You can validate an XML file with a document type definition (DTD). DTDs use a formal grammer to describe the structure and syntax of compliant XML documents; they specify content and values allowed for the XML document. You define the DTD of your XML file in its DOCTYPE declaration. The DOCTYPE declaration can either point to an inline DTD or can be a reference to an external DTD file.
Our mydvd XML file can be validated against the following DTD:
<!ELEMENT sales (summary, data)>
<!ELEMENT summary (heading, subhead, description, author)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT subhead (#PCDATA)>
<!ELEMENT description (#PCDATA)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT data (month*)>
<!ELEMENT month (name, week*)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT week (#PCDATA)>
<!ATTLIST week number CDATA #REQUIRED>
<!ATTLIST week dvds_rented CDATA #REQUIRED>
<!ENTITY preparedby "John Smith">
<!ENTITY month "April">
Here is our mydvd XML file:
<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="mydvd7.xsl"?>
<!DOCTYPE sales SYSTEM "mydvd7.dtd">
<sales>
<summary>
<heading>MyDVD Rental Store</heading>
<subhead>Periodical Sales Report</subhead>
<description>Sales Report for January, February, and March of 2001</description>
<author>author: John Smith</author>
</summary>
<data>
<month>
<name>January 2001</name>
<week number="1" dvds_rented="12000" />
<week number="2" dvds_rented="15000" />
<week number="3" dvds_rented="18000" />
<week number="4" dvds_rented="11800" />
</month>
<month>
<name>February 2001</name>
<week number="1" dvds_rented="11500" />
<week number="2" dvds_rented="12390" />
<week number="3" dvds_rented="19050" />
<week number="4" dvds_rented="11200" />
</month>
<month>
<name>March 2001</name>
<week number="1" dvds_rented="15300" />
<week number="2" dvds_rented="12390" />
<week number="3" dvds_rented="10050" />
<week number="4" dvds_rented="11230" />
</month>
</data>
</sales>
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