XML

Draft

{{Comment | This page is juste a beginning. Several sub-topics will be factored out to other pages.

Definition

• XML means "Extended markup language". XML is designed as a machine readable self describing text editable persistent store for data. XML is a formalism or a meta-language (not to be confounded with HTML, a language to describe the structure of Web pages)

History

• XML is a subset of SGML (Standardized Generalized Markup Language). SGML has been used to define HTML whereas XHTML is defined with XML (This is why empty tags are not allowed anymore in XHTML).

• XML was formally defined in 1998 as W3C's XML Recommendation 1.0

• Since then, hundreds of XML languages have been defined and few dozens are popular and in production.

Ken Sall's famous Big Picture only list some, e.g. he misses out all the IMS e-learning standards.

Some technical XML concepts

An XML document can refer to a physical file, a database entry, a datastream (any appropriate "text" that is delimited).

Wellformedness

An XML document is well formed if and only if

There is an appropriate XML declaration at the beginning

• The document starts with an XML declaration that includes a version number (currently 1.0).

<?xml version="1.0"?>

This declaration can also contain encoding information. By default encoding isUTF-8):

<?xml version="1.0" encoding="ISO-8859-1"?>

XML documents are hierarchical

• begin-tags and end-tags that match
• No tags crossing like

  <i>...<b>...</i> .... </b> 

• There must be single root
  • It can only appear once and can not be used within other elements

Other features

• XML is case sensitive, "LI" is not "li" for example
• "Empty" tags must be self closing, e.g.

• Attribute values are quoted

<a href= " http://tecfa.unige.ch:8080/xml.html " >)

• Special caracters: <, &, >,", '
  • Use < & > &aquot; ' instead of <, &, >,", '
  • Including URLs !!

Valid

An XML document is said valid if it conforms to some kind of grammar also called schema.

The most popular ones are in this order:

• DTD
• XML Schema
• Relax NG

Text-centric vs. data-centric XML

Data-centric XML as opposed to the text-centric XML refers to XML whose primary audience is not a human reader, but a computer program which will process the information, respond to it, store data items in a database, and so on.

Software

(longer entries have their own page)

XML creation

• See XML editor

Validation

Off-line validation

• Most decent XML editors do offer validation functionality. However, some free XML editors do not. Some (like Xemacs) only offer limited verification.

• xmllint, a command line tool which is distributed as part of the libxml2 C parser developed for the Gnome project. This means that it ships with most Linux installations, but there also distributions for Windows and other OSs.
• xmlTester.jar. This tools is based on the Xerxes parser.
• XML Nanny. XML Nanny is a Free Mac OS X developer tool that provides an Aqua interface for checking XHTML and XML documents for Well-Formedness and Validity either locally or across the network. (Tiger OS X 10.4) [sept 2005]

On-line validation

Note: You may need to change DTD's local system identifier. These programs must be able to get the DTD. I rather suggest installing a local program on your machine (like xmllint or xmlTester).

• STG XML Validation Form, curtosy of Scholarly Technology Group, Brown University
• XML well-formedness checker and validator, Richard Tobin, University of Endinburgh (RXP parser)
• XML.com's (simple well-formedness)

On-line validation for specific XML applications

• W3C HTML Validation Service This validator doesn't work with your own DTD's. Its primary function is to validate W3C vocabularies (HTML, XHTML, SVG, MathML, ... )

• FEED Validator. Validates various RSS Formats plus PIE

Links

Tutorials

• XML:Managing Data Exchange. Thi Wikibook project introduced XML from a Data exchange perspective.

News

• Cafe con Leche XML News and Resources (Best resource to keep in touch with XML-related news)

• TECFA's XML Page (DSchneider's "old" XML pointers page).

References

• Elliotte Rusty Harold, (2004). XML in a Nutshell, O'Reilly, Abstract/TOC ISBN 0-596-00764-7 (Best buy according to DSchneider).