Perl XML Quickstart: The Perl XML Interfaces

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Perl XML Quickstart: The Perl XML Interfaces

by Kip Hampton

April 18, 2001

Introduction

A recent flurry of questions to the Perl-XML mailing list points to the need for a document that gives new users a quick, how-to overview of the various Perl XML modules. For the next few months I will be devoting this column solely to that purpose.

The XML modules available from CPAN can be divided into three main categories: modules that provide unique interfaces to XML data (usually concerned with translating data between an XML instance and Perl data structures), modules that implement one of the standard XML APIs, and special-purpose modules that seek to simplify the execution of some specific XML-related task. This month we will be looking the first of these, the Perl-specific XML interfaces.

use Disclaimer qw(:standard);

This is not an exercise in comparative performance benchmarking, nor is it my intention to suggest that any one module is inherently more useful than another. Choosing the right XML module for your project depends largely upon the nature of the project and your past experience. Different interfaces lend themselves to different kinds of tasks and to different kinds of people. My only goal is to offer working examples of the various interfaces by defining two simple tasks, and then showing how to achieve the same net result using each of the selected modules.

The Tasks

While the uses for XML are rich and varied, most XML-related tasks can be divided into two groups: those related to extracting data from existing XML documents, and those related to creating a new XML documents using data from other sources. With this in mind, the examples that we will use for our module introductions will consist of extracting a specific set data from an XML file, and and marking up a Perl data structure in a specific XML format.

Task One: Extracting Information

First, consider the following XML fragment:

<?xml version="1.0"?> <camelids> <species> <common-name>Dromedary, or Arabian Camel</common-name> <physical-characteristics> <mass>300 to 690 kg.</mass> <appearance> The dromedary camel is characterized by a long-curved neck, deep-narrow chest, and a single hump. ... </appearance> </physical-characteristics> <natural-history> <food-habits> The dromedary camel is an herbivore. ... </food-habits> <reproduction> The dromedary camel has a lifespan of about 40-50 years ... </reproduction> <behavior> With the exception of rutting males, dromedaries show very little aggressive behavior. ... </behavior> <habitat> The camels prefer desert conditions characterized by a long dry season and a short rainy season. ... </habitat> </natural-history> <conservation status="no special status"> <detail> Since the dromedary camel is domesticated, the camel has no special status in conservation. </detail> </conservation> </species> ... </camelids>

Now let's say that the complete document (available with this month's sample code) contains the same information for all the members of Camelidae family, not just our friend the single-humped Dromedary Camel. To illustrate how each module might be used to extract a subset of the data stored in this document, we will write a tiny script that parses the camelids.xml document and, for each species found, prints a line to STDOUT containing that species' common name, Latin name (in parentheses), and conservation status. So, having processed the entire document, the output of each script should yield the following result:

Bactrian Camel (Camelus bactrianus) endangered Dromedary, or Arabian Camel (Camelus dromedarius) no special status Llama (Lama glama) no special status Guanaco (Lama guanicoe) special concern Vicuna (Vicugna vicugna) endangered Task Two: Creating An XML Document

To demonstrate how each of the selected modules may be used to create XML documents from other data sources, we will write a small script that marks up a simple Perl hash containing URLs to a few cool camelid-related pages on the Web as a simple XHTML document.

Here's the hash:

my %camelid_links = ( one => { url => ' ', description => 'Bactrian Camel in front of Great ' . 'Pyramids in Giza, Egypt.'}, two => { url => 'http://www.fotos-online.de/english/m/09/9532.htm', description => 'Dromedary Camel illustrates the ' . 'importance of accessorizing.'}, three => { url => 'http://www.eskimo.com/~wallama/funny.htm', description => 'Charlie - biography of a narcissistic llama.'}, four => { url => 'http://arrow.colorado.edu/travels/other/turkey.html', description => 'A visual metaphor for the perl5-porters ' . 'list?'}, five => { url => 'http://www.galaonline.org/pics.htm', description => 'Many cool alpacas.'}, six => { url => 'http://www.thpf.de/suedamerikareise/galerie/vicunas.htm', description => 'Wild Vicunas in a scenic landscape.'} );