THE XML REVOLUTION - TECHNOLOGIES FOR THE FUTURE WEB

DTD - Document Type Definition

Recall from earlier that XML 1.0 contains a built-in schema language: Document Type Definition

<!DOCTYPE root-element [ doctype-declaration... ]>
determines the name of the root element and contains the document type declarations
<!ELEMENT element-name content-model>
associates a content model to all elements of the given name
content models:
- EMPTY: no content is allowed
- ANY: any content is allowed
- (#PCDATA|element-name|...)*: "mixed content", arbitrary sequence of character data and listed elements
- deterministic regular expression over element names: sequence of elements matching the expression
  - choice: (...|...|...)
  - sequence: (...,...,...)
  - optional: ...?
  - zero or more: ...*
  - one or more: ...+
<!ATTLIST element-name attr-name attr-type attr-default ...>
declares which attributes are allowed or required in which elements
attribute types:
- CDATA: any value is allowed (the default)
- (value|...): enumeration of allowed values
- ID, IDREF, IDREFS: ID attribute values must be unique (contain "element identity"), IDREF attribute values must match some ID (reference to an element)
- ENTITY, ENTITIES, NMTOKEN, NMTOKENS, NOTATION: just forget these... (consider them deprecated)
attribute defaults:
- #REQUIRED: the attribute must be explicitly provided
- #IMPLIED: attribute is optional, no default provided
- "value": if not explicitly provided, this value inserted by default
- #FIXED "value": as above, but only this value is allowed

This is a simple subset of SGML DTD.

Validity can be checked by a simple top-down traversal of the XML document (followed by a check of IDREF requirements).