Document Type Definitions
A DTD specifies how elements inside an XML document should relate to each other. It also provides grammar rules for the document and each of its elements. A document that adheres to the specifications outlined by its DTD is considered to be valid. (Don’t confuse this with a well-formed document, which adheres to the XML syntax rules outlined earlier.)
You must declare each of the elements that appear inside your XML
document within your DTD. You can do so with the
declaration, which uses the this format:
<!ELEMENT elementname rule>
This declares an XML element and an associated rule, which relates
the element logically in the XML document. The element name
should not include
<> characters. An element name must
start with a letter or an underscore. After that, it can have any number of
letters, numbers, hyphens, periods, or underscores in its name. Element
names may not start with the string
xml, in any variation of
upper- or lowercase. You can use a colon in element names only if you are
using namespaces; otherwise, it is forbidden.
ANY and PCDATA
The simplest element declaration states that between the opening and closing tags of the element, anything can appear:
<!ELEMENT library ANY>
ANY keyword allows you to include both other tags
and general character data within the element. However, you may want to
specify a situation where you want only general characters appearing.
This type of data is better known ...