A Simple DTD Example

Recall Example 2-2 from the last chapter, which described a person. The person had a name and three professions. The name had a first name and a last name. The particular person described in that example was Alan Turing. However, that’s not relevant for DTDs. A DTD only describes the general type, not the specific instance. A DTD for person documents would say that a person element contains one name child element followed by zero or more profession child elements. It would further say that each name element contains exactly one first_name child element followed by exactly one last_name child element. Finally it would state that the first_name, last_name, and profession elements all contain text. Example 3-1 is a DTD that describes such a person element.

Example 3-1. A DTD for the person
<!ELEMENT person     (name, profession*)>
<!ELEMENT name       (first_name, last_name)>
<!ELEMENT first_name (#PCDATA)>
<!ELEMENT last_name  (#PCDATA)>
<!ELEMENT profession (#PCDATA)>

This DTD would probably be stored in a separate file from the documents it describes. This allows it to be easily referenced from multiple XML documents. However, it can be included inside the XML document if that’s convenient, using the document type declaration we discuss later in this section. If it is stored in a separate file, then that file would most likely be named person.dtd, or something similar. The .dtd extension is fairly standard although not specifically required by the XML specification. If this ...

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