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<h1>Introduction to DTD</h1>
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<p class="intro">A Document Type Definition (DTD) defines the legal building blocks of an XML
document. It defines the document structure with a list of legal elements and
attributes.</p>
<p class="intro">A DTD
can be declared inline inside an XML document, or as an external reference.</p>
<hr />
<h2>Internal DTD Declaration</h2>
<p>If the DTD is declared inside the XML file, it should be wrapped in a DOCTYPE
definition with the following syntax:</p>
<table border="1" width="100%" class="ex" cellspacing="0">
<tr><td width="100%"><pre><!DOCTYPE root-element [element-declarations]></pre></td></tr>
</table>
<p>Example XML document with an internal DTD:</p>
<table border="1" width="100%" class="ex" cellspacing="0">
<tr><td width="100%"><pre><?xml version="1.0"?>
<!DOCTYPE note [
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend</body>
</note></pre></td></tr>
</table>
<br />
<a target="_blank" href="note_in_dtd.xml">Open
the XML file above in your browser</a>, and select view source or view page
source to view the DTD.<p>The DTD above is interpreted like this:<br />
<b><br />
!DOCTYPE note</b> defines that the root element of this document is <b>note.<br />
</b><b>!ELEMENT note</b> defines that the <b>note</b> element contains
four elements: "to,from,heading,body".<br />
<b>!ELEMENT to</b> defines the <b>to</b> element to be of
the type "#PCDATA".<br />
<b>!ELEMENT from</b> defines the <b>from</b> element to be of the
type "#PCDATA".<br />
<b>!ELEMENT heading</b> defines the <b>heading</b> element to be of the
type "#PCDATA".<br />
<b>!ELEMENT body</b> defines the <b>body</b> element to be of the
type "#PCDATA".</p>
<hr />
<h2>External DTD Declaration</h2>
<p>If the DTD is declared in an external file, it should be wrapped in a DOCTYPE
definition with the following syntax:</p>
<table border="1" width="100%" class="ex" cellspacing="0">
<tr><td width="100%"><pre><!DOCTYPE root-element SYSTEM "filename"></pre></td></tr>
</table>
<p>This is the same XML document as above, but with an external DTD (<a target="_blank" href="note_ex_dtd.xml">Open
it</a>, and select view source):</p>
<table width="100%" border="1" class="ex" cellspacing="0">
<tr>
<td>
<pre><?xml version="1.0"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note> </pre>
</td>
</tr>
</table>
<p>And this is the file "note.dtd" which contains the DTD:</p>
<table width="100%" border="1" class="ex" cellspacing="0">
<tr>
<td>
<pre><!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)></pre>
</td>
</tr>
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<br />
<hr />
<h2>Why Use a DTD?</h2>
<p>With a DTD, each of your XML files can carry a description of its own format.
</p>
<p>With a DTD,
independent groups of people can agree to use a standard DTD for interchanging
data.
</p>
<p>Your application can use a standard DTD to verify that the data you
receive from the outside world is valid.
</p>
<p>You can also use a DTD to verify your own data.
</p>
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