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XML stands for Extensible Markup Language, and is the defacto standard data interchange language. The goal of XML is do for data interchange what HTML did for (hyperlinked text).

XML defines a set of rules that all conformant XML documents must follow. All XML documents must be well-formed.

  1. All element tags in a well-formed XML document must either have matching start tags and end tags (e.g. <element-name;> ... <element-name>), or follow the correct format for an empty element (e.g. <element-name />).
  2. Special characters such as <, >, &, ' and " must be properly escaped as an entity reference in a well-formed document.
  3. All elements must be properly contained in the parent element. Constructs such as <element-a> ... <element-b> ... </element-a> ... </element-b> are not allowed in a well-formed XML document.
These stricter rules on a well-formed XML document makes XML easier to parse than HTML documents.

Furthermore, an XML document can be valid if

  1. A set of rules defining the valid elements and attributes that can be present in the document has been defined. There are currently 2 ways of defining these rules, through DTD and XML Schema allows anyone to define a new set of XML based vocabulary for the domain concerned.

"Vocabularies" from different domain can be mixed in the same document through the use of namespaces.

Advantages of XML

Disadvantages of XML

Useful Links

Applications and Uses of XML

XML has been applied and used in many different domains, in areas as diverse as data store, ontologies, graphics and remote procedure calls. Some examples are listed in this section.


Written by Mike Kwong