XML Tools for Perl

XML Tools in Perl Geir Aalberg geir@aalberg.com Nordic Perl Workshop 2006

Myths about XML  “Unicode with pointy brackets”  Too hard to parse  All data must be put inside CDATA blocks  Namespaces don't work  XSLT will never take off  What's wrong with using Perl data structures?

What is XML?  A syntax − Simplified SGML, much easier to parse  A data structure − tree-based, cross.platform − industry standard tools  A technology family − SAX, DOM, XPath, XSLT, XQuery − XHTML, WML, RDF/XML, RSS/Atom, SOAP, ODF, ebXML

The 10 XML Commandments 1. Thou shalt think of XML as a tree structure, not as a string

History of XML  Generalized Markup Language (GML) − 1969: Invented by Goldfarb, Mosher and Lorie at IBM − Over 90% of all IBM documents produced using GML  Simple Generalized Markup Language (SGML) − 1980: First draft by ANSI − 1986: ISO standard 8879 − Major users include US DoD, AAP − 1988-96: DSSSL developed into ISO 10179 − 1991: O'Reilly and HaL Computer Systems design DocBook − 1992: Tim Berners-Lee designs HTML

History of XML  Extensible Markup Language (XML) − 1996: XML Working Group − 1998: XML 1.0 W3C Recommendation − 1998: DOM W3C Recommendation − 1999: XSLT and XPath W3C Recommendations − 2000: XHTML 1.0 W3C Recommendation − 2001: XML Schema W3C Recommendation − 2001: RELAX NG OASIS spec + part of ISO 19757 − 2006: XQuery W3C Recommendation Candidate

The 10 XML Commandments 2. Thou shalt not make unto thee any illegal markup

XML Syntax  Wellformed (legal) XML − correctly nested opening and closing tags <foo><bar><baz/></bar></foo> − [&<>”] must be encoded as entities (or CDATA) & < > " − parsing non-wellformed documents must cause fatal error  Encoding − ASCII, ISO-8859-1 or (default) UTF-8 − Always UTF-8 internally

The 10 XML Commandments 3. Thou shalt not XML namespaces in vain

Namespaces  Motivation − To avoid tag name collisions − To allow processor handlers in pipeline (e.g. XSLT)  Namespace determined by scope − much like Perl  Namespace is empty string unless stated otherwise − Common pitfall when using XPath  The prefix is irrelevant after parsing − Only the tag name and namespace URI counts

Namespace prefixes <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:template match="/"> <html xmlns="http://www.w3.org/1999/xhtml"> <body> <p>This page intentionally left blank.</p> </body> </html> </xsl:template> </xsl:stylesheet> is the same as <stylesheet version="1.0" xmlns="http://www.w3.org/1999/XSL/Transform"> <template match="/"> <html:html xmlns:html="http://www.w3.org/1999/xhtml"> <html:body> <html:p>This page intentionally left blank.</html:p> </html:body> </html:html> </template> </stylesheet>

Namespace prefixes Or indeed <stylesheet version="1.0" xmlns="http://www.w3.org/1999/XSL/Transform"> <template match="/"> <html xmlns="http://www.w3.org/1999/xhtml"> <body> <p>This page intentionally left blank.</p> </body> </html> </template> </stylesheet>  These are all exactly similar! − Try transforming any XML document with them and load into Firefox. Use .xhtml extension to force correct MIME type (application/xhtml+xml).

The 10 XML Commandments 4. Honor thy DTD and XML Schemas: that thy working days may be short upon the land which the BOSS giveth thee

Validation  DTD − Legacy from SGML − Does not follow XML syntax (but can be included inline) − Does not understand namespaces − Can define entities (unlike schemas)  XML Schema − Schema used by W3C  RELAX NG − Schema used by most others − Both XML and simpler non-XML syntax

DTD/Schema generators  Very useful as a starting point − DTD syntax is pretty arcane and hard to remember  Generate when needed − may catch typos that will take a long time to debug  Online tools − http://www.hitsw.com/xml_utilites/

The 10 XML Commandments 5. Thou shalt cache thy DTDs and Schemas locally to avoid unneccesary HTTP requests

XML Catalogs (libxml example)  Local repository of DTD/Schemas − Resolves official URIs to local files $ cat /etc/xml/catalog <?xml version="1.0"?> <!DOCTYPE catalog PUBLIC "//OASIS//DTD XML Catalogs V1.0//EN" "file:///usr/share/xml/schema/xmlcore/catalog.dtd"> <catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog"> <delegatePublic publicIdStartString="//Norman Walsh//DTD Slides" catalog="file:///etc/xml/docbookslides.xml"/> <delegateSystem systemIdStartString="http://docbook.org/xml/" catalog="file:///etc/xml/docbookxml.xml"/>...  Use xmlcatalog tool to add/remove $ xmlcatalog /etc/xml/catalog "//W3C//DTD XHTML 1.0 Transitional//EN" file:///usr/share/xml/xhtml/schema/dtd/1.0/xhtml1transitional.dtd

The 10 XML Commandments 6. Thou shalt not parse thy XML with regular expressions

Processing XML  Stream-based parsing − SAX  Tree-based parsing − DOM − XPath − XQuery  Convert to Perl structure

Streaming vs treebased parsing

Simple API for XML (SAX)  Stream-based parsing  Emphasis on simple  Suitable for large documents  Event handlers for each node (start, content, end)  No way to backtrack/lookahead  Namespace support from v.2 (SAX2)

Document Object Model (DOM)  Cross-platform API for processing XML tree − Same in Perl, C, Java, Javascript et al − Familiar to AJAX programmers  Set of standard methods getElementById() setAttribute() createElement() replaceChild()  DOM Level 2 adds namespace support  verbose compared to XPath and XSLT

XPath  Developed in conjuction with XSLT spec  Functional query language /html/body/div[@class="sect"]/h1[count(followingsibling())>1]  One line XPath = 10 lines of Perl

DOM/XPath example (Javascript) var rightcol = document.evaluate("/html/body/table[7]", document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue; var mybox = document.evaluate("/html/body/table[7]/tbody/tr[1]/td[6]", document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue; if (rightcol) { var holder = rightcol.parentNode; if (mybox) { var gone = mybox.parentNode.removeChild(mybox); var newtable = document.createElement("table"); holder.appendChild(newtable); var newtr = document.createElement("tr"); newtable.appendChild(newtr); newtr.appendChild(gone); } holder.replaceChild(newtable, rightcol); }

Same example in XSLT <xsl:template match="/html/body/table[7]"> <table> <tr> <xsl:copyof select="tbody/tr[1]/td[6]"/> </tr> </table> </xsl:template>

XQuery  Similar to SQL, but for XML trees instead of tables for $b in $books/book[price < 100] order by $b/title return $b  Not yet an official W3C Recommendation  Few tools support it yet

The 10 XML Commandments 7. Thou shalt choose thy XML parser wisely

XML parser libraries  James Clark's expat − C. Non-standard, stream-based API (but not SAX)  GNOME libxml − C, with OO Perl bindings  Apache Xerces − C++ (Also Java)  Platform specific − .NET (MSXML), Apple Cocoa NSXML  Pure Perl

Parser features expat libxml Xerces .NET NSXML DTD validation N Y Y Y XML Schema N Y Y Y? RELAX NG N Y N ? Namespaces ? Y Y SAX2 N Y Y DOM N Y Y XPath 1.0 ? Y XPath 2.0 N ? N N XQuery N N Partly gzip N Y

command line tools  xmlwf (expat) − check for wellformedness  xml_pp (XML::Twig) − code reformatter  xmllint (libxml) − check/validate documents format # code reformat/indent compress # output gzip data xinclude # process XIncludes valid # validate before XInclude postvalid # validate XIncluded document shell # this is cool!

XML::Parser  Perl granddaddy of XML  Based on James Clark's expat  Non-standard API  Expects string input, returns string output  Not suitable for pipeline processing

XML::SAX use XML::SAX; use MySAXHandler; my $parser = XML::SAX::ParserFactory>parser( Handler => MySAXHandler>new ); $parser>parse_uri("foo.xml"); package MySAXHandler; use base qw(XML::SAX::Base); sub start_document { my ($self, $doc) = @_; # process document start event } sub start_element { my ($self, $el) = @_; # process element start event }

XML::Twig  SAX-like interface on top of expat  Discards nodes after use, suitable for large files my $twig=XML::Twig>new( twig_handlers => { title => sub { $_>set_tag( 'h2') }, # change title tags to h2 para => sub { $_>set_tag( 'p') }, # change para to p hidden => sub { $_>delete; }, # remove hidden elements list => &my_list_process, # process list elements div => sub { $_[0]>flush; }, # output and free memory }, pretty_print => 'indented', # output formatted empty_tags => 'html', # outputs <empty_tag /> );

XML::LibXML  Implements SAX, DOM, XPath (but not XQuery)  Faster and more robust than anything else  Plugins for XUpdate  Mix and match DOM, XPath and XSLT on same tree  Works hand-in-hand with XML::LibXSLT and other libxml-based modules

XML::LibXML example use XML::LibXML; my $parser = XML::LibXML>new(); my $tree = $parser>parse_file('text.xhtml'); my $root = $tree>getDocumentElement; foreach ($root>findnodes('/html/body/div[@class=”sect”]')) { printf “%s (%s chars)n”, $_>findvalue('h1[1]'), length($_>findvalue('.')); }

XML::XPath  More unwieldy than XML::LibXML use XML::XPath; use XML::XPath::XMLParser; my $xp = XML::XPath>new(filename => 'test.xhtml'); my $nodeset = $xp>find('/html/body/div[@class="sect"]/h1'); foreach ($nodeset>get_nodelist) { printf "%sn", XML::XPath::XMLParser::as_string($_); }  xpath utility can be handy for debugging $ xpath transitional.html '/html/head/title/text()' Found 1 nodes: NODE Quick Example $

XML::Xerces  Little or no Perl documentation − See C++ API at Apache site

Pure Perl parsers  XML::SAX::PurePerl − From author: “XML::SAX::PurePerl is slow. Very slow. I suggest you use something else in fact.”  XML::Stream::Parser − 50 % slower than XML::Parser − Could be useful where installing libraries not possible

The 10 XML Commandments 8. Thou shalt not convert XML to Perl data structures without good reason

XML::Simple  Convert XML into hashes and arrays <config logdir="/var/log/foo/" debugfile="/tmp/foo.debug"> <server name="sahara" osname="solaris" osversion="2.6"> <address>10.0.0.101</address> <address>10.0.1.101</address> </server> </config> { 'logdir' => '/var/log/foo/', 'debugfile' => '/tmp/foo.debug', 'server' => { 'sahara' => { 'osversion' => '2.6', 'osname' => 'solaris', 'address' => [ '10.0.0.101', '10.0.1.101' ] } } }

XML::Smart  Similar to XML::Simple − each point in the tree work as a hash and an array at the same time  Caveat − Some users report encoding problems

The 10 XML Commandments 9. Thou shalt not generate thy XML output using print statements

XML::API  Uses XML Schema to generate methods  XHTML API available use XML::API::XHTML; my $x = new XML::API::XHTML(); $x>head_open(); $x>title('Test Page'); $x>head_close(); $x>body_open(); $x>div_open({id => 'content'}); $x>p('A test paragraph'); $x>div_close(); $x>body_close(); $x>_print;

XML::Writer use XML::Writer; use IO::File; my $output = new IO::File(">output.xml"); my $writer = new XML::Writer(OUTPUT => $output); $writer>startTag("greeting", "class" => "simple"); $writer>characters("Hello, world!"); $writer>endTag("greeting"); $writer>end(); $output>close();  Warning − Does not check for illegal characters; may produce incorrect XML

XML::LibXML::Tools my $dom = $lxt>complex2Dom( data => [ document => [ node => [ deeper_content => [ $tools>attribute("attribute", "value"), "deep content" ], ], node => [ "content" ] ] ] );  This is now ready to process further − eg with LibXSLT  Fails tests... needs more work?

XML::RSS  Parser/generator  Supports RSS 0.9, 0.91 and 1.0  XML::RSS::LibXML recommended − easier to extend with own namespaces − can be processed further with LibXSLT

XML::PYX  Use standard UNIX filters on XML $ pyxhtml dirty.html | pyxw > clean.html  Can clean up “dirty” HTML $ pyxhtml dirty.html | pyxw > clean.html

XML::XSH  Shell for working inside XML documents − Similar to xmllint –shell − Seems to have namespace parsing problems  Use pipes to add remote functionality xsh> ls DOC:/ │ ssh my.remote.org ’cat > test.xml’

The 10 XML Commandments A. Six days shalt thou labour, unless using XSLT

XSLT  XML::XSLT − Perl. Alpha versjon; incomplete. Dead?  XML::LibXSLT − C. Fast (twice as fast as Sablotron). xsltproc  XML::Sablotron − C++  XML::Xalan − Java? Commited to XSLT 2.0. Slower than Saxon

The 10 XML Commandments B. Thou shalt not make unto thee any more old- style HTML

Why XHTML?  Faster parsing in browser and spiders − Said to improve Google PageRank  Better suited for mobile devices − smaller memory footprint  It's the future!  XHTML 2.0 brings cool stuff − <section> and <h> for better structuring − any tag can contain href and src − XForms

XHTML requirements  Must be 100 % legal XML − Browsers will croak if illegal <img alt=”Bang & Olufsen 15" speakers”/>  Serve as application/xhtml+xml − text/html is reserved for SGML  Use correct DTD and namespace <!DOCTYPE html PUBLIC "//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml">

Template systems  Model-View-Controller (applied on web apps) − Model = your data − View = HTML markup − Controller = everything else?  MVC is only relevant for GUI applications − “Controllers contain the interface between their associated models and views and the input devices (e.g., keyboard, pointing device, time).” http://c2.com/cgi/wiki?WhatsaControllerAnyway

Web application layers Presentation Markup (HTML, WML, RSS) Logic Perl code Data Class::DBI, webservice

Separating logic from presentation  Hardcoding HTML in Perl print <<EOT <p>$name<br/>$address</p> EOT  Hardcoding Perl in HTML (Mason) <ul> % foreach $item (@list) { <li><% $item %> % } </ul>  Both are equally bad  Neither handles entity encoding

Common template systems  Must encode entities automatically  Template Toolkit − Template::Plugin::XML (hopefully) − Template::Plugin::XML::LibXML (probably)  HTML::Mason − Does not encode; has no grasp of XML  HTML::Template − Ditto

Conclusion Now you know why Jesus had 10 disciples

XML Tools for Perl

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