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DTD - XML Constituent

The constituent components of XML DTD Documents.

DTD - XML Constituent


The constituent components of XML DTD Documents.

 DTDs are made up by the following integrants:

  • Elements
  • Attributes
  • Entities

Brief explanation of each of the integrants :


Elements are the main constituent components of both XML documents.

Elements can contain text, other elements, or be empty eg..



Attributes provide extra information about elements.

Attributes are always placed inside the opening tag of an element. Attributes always come in name/value pairs. The following "img" element has additional information about a source file:

<img src="computer.gif" />

The name of the element is "img". The name of the attribute is "src". The value of the attribute is "computer.gif". Since the element itself is empty it is closed by a " /".


Entities are expanded when a document is parsed by a XML parser. Some characters have a special meaning in XML, like the less than sign (<) that defines the start of an XML tag , the greater than sign (>) that defines the end of a XML tag.

The following entities are predefined in XML:

Entity References    Character
&lt;   <
&gt;   >
&amp;    &
&quot;   "
&apos;   '


PCDATA means parsed character data. It can be thought as the character data ( text ) found between the start tag and the end tag of a XML element.

PCDATA is a text to be parsed by a parser. The text is checked by the parser for entities and markup.

Tags inside the text will be treated as markup and entities will be expanded. However, parsed character data should not contain any &, <, or > characters. These should  be represented by the &amp , &lt, and &gt entities, respectively.


CDATA is character data that will NOT be parsed by a parser. Tags inside the text will NOT be treated as markup and entities will not be expanded.




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