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===== Document Type Definition (DTD) =====

* 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. <code>
<!DOCTYPE root-element [
  element-declarations
]>
</code> or externally defined e.g. <code>
<!DOCTYPE root-element SYSTEM "filename.dtd">
</code> where the element-declarations are in filename.dtd

* A simple example of DTD element-declarations <code>
  <!ELEMENT note (to,from)>
  <!ELEMENT to      (#PCDATA)>
  <!ELEMENT from    (#PCDATA)>
</code> 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 <code><!ELEMENT element-name category></code> or <code><!ELEMENT element-name (element-content)></code>

* category can be either EMPTY or ANY

* element-content can be any combination of #PCDATA or child elements

* child_element+ - one or more occurrences

* child_element* - zero or more occurrences

* child_element? - zero or one occurrences

* (a|b) - Either a or b (can be #PCDATA)

* a,b - a and then b in that order

==== DTD Attributes ====

* An attribute specified in a DTD takes the form <code><!ATTLIST element-name attribute-name attribute-type default-value></code> for example:<code>
<!ATTLIST payment
   currency CDATA "US Dollars"
   amount CDATA #REQUIRED>
   form (Cash|Cheque) "Cheque" </code>

* 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. <code>
<!ENTITY writer "Donald Duck."> </code> or external e.g. <code><!ENTITY writer SYSTEM "http://a.com/entities.dtd"></code>

* Entity references are references to entities and may be used in XML documents.

* An entity reference has three parts: an ampersand (&), an entity name, and a semicolon (;). e.g. <code><author>&writer;</author></code>

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