XML Document Type Definitions
Extensible Markup Language (XML) has developed in a major way in the last few years. It has become
one of the most important means of exchanging information of all kinds on the Internet. And there are
now a number of different specialized versions of XML. These range from markup for chemistry
documents to a quick way to distribute news releases.
Specialized versions are accompanied by document type definitions (DTDs) and/or schema. Both provide
some information about the tags used in the field, but schema, being more recent, are more extensive.
However, we will begin with a look at DTDs.
Document Type Definitions
A DTD for an xml file is a list of elements (tags) used in the file, together with some information about
how they are defined. The document must have a single root node. This is followed by the children of
the root and either their children or data type. In a DTD there are only two data types, PCDATA (parsed
character data) or CDATA, unparsed character data. Most of the examples will use parsed character data.
A DTD also indicates how many times an element can occur in the file. The default is once. But most
files use the same tag names a number of times. The notation used is similar for that used in regular
expressions.
• * means zero or more occurrences.
• + means one or more occurrences.
• ? means zero or one occurrence.
A DTD also allows for choice. A vertical bar ( | ) is used to indicate one element or another.
A DTD follows for a grocery store application.
grocery.dtd
<!-- A document type definition for grocery.xml. -->
<!ELEMENT grocery (heading+, fruit*, vegetables*, bakery*)>
<!-- The elements that have children. -->
<!ELEMENT heading (name, id, quantity, price)>
<!ELEMENT fruit (name, id, quantity, price)>
<!ELEMENT vegetables (name, id, quantity, price)>
<!ELEMENT bakery (name, id, quantity, price)>
<!-- Definition of the data types. -->
<!ELEMENT name (#PCDATA)>
<!ELEMENT id (#PCDATA)>
<!ELEMENT quantity (#PCDATA)>
<!ELEMENT price (#PCDATA)>
From this DTD you can see that the root element is <grocery>. This element has four different kinds of
children. There can be zero or one heading. The heading can be used in a table that displays the file. The
DTD also indicates that there can be zero or more fruit, vegetables, and bakery elements. But it also
mandates that all fruit elements come first, vegetable elements next, and bakery elements last.
A file that satisfies all these requirements follows:
