Chapter 7

XML::SAX

The Perl XML

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 community has come a long way in recent years. In the beginning 

there was XML::Parser, created by Larry Wall and maintained by Clark Cooper. 

XML::Parser provides a thin wrapper around the Expat XML parsing library 

written in C.

22

 At the same time, the Perl XML mailing list, 

perl xml@listserv.ActiveState.com, got started to provide a place to discuss using 

XML with Perl.

XML::Parser did (and still does) an excellent job of parsing XML. But it suffers 

from a quirky interface that is difficult to learn to use effectively. As a result, many 

wrapper modules grew up around XML::Parser XML::Twig, XML::Simple, and 

XML::TokeParser, to name a few. These modules have helped the situation a great 

deal and are basically the  state of the art  in Perl XML usage.

XML::SAX

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 may not be the most popular module at present, but it represents 

an evolutionary step forward in the development of Perl's capabilities. It provides an 

interface for XML parser usage in the same way that DBI provides an interface for 

SQL database usage. Individual XML parsers can be plugged into the back end API 

provided by XML::SAX. Much like using MySQL through DBI means loading the 

DBD::MySQL module, using Expat through XML::SAX loads XML::SAX::Expat. On 

the front end modules can make use a of consistent and standardized

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 interface 

to whichever parser is in use.

XML::SAX is an object oriented module, and it provides much of its function 

ality through inheritance. To start building a SAX parser or filter, the user creates a 

module that inherits from XML::SAX::Base and overrides methods as required to 

implement the desired functionality. It is interesting to note that although XML::SAX is 

tackling a similar problem to DBI, the choice of object oriented methods differs 

DBI chooses a complex mix of composition and inheritance, whereas XML::SAX 

chooses a pure inheritance model.

One of the most interesting front end modules to make use of XML::SAX thus 

far is XML::SAX::Machines by Barrie Slaymaker. XML::SAX::Machines provides a 

layer over XML::SAX based parsers and filters that allows the end user to easily 

construct XML processing pipelines. Using XML::SAX::Machines, XML::SAX com 

ponents can be assembled into systems of almost limitless capability. Typically, 

input XML enters one end of the pipeline through a SAX parser, is transformed and 

processed by the configured SAX filters, and is output by a SAX writer at the end. 

21. XML stands for the eXtensible Markup Language. See http://www.w3c.org/XML for details.

22. Written by James Clark. See http://expat.sourceforge.net for details.

23. Written by Matt Sergeant. See http://sergeant.org.

24. XML::SAX implements the SAX standard, versions 1 and 2. See http://www.saxproject.org

for details.

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