

    If the XML data doesn't have a value, an error will occur.



  • DTD Examples
    Here are some sample DTD statements. Move the mouse over the text for more information.

    <!ATTLIST person gender CDATA #DEFAULT "male">

    <!ATTLIST person gender CDATA #FIXED "male">

    <!ATTLIST person gender CDATA #REQUIRED>

    <!ATTLIST person gender CDATA #IMPLIED>

    <!ATTLIST person gender (male|female) "male">

    Here is an XML statement that will satisfy all of the above DTD statements.

    <person gender="male">

    This XML statement does not satisfy DTD rule 2 which requires a value of "male".

    <person gender="female">

    This XML statement fails DTD rule 5 because "unknown" is not an acceptable value.

    <person gender="unknown">

DTD Entity Declaration

Recall, entities are variables that represent other values. The value of the entity is substituted for the entity when the XML document is parsed.

Entities can be defined internally or externally to your DTD.

 

Internal declaration:
<!ENTITY entity-name entity-value>

Example:
<!ENTITY website "http://www.TheScarms.com">

External declaration:
<!ENTITY entity-name SYSTEM "entity-URL">

Example:
<!ENTITY website SYSTEM "http://www.TheScarms.com/entity.xml">

The above entity make this line of XML valid.

 

XML line:
<url>&website</url>

Evaluates to:
<url>http://www.TheScarms.com</url>

 

Complete DTD Example

Click this link to view a complete DTD example.

 








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