XML in a Nutshell, 3rd Edition

Internal DTD Subsets

When you’re first developing a DTD, it’s often useful to keep the DTD and the canonical example document in the same file so you can modify and check them simultaneously. Therefore, the document type declaration may contain the DTD between square brackets rather than referencing it at an external URL. Example 3-4 demonstrates.

Example 3-4. A valid person document with an internal DTD

<?xml version="1.0"?>
<!DOCTYPE person [
  <!ELEMENT first_name (#PCDATA)>
  <!ELEMENT last_name  (#PCDATA)>
  <!ELEMENT profession (#PCDATA)>
  <!ELEMENT name       (first_name, last_name)>
  <!ELEMENT person     (name, profession*)>
]>
<person>
  <name>
    <first_name>Alan</first_name>
    <last_name>Turing</last_name>
  </name>
  <profession>computer scientist</profession>
  <profession>mathematician</profession>
  <profession>cryptographer</profession>
</person>

Some document type declarations contain some declarations directly but link in others using a SYSTEM or PUBLIC identifier. For example, this document type declaration declares the profession and person elements itself but relies on the file name.dtd to contain the declaration of the name element:

<!DOCTYPE person SYSTEM "name.dtd" [
  <!ELEMENT profession (#PCDATA)>
  <!ELEMENT person (name, profession*)>
]>

The part of the DTD between the brackets is called the internal DTD subset. All the parts that come from outside this document are called the external DTD subset. Together they make up the complete DTD. As a general rule, the two different subsets must be ...