Chapter 3
Validating XML with the
Document Type Definition (DTD)
XML is a meta-markup language
that is fully extensible. As long as it is well formed, XML authors can create
any XML structure they desire in order to describe their data. However, an XML
author cannot be sure that the structure he poured so much time and effort into
creating won’t be changed by another XML author or for that matter an
application. There needs to be a way to ensure that the XML structure cannot
be changed at random. This type of assurance for XML document structure is
vital for e-commerce applications and business-to-business processing, among
other things. This is where the Document Type Definition (DTD) steps in. A DTD
provides a roadmap for describing and documenting the structure that makes up
an XML document. A DTD can be used to determine the validity of an XML
document.
In this chapter we will start
with several examples and a brief overview of the DTD and what it does. Then we
will break down the different items that make up the structure of the DTD. The
coverage of the DTD structure will begin with a discussion of the Document Type
Declaration. Then we will move on to the functional items that make up the DTD.
The DTD includes element definitions, entity definitions, and parameters.
Finally, before closing the chapter, we will explore some of the drawbacks of
DTDS and emerging alternatives for validation. Now, let’s start by defining the
Document Type Definition.
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