Random Walk - Perl

Converted your little black book into XML, and don't know what to do next? This article gets you started on the path to being an XML guru, demonstrating how to use Perl's SAX parser to parse and convert your XML into Web-friendly HTML.

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By: icarus, (c) Melonfire

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January 15, 2002

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In addition to elements and CDATA, Perl also allows you to set up handlers for other types of XML structures, most notably PIs, entities and notations (if you don't know what these are, you might want to skip this section and jump straight into another, more complex example on the next page). As demonstrated in the previous example, handlers for these structures are set up by specifying appropriate callback functions via a call to the setHandlers() object method.

Here's a quick list of the types of events that the parser can handle, together with a list of their key names (as expected by the setHandlers() method) and a list of the arguments that the corresponding callback function will receive.


Key        Arguments                     Event
to callback
------------------------------------------------------------------------
Final      parser handle                 Document parsing completed
Start      parser handle,                Start tag found
element name,
attributes
End        parser handle,                End tag found
element name
Char       parser handle,                CDATA found
CDATA
Proc       parser handle,                PI found
PI target,
PI data
Comment    parser handle,                Comment found
comment
Unparsed   parser handle, entity,        Unparsed entity found
base, system ID, public
ID, notation
Notation   parser handle, notation,      Notation found
base, system ID, public
ID
XMLDecl    parser handle,                XML declaration found
version,  encoding,
standalone
ExternEnt  parser handle, base,          External entity found
system ID, public ID
Default    parser handle, data           Default handler

As an example, consider the following example, which uses a simple XML document,

<?xml version="1.0"?>
<random>
<?perl print rand(); ?>
</random>

in combination with this Perl script to demonstrate how to handle processing instructions (PIs):

#!/usr/bin/perl
# include package
use XML::Parser;
# initialize parser
$xp = new XML::Parser();
# set PI handler
$xp->setHandlers(Proc => \&pih);
# output some HTML
print "Content-Type: text/html\n\n";
print "<html><head></head><body>And the winning number is: ";
$xp->parsefile("pi.xml");
print "</body></html>";
# this is called whenever a PI is encountered
sub pih()
{
# extract data
my ($parser, $target, $data) = @_;
# if Perl command
if (lc($target) == "perl")
{
# execute it
eval($data);
}
}
# end

In this case, the setHandlers() method knows that it has to call the subroutine pih() when it encounters a processing instruction in the XML data; this user-defined pih() function is automatically passed the PI target and the actual command to be executed. Assuming the command is a Perl command - as indicated by the target name - the function passes it on to eval() for execution.

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