XML Reference Guide

Mar 14, 2003

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Document Type Definitions (DTDs) are actually a holdout from XML's legacy as a special case of SGML. They were intended to make certain that an application could read an SGML file by providing a way for the application to know what to expect in terms of the document's structure.

A DTD can be contained in an external file or as part of the XML document itself. The rules in an external file are known as the external subset, while those within the document are the internal subset. In case of a conflict, the internal subset always takes precedence.

For example, our validation example can be written as an external subset:

<?xml version="1.0"?>
<!DOCTYPE order SYSTEM "orders.dtd">
<order orderid="THX1138" customerNumber="3263827">
  <lineitem itemid="C33">
    <quantity>36</quantity>
    <unitprice currency="dollars">.35</unitprice>
  </lineitem>
  <lineitem itemid="M48">
    <quantity>1</quantity>
    <unitprice currency="dollars">2200</unitprice>
  </lineitem>
</order>

In this case, the DOCTYPE declaration (shown in bold) indicates that the root element of the document must be named order, and that the DTD is located in the SYSTEM file orders.dtd. A SYSTEM identifier can also be a fully-qualified URL such as http://www.example.com/orders.dtd. (A DOCTYPE declaration can also reference a PUBLIC identifier, which is more of a lookup that points to the actual location of the file.)

A DTD defines the name of each element, its content model, or the contents it must contain, and the name and type of any attributes it may have. For example, orders.dtd might define the quantity element as follows:

<!ELEMENT quantity (#PCDATA)>

to indicate that it can contain only text (parsed character data), while the lineitem element would be defined like this to indicate that lineitem must contain exactly one quantity element, followed by exactly one unitprice element:

<!ELEMENT lineitem (quantity, unitprice)>
<!ATTLIST lineitem itemid CDATA #REQUIRED>

It must also have a single attribute, itemid, which can be any character data allowed in an attribute. (Some characters, such as <, can't appear in an attribute.)

In some cases, you might use modifiers, which control the number of times an element can appear within another element. For example, in the following definition, the order element must contain one or more lineitem elements, as indicated by the plus (+) modifier:

<!ELEMENT order (lineitem+)>
<!ATTLIST order orderid ID #REQUIRED
        customerNumber CDATA #REQUIRED>

Other modifiers include the question mark (?), which means that an item is optional, but if present can only appear once, and the asterisk (*), which means that an element is optional, but if present can appear an unlimited number of times.

DTDs are limited in the control they provide over the content of elements and attributes, but they do enable you to create a rule that provides set of enumerated values for an attribute. For example, the following currency attribute can contain only one of two values:

<!ATTLIST unitprice currency (dollars|pounds) #IMPLIED dollars>

In this case, the value of currency may be dollars or pounds, and if it's not present, the value of dollars is implied.

This ability isn't available for elements, but you can use the pipe symbol (|) as an "or" indicator to specify a choice of elements. For example, if you didn't care about the order of elements within lineitem, you could define it like this:

<!ELEMENT lineitem (quantity|unitprice)+>
<!ATTLIST lineitem itemid CDATA #REQUIRED>

Note that this really only gives you part of the definition you want; it says that you must have at least one quantity or unitprice, but that you can have more than one. This does allow an author to create a lineitem element with the unitprice and then the quantity, but it also allows the author to create a lineitem element with three quantity elements and no unitprice. For more control over the content of an element or attribute, you need to use XML Schema or one of the other schema proposals available, such as RELAX NG.

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