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Is it possible to include a DTD in another DTD? (I don't mean copy-and-paste the second DTD into the first DTD. I mean to have something like a pointer to the second DTD in the first DTD.)
557
Yes, It's possible. One way is to use parameter entity, which can be used within the DTD. Let's look at example:
XML file:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note SYSTEM "first.dtd" [
<!ELEMENT type (#PCDATA)> <!-- you can also add DTD here -->
]>
<note>
<type>business</type>
<to>George W.</to>
<from>Me</from>
<heading>meeting</heading>
<body>Meet me in central park at 16</body>
</note>
First (referencing) DTD:
<!ELEMENT note (type,to,from,heading,body)>
<!ENTITY % elements SYSTEM "second.dtd">
%elements;
Second (referenced) DTD:
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
As checked with Oxygen XML you can even make third DTD, which is referenced from second and so on. However you can't use recursive entity references e.g.:
elements1.dtd:
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ENTITY % elements2 SYSTEM "elements2.dtd">
%elements2;
elements2.dtd:
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
<!ENTITY % elements1 SYSTEM "elements1.dtd">
%elements1;
[Xerces] Recursive entity reference "%elements1". (Reference path: %elements1 -> %elements2 -> %elements1),
153
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