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http://www.xml.com/pub/a/2002/04/17/perl-xml.html
by Kip Hampton April 17, 2002
Over the last several months we have explored some the of ways that Perl's XML modules can by used to create complex, modern Web publishing systems. Also, the growing success of projects like AxKit, Bricolage, and others shows the combination of Perl and XML to be quite capable for creating large-scale applications. However, in looking at more conceptual topics here recently, together with the fact that the Perl/XML combination is often seen in complex systems, seems to give the impression to the larger Perl community that processing XML with Perl tools is somehow complex and only worth the effort for big projects.
The truth is that putting Perl's XML processing facilities to work is no harder than using any other part of Perl; and if the applications that feature Perl/XML in a visible way are complex, it is because the problems that those applications are designed to solve are complex. To drive this point home, this month we will get back to our Perlish roots by examining how Perl can be used on the command line to perform a range of common XML tasks.
For our first few examples we will focus on those modules that ship with command line tools as part of their distributions.
Requires: XML::Path, XML::Parser
Matt Sergeant's fine XML::XPath module provides a way access the contents of XML documents using the W3C-recommended XPath Language. This module also installs a Perl utility called xpath, which allows XPath expressions to be used to examine the contents of XML documents. The XML document can be specified either by passing in a path to the file as the first argument or by piping the document via STDIN.
Find all section titles in a DocBook XML:
xpath mybook.xml //section/title
The same command using a pipe:
cat files/mybook.xml | xpath //section/title
Retrieve just the significant text (not including nodes containing all-whitespace) from a given document:
xpath somefile.xml "//text()[string-length(normalize-space(.)) > 0 ]"
Requires DBIx::XML_RDB, DBI
Fans of Matt's popular DBIx::XML_RDB module will be pleased to know that it too ships with a command line tool, sql2xml, that returns an entire database table as a single XML document.
Requires: XML::Handler::YAWriter, XML::Parser::PerlSAX
No matter how carefully XML document are edited, they often need reformatting to be reasonably called "human-readable". Michael Koehne's XML::Handler::YAWriter SAX Handler installs an XML pretty-printer called xmlpretty which reduces this task to a quick one-liner.
Requires: XML::SemanticDiff, XML::Parser
Unfortunately, standard command line text-processing tools like diff often fall short when dealing with XML documents. My XML::SemanticDiff was designed to make comparing the relevant parts of two XML documents (while ignoring things like extra whitespace, or having the same namespace URI bound to different prefixes) easy and straightforward. Newer versions of this module install the xmlsemdiff tool, which allows simple access from the shell.
The Apache Software Foundation's Xerces-Perl project offers a Perl interface to the Xerces C++ XML parser. Xerces-Perl ships with several sample scripts that can be copied into your favorite bin directory. The most notable difference between Xerces and the other XML parsers available to Perl is that it provides a way to validate XML documents against W3C XML Schemas.
Developers using XML::LibXML often aren't aware of the feature-rich command line XML processing tool, xmllint, which is installed with the C libraries that XML::LibXML depends upon. No, xmllint is not a Perl tool, but its many features, and the fact that it can be easily piped together with other tools, makes it more than worthy of mention here.
Use the built-in HTML parser to convert ill-formed HTML to XML before further processing:
xmllint --html khampton_perl_xml_17.html | xpath "//a[@href]"
Or the same thing, but using the DocBook SGML parser:
xmllint --sgml ye-olde.sgml | xpath "//chapter[@id='chapt4']"
Using xmllint as a pretty-printer:
cat some.xml | xmllint --format
Using xmllint to validate a document against an external DTD:
cat some.xml | xmllint --postvalid --dtdvalid my.dtd
Requires: Devel::TraceSAX, XML::SAX, XML::SAX::Machines
While the syntax may be a bit verbose, it is entirely possible to use XML::SAX::Machines to bring the power of Perl SAX2 to the command line.
Using XML::SAX::Machines to produce an XML document to STDOUT after applying a SAX filter:
perl -MXML::SAX::Machines=Pipeline -e
'Pipeline("XML::MyFilter", \*STDOUT)->parse_uri("files/camelids.xml");'Or, reading from STDIN,
cat files/camelids.xml | perl -MXML::SAX::Machines=Pipeline -e
'Pipeline("XML::MyFilter", \*STDOUT)->parse_string(join "", <STDIN>);'It is often very helpful when writing custom SAX Filters to be able to examine what events are being generated and forwarded to which classes. Barrie Slaymaker's Devel::TraceSAX makes this painless.
Debugging SAX events by tracing them through multiple filters:
perl -d:TraceSAX -MXML::SAX::Machines=Pipeline -e
'Pipeline("XML::Filter1", "XML::Filter2")->parse_uri("file.xml");'Processing XML with Perl does not have to mean buying into a huge XML-centric application with a steep learning curve or departing from Perl's long history as a command line tool. You may not use all of the tools or techniques described here, but it is nice to know that they are available when and if you need them.
Download the sample code. http://www.xml.com/2002/04/17/examples/kip.zip
2002-04-26 03:37:55 Grant McLean
Thanks Kip, I certainly picked up few handy tips from this article.
Matt's XML::PYX is another module that provides command line functionality. For example this one-liner prints statistics on how many times each element type occurs in a document:
pyx doc.xml | sed -n 's/^(//p' | sort | uniq -c
2002-04-23 06:04:52 Matt Sergeant [Reply]
The perl XML tools are very cool for screen scraping. Here's something that gets you the top-ten viruses from our virus-eye service:
lwp-request http://www.messagelabs.com/VirusEye/ | perl -pe 's/--->/-->/' | xmllint --html --format - | xpath '/html/body/table[2]/tr[1]/td[1]/table[1]/tr[1]/td[2]/table[1]/tr[3]/td[2]/table[3]/tr[1]/td[1]/table[1]/tr[2]/td[1]/table[1]/tr[1]/td[2]/table[1]/tr/td[3]/a/text()'
(note that normally you wouldn't need the perl -pe stuff to fix up the broken HTML assuming it parses cleanly - I think this may be a bug in libxml2)
2003-09-14 20:43:42 Mikhail Grushinskiy
> Thanks for this article. I have been looking for this type of functionality > for a while and thought I would have to roll my own. Now I can return to > being lazy!
You might also take a look at this one (XmlStarlet Command Line XML Toolkit)
http://xmlstar.sourceforge.net/
The toolkit's feature set includes options to: