While RSS can be parsed directly using text-processing tools, XML parsers are often more convenient. Many parsers exist for using XML with many different programming languages. Most are freely available, and the majority are open source.
An XML parser typically takes the form of a library of code that you interface with your own program. The RSS program hands the XML over to the parser, and the parser hands back information about the contents of the XML document. Typically, parsers do this either via events or via a document object model.
With event-based parsing, the parser calls a function in your program whenever a parse event is encountered. Parse events include things like finding the start of an element, the end of an element, or a comment. Most Java event-based parsers follow a standard API called SAX, which is also implemented for other languages such as Python and Perl. You can find more about SAX at http://www.saxproject.org.
Document object model (DOM)-based parsers work in a markedly different way. They consume the entire XML input document and hand back a tree-like data structure that the RSS software can interrogate and alter. The DOM is a W3C standard; documentation is available at http://www.w3.org/DOM.
Choosing whether to use an event- or DOM-based model depends on the application. If you have a large or unpredictable document size, it is better to use event-based parsing for reasons of speed and memory consumption (DOM trees can get very large). If you have small, simple XML documents, using the DOM leaves you less programming work to do. Many programming languages have both event-based and DOM support.
As XML matures, hybrid techniques that give the best of both worlds are emerging. If you're interested in finding out what's available and what's new for your favorite programming language, keep an eye on the following online sources:
Many XML applications involve transforming one XML document into another or into HTML. The W3C has defined a special language, called XSLT, for doing transformations. XSLT processors are becoming available for all major programming platforms.
XSLT works by using a style sheet, which contains templates that describe how to transform elements from an XML document. These templates typically specify what XML to output in response to a particular element or attribute. Using a W3C technology called XPath gives you the flexibility not only to say "do this for every person element," but also to give instructions as complex as "do this for the third person element, whose name attribute is Fred."
Because of this flexibility, some applications have sprung up for XSLT that aren't really transformation applications at all, but take advantage of the ability to trigger actions on certain element patterns and sequencers. Combined with XSLT's ability to execute custom code via extension functions, the XPath language has enabled applications such as document indexing to be driven by an XSLT processor.
The W3C specifications for XSLT and XPath can be found at http://w3.org/TR/xslt and http://w3.org/TR/xpath, respectively. For more information on XSLT, see Doug Tidwell's XSLT (O'Reilly). For more on XPath, see John Simpson's XPath and XPointer (O'Reilly).