The document discusses two ways to validate XML documents: Document Type Definitions (DTDs) and XML Schema. DTDs define the legal structure of an XML document using elements, tags, attributes, entities, and other building blocks. A DTD can be internal or external and is referenced in the DOCTYPE declaration. DTDs allow XML files to carry their own validation description and are used to verify that external data is valid.

1. Validating XML
There are different ways to define the structure
of a XML document and validate a given XML
against the definition.
Two of them are:
Document Type Definition (DTD)
XML Schema

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Document Type Definition
 The purpose of a Document Type
Definition is to define the legal building
blocks of an XML document.
 It defines the document structure with
a list of legal elements.
 A DTD can be declared inline in your
XML document, or as an external
reference.

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Internal DOCTYPE Declaration
 If the DTD is included in your XML
source file, it should be wrapped in a
DOCTYPE definition with the following
syntax:
<!DOCTYPE rootElement [element-declara]>

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External DOCTYPE Declaration
 If the DTD is external to your XML
source file, it should be wrapped in a
DOCTYPE definition with the following
syntax:
<!DOCTYPE rootElement SYSTEM "filename">

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Why use a DTD ?
 With DTD, each of your XML files can carry a
description of its own format with it.
 With a DTD, independent groups of people can
agree to use a common DTD for interchanging
data.
 Your application can use a standard DTD to
verify that the data you receive from the
outside world is valid.
 You can also use a DTD to verify your own
data.

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DTD and XML Blocks
 Seen from a DTD point of view, all XML
documents are made up by the
following simple building blocks:
 Elements
 Tags
 Attributes
 Entities
 PCDATA
 CDATA

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Element block:
 In the DTD, XML elements are declared
with an element declaration. An
element declaration has the following
syntax:
<!ELEMENT element-name category>
or
<!ELEMENT element-name (content)>
Or
<!ELEMENT element-name (child-name, child-
name,.....)>

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Element Categories
 Some of the categories of element block are:
 EMPTY :no content inside
 ANY : can contain tags as well as data
 (#PCDATA) : contains parsable text, no nodes