XML is self-describing data in the form of tagged text. XML stands for Extensible Markup Language. An XML file resembles an HTML file if you open it in a text editor. The difference is that XML allows arbitrary tags. Any given XML file, however, will follow a DTD (Document Type Definition) or an XML Schema, which describes the tags it may include and whose elements may contain other elements. XML is growing in importance because of its use as a format for the exchange of information in e-commerce applications, and because of its usefulness as a base format from which to generate multiple versions of the same document (print, web, plain text, and so on).
As with HTML, if you need to make major edits to XML files, the best way is probably to use emacs together with the appropriate mode (see Figure 13-5). You will need to have the psgml package installed for this. The emacs editor will then be able to validate the XML document, load a DTD, offer you only the tags that are available at the current point in the document, close the open tag with a single keystroke, and much more.
For more information about emacs, see Chapter 11.
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