===== Document Type Definition (DTD) =====
This is a cheat sheet for the DTD language. It is a follow up to the [[XML Cheat Sheet]] and summarizes [[http://www.w3schools.com/dtd/default.asp | this DTD Tutorial]]. There is also an [[XML Schema Cheat Sheet]].
For reference this is the [[http://www.w3.org/TR/REC-xml/ | XML Specification]] (which also documents DTD) and the [[http://www.xml.com/axml/testaxml.htm | version annotated by Tim Gray]].
* A **D**ocument **T**ype **D**efinition (DTD) defines the legal building blocks of an XML document. It defines the document structure with a list of legal elements and attributes.
* A DTD can be internally defined e.g.
or externally defined e.g.
where the element-declarations are in filename.dtd
* A simple example of DTD element-declarations
This says that note contains 2 child elements and to and from are of type #PCDATA .
* A DTD for an XML document may contain the following building blocks
* elements
* attributes
* entities (e.g. character entities)
* PCDATA (parsed character data parsed for entities and markup)
* CDATA (character data that is not parsed)
==== DTD Elements ====
* An element is defined as
for example:
* attribute-type can be one of the following
* CDATA - character data
* (en1|en2|..) - one from an enumerated list
* ID - a unique id
* IDREF - the id of another element
* IDREFS - a list of other ids
* NMTOKEN - a valid XML name
* NMTOKENS - a list of valid XML names
* ENTITY - an entity
* ENTITIES - a list of entities
* NOTATION - a name of a notation
* xml: - a predefined xml value
* default-value can be one of the following
* value - the default value of the attribute
* #REQUIRED - attribute is required but with no default
* #IMPLIED - attribute is optional and with no default
* #FIXED value - the attribute value is fixed
==== DTD Entities ====
* Entities are variables used to define shortcuts to standard text or special characters.
* Entities are declared in DTDs and can be declared internal e.g.
or external e.g. &writer;