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<html><head><title>XML and Perl (Perl in a Nutshell, 2nd Edition)</title><link rel="stylesheet" type="text/css" href="../style/style1.css" /><meta name="DC.Creator" content="Stephen Spainhour" /><meta name="DC.Format" content="text/xml" scheme="MIME" /><meta name="DC.Language" content="en-US" /><meta name="DC.Publisher" content="O'Reilly & Associates, Inc." /><meta name="DC.Source" scheme="ISBN" content="0596002416L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="Perl in a Nutshell, 2nd Edition" /><meta name="DC.Type" content="Text.Monograph" /></head><body bgcolor="#ffffff"><img src="gifs/smbanner.gif" usemap="#banner-map" border="0" alt="Book Home" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Java and XSLT" /><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="part6.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td align="center" valign="top" width="228" /><td align="right" valign="top" width="228"><a href="ch13_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr></table></div><h1 class="chapter">Chapter 13. XML and Perl</h1><div class="htmltoc"><h4 class="tochead">Contents:</h4> <p> <a href="#perlnut2-CHP-13-SECT-1">XML Parsing and Validation</a><br /><a href="ch13_02.htm">XML::Parser Methods</a><br /><a href="ch13_03.htm">Expat Handlers</a><br /><a href="ch13_04.htm">XML::Parser Styles</a><br /><a href="ch13_05.htm">Expat Encodings</a><br /><a href="ch13_06.htm">XML::Parser::ContentModel Methods</a><br /></p></div><p>The <a name="INDEX-1801" /></a>Extensible Markup Language (XML) is ametalanguage for providing complete, configurable information fordocuments and other types of data. XML is based loosely on SGML, buthas divorced itself of much of the complexity that made SGMLunsuitable for everyday use. Without opening a can of worms about thedifferences between SGML and XML, suffice it to say that SGML is thepredecessor to XML, and XML is a subset of SGML, with extensions.</p><p>With XML, you aren't bound by a fixed format, butcan mark up a document to make it easily adaptable to whatever finalformat you later decide to apply it to. In fact, this book is writtenin XML, to be produced later not only in a print format but in anonline format as well.</p><p>XML is often associated with web content, but it is much moreflexible than that. Lately, XML's application to webservices such as SOAP and XML-RPC has given it a chance to flex itsmuscles and show what it's capable of. XML gives youthe structure to hold any content you'd like,whether it be the pages of this book in their rawest form, a list ofyour favorite recipes, or the ledger from your checkbook. XML isstructured so that you can represent any kind of data in XML.XML's openness means that you can implement anXML-based application on any platform.</p><p>This chapter is focused on parsing, checking and delivering XMLcontent. <a href="ch14_01.htm">Chapter 14, "SOAP"</a> covers SOAP programming in XML.</p><div class="sect1"><a name="perlnut2-CHP-13-SECT-1" /></a><h2 class="sect1">13.1. XML Parsing and Validation</h2><p>The two most common tasks that you'll perform onyour XML content are likely to be parsing and validating. Ifyou've ever combed around on CPAN for XML-relatedmodules, you probably already know that there's noshortage of resources when it comes to Perl and XML. On the contrary,the sheer volume of modules available for Perl and XML is ratherdaunting, so you might be looking for a place to start.</p><p>This chapter covers two Perl/XML modules in particular: XML::Simpleand XML::Parser. These modules were selected because they allow youto parse and manipulate most XML. While these modules themselvesdon't validate XML, we'll resort toa little bit of trickery to show you how you can do exactly that.</p><p><a name="INDEX-1802" /></a>XML::Simpleprovides an easy API that allows you to read and write XML. It isbuilt on top of XML::Parser, which will be covered shortly. As itsname suggests, XML::Simple implements only two methods:<tt class="literal">XMLin()</tt><a name="INDEX-1803" /></a> and<tt class="literal">XMLout( )</tt>. But don't let itsapparent lack of methods fool you; XML::Simple lets you do greatthings simply, such as parsing XML-written configuration files.</p><p>Let's say that your company keeps a log of its SunMicrosystems servers and their respective operation-system versions,IP addresses, and current patch levels. While you could keep thisinformation in your home directory as a delimited file (which you canparse and analyze when you need to, or import into a database, suchas PostGresSQL or MySQL), why not just write it in XML?You'll find that by writing this information in anXML document, you'll be able to operate on thisinformation just as flexibly as you can with one of theaforementioned strategies. In addition, your information would residein a structure in which the relationships between items and theirmeanings are clear.</p><p>Let's say that your flat text log file looks likethis:</p><blockquote><pre class="code"># sunhosts - patches and levelsatlas|solaris|2.8|192.168.0.2|Generic_108528-10|5.6.1carrie|solaris|2.8|192.168.1.10|Generic_108527-12|5.6.0not4sun|solaris|2.8|192.168.0.25|Generic_108482-06|5.005_03</pre></blockquote><p>While you could parse this configuration file easily, and generatesome kind of report from it, your configuration filedoesn't really tell you anything about the data thatit represents. You can represent the same data in XML like so:</p><blockquote><pre class="code"><config servertype="sunhosts" reporttype="patches and levels"> <server name="atlas" osname="solaris" osversion="2.8"> <address>192.168.0.2</address> <patchlevel>Generic_108528-10</patchlevel> <perlversion>5.6.1</perlversion> </server> <server name="carrie" osname="solaris" osversion="2.8"> <address>192.168.1.10</address> <patchlevel>Generic_108527-12</patchlevel> <perlversion>5.6.0</perlversion> </server> <server name="not4sun" osname="solaris" osversion="2.8"> <address>192.168.0.25</address> <patchlevel>Generic_108482-06</patchlevel> <perlversion>5.005_03</perlversion> </server></config></pre></blockquote><p>In the above XML, all of the entries will be keyed on<tt class="literal">server</tt> so that for each entry in your XMLthat's called <tt class="literal">server</tt>,you'll be able to view its information for address,patchlevel, and perlversion. The following XML::Simple code does justthat:</p><blockquote><pre class="code">#!/usr/local/bin/perl -wuse XML::Simple;my $config = XMLin('./myconfig.xml');# Simply show us the IP address of each server in our tableforeach my $server (keys %{$config->{server}}) { print "$server -> $config->{server}{$server}{address}\n";}</pre></blockquote><p>Now that you've parsed your configuration file,let's say you want to write XML back to yourconfigration. To do this, you should use <tt class="literal">XMLout()</tt>, which is covered in the XML::Simple documentation.</p><p>While XML::Simple doesn't provide a thorough methodfor validating XML, it does insist that a document is compliant XML.This means that if you present XML::Simple with a documentthat's not syntatically correct, XML::Simple willstop parsing the XML document and store the error message in<tt class="literal">$@</tt>. For example:</p><blockquote><pre class="code"><config servertype="sunhosts" reporttype="patches and levels"> <server name="atlas" osname="solaris" osversion="2.8"> <address>192.168.0.2</address> <patchlevel>Generic_108528-10</patchlevel> <perlversion>5.6.1</perlversion></config></pre></blockquote><p>The following code would find the error in your XML and exit uponfinding it:</p><blockquote><pre class="code">#!/usr/local/bin/perl -wuse XML::Simple;my $config = eval { XMLin('./mybadconfig.xml') };print("I found an error in your XML: $@") if $@;foreach my $server (keys %{$config->{server}}) { print "$server -> $config->{server}{$server}\n";}</pre></blockquote><p>This gives you: </p><blockquote><pre class="code">I found an error in your XML: mismatched tag at line 6, column 2, byte 241 at/usr/local/lib/perl5/site_perl/5.6.1/sun4-solaris/XML/Parser.pm line 185</pre></blockquote><p><a name="INDEX-1804" /></a>Twocomponents are very useful for parsing XML with Perl:<a name="INDEX-1805" /></a>Expat and <a name="INDEX-1806" /></a>XML::Parser.Although there are several XML parsing options for Perl, such asGNOME's <em class="emphasis">libxml</em> and offeringsfrom the PerlSAX2 project, we stick to Expat and XML::Parser in thischapter.</p><p>Expat is a nonvalidating (that is, it does not check XML forcorrectness) XML parser that was written by<a name="INDEX-1807" /></a>James Clark.XML::Parser, a Perl wrapper around Expat, was originally written by<a name="INDEX-1808" /></a>Larry Wall andlater developed by <a name="INDEX-1809" /></a>Clark Cooper. If you'reusing the ActivePerl, you should be able to install XML::Parser with<em class="emphasis">ppm</em>. Otherwise, you'll have tobuild Expat first, then link XML::Parser against it.</p><p>Each call to one of the XML::Parser parsing methods creates a newinstance of XML::Parser::Expat, which is then used to parse thedocument. Expat options may be provided when the XML::Parser objectis created. These options are then passed on to the Expat object oneach parse call. They can also be given as extra arguments to theparse methods, in which case they override options given atXML::Parser creation time.</p><p>The XML parser takes your XML document and turns it into a datastructure that you can operate on. XML::Parser gives you low-levelcontrol (and precision) over the data structure that it created, fromwhich you can build parsers on top of it. XML::Simple is an exampleof this.</p><p>The behavior of the parser is controlled either by<tt class="literal">Style</tt> and/or <tt class="literal">Handlers</tt> optionsor by the <tt class="literal">setHandlers</tt> method. These all providemechanisms that XML::Parser can use to set the handlers needed byXML::Parser::Expat. If neither <tt class="literal">Style</tt> nor<tt class="literal">Handlers</tt> are specified, then parsing just checksif the document is well-formed.</p><p>When underlying handlers get called, they receive as their firstparameter the Expat object, not the Parser object.</p><p>You can show the relationships between entries in a configurationfile with XML::Parser as well. For example:</p><blockquote><pre class="code">#!/usr/local/bin/perl -wuse XML::Parser;use Data::Dumper;# Simply dump all of the entries and their relationshipsmy $p1 = XML::Parser->new(Style => 'Tree');my $tree = $p1->parsefile('myconfig.xml');print Dumper($tree);</pre></blockquote></div><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="part6.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0" /></a></td><td align="right" valign="top" width="228"><a href="ch13_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr><tr><td align="left" valign="top" width="228">VI. XML and SOAP</td><td align="center" valign="top" width="228"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0" /></a></td><td align="right" valign="top" width="228">13.2. XML::Parser Methods</td></tr></table></div><hr width="684" align="left" /><img src="../gifs/navbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links" /><p><p><font size="-1"><a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. 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