This is a cheat sheet for the DTD language. It is a follow up to the XML Cheat Sheet and summarizes this DTD Tutorial. There is also an XML Schema Cheat Sheet.

• A Document Type Definition (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.

  <!DOCTYPE root-element [
    element-declarations
  ]>

  or externally defined e.g.

  <!DOCTYPE root-element SYSTEM "filename.dtd">

  where the element-declarations are in filename.dtd

• A simple example of DTD element-declarations

    <!ELEMENT note (to,from)>
    <!ELEMENT to      (#PCDATA)>
    <!ELEMENT from    (#PCDATA)>

  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)

• An element is defined as

  <!ELEMENT element-name category>

  or

  <!ELEMENT element-name (element-content)>

• 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

• An attribute specified in a DTD takes the form

  <!ATTLIST element-name
    attribute-name attribute-type default-value
    attribute-name2 attribute-type2 default-value2
       ... >

  for example:

  <!ATTLIST payment
     currency CDATA "US Dollars"
     amount CDATA #REQUIRED
     form (Cash|Cheque) "Cheque"> 

• 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

• 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.

  <!ENTITY writer "Donald Duck."> 

  or external e.g.

  <!ENTITY writer SYSTEM "http://a.com/entities.dtd">

• 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.

  <author>&writer;</author>