We all agree that Perl does a really good job when it comes to text extraction, particulary with regular expressions.
The XML is based on text, so one might think that it would be dead easy to take any XML input and have it converted in the way one wants.
Unfortunately, that is wrong. If you think you'll be able to parse a XML file with your own homegrown parser you did overnight, think again, and look at the XML specs closely. It's as complex as the CGI specs, and you'll never want to waste precious time trying to do something that will surely end up wrong anyway. Most of the background discussions on why you have to use instead of your own CGI-parser apply here.

The aim of this tutorial is not to show you how XML should be structured and why you shouldn't parse it by hand but how to use the proper tool to do the right job.
I'll focus on the most basic XML module you can find, XML::Parser. It's written by Larry Wall and Clark Cooper, and I'm sure we can trust the former to make good software (rn and patch are his most famous programs)
Okay, enough talk, let's jump into the module!

This tutorial will only show you the basics of XML parsing, using the easiest (IMHO) methods. Please refer to the perldoc XML::Parser for more detailed info.
I'm aware that there are a lot of XML tools available, but knowing how to use XML::Parser can surely help you a lot when you don't have any other module to work with, and it also helped me to understand how other XML modules worked, since most of them are built on top of XML::Parser.
The example I'll use for this tutorial is the Perlmonks Chatterbox ticker that some of you may have already used. It looks like this:

<CHATTER><INFO site="" sitename="Perl Monks">
Rendered by the Chatterbox XML Ticker</INFO>
	<message author="OeufMayo" time="20010228112952">
	<message author="deprecated" time="20010228113142">
	<message author="OeufMayo" time="20010228113153">
/me test again; :)</message>
	<message author="OeufMayo" time="20010228113255">
&lt;a href="#"&gt;please note the use of HTML 

Thanks to deprecated for his unaware intervention here

( The astute reader will notice that in the CB ticker, a 'user_id' has shown up recently. Since it wasn't there when I took my 'snapshot' of the CB, I'll ignore it, but don't worry the code below won't break at all, precisely because I used a proper parser to handle that for me! )

Let's assume we want to output this file in a readable way (though it'll still be barebone). It doesn't handles links and internal HTML entities. It only gets the CB ticker, parses it and prints it, you have to launch it again to follow the wise meditations and the brilliant rethoric of the other fine monks present at the moment.

1 #!/usr/bin/perl -w 2 use strict; 3 use XML::Parser; 4 use LWP::Simple; # used to fetch the chatterbox ticker 5 6 my $message; # Hashref containing infos on a message 7 8 my $cb_ticker = get(" +xml+ticker"); 9 # we should really check if it succeeded or not 10 11 my $parser = new XML::Parser ( Handlers => { # Creates our parse +r object 12 Start => \&hdl_start, 13 End => \&hdl_end, 14 Char => \&hdl_char, 15 Default => \&hdl_def, 16 }); 17 $parser->parse($cb_ticker); 18 19 # The Handlers 20 sub hdl_start{ 21 my ($p, $elt, %atts) = @_; 22 return unless $elt eq 'message'; # We're only interrested in +what's said 23 $atts{'_str'} = ''; 24 $message = \%atts; 25 } 26 27 sub hdl_end{ 28 my ($p, $elt) = @_; 29 format_message($message) if $elt eq 'message' && $message && $ +message->{'_str'} =~ /\S/; 30 } 31 32 sub hdl_char { 33 my ($p, $str) = @_; 34 $message->{'_str'} .= $str; 35 } 36 37 sub hdl_def { } # We just throw everything else 38 39 sub format_message { # Helper sub to nicely format what we got fro +m the XML 40 my $atts = shift; 41 $atts->{'_str'} =~ s/\n//g; 42 43 my ($y,$m,$d,$h,$n,$s) = $atts->{'time'} =~ m/^(\d{4})(\d{2})( +\d{2})(\d{2})(\d{2})(\d{2})$/; 44 45 # Handles the /me 46 $atts->{'_str'} = $atts->{'_str'} =~ s/^\/me// ? 47 "$atts->{'author'} $atts->{'_str'}" : 48 "<$atts->{'author'}>: $atts->{'_str'}"; 49 $atts->{'_str'} = "$h:$n " . $atts->{'_str'}; 50 print "$atts->{'_str'}\n"; 51 undef $message; 52 }

Step-by-step code walkthrough:

Lines 1 to 4
Initialisation of the basics needed for this snippet, XML::Parser, of course, and LWP::Simple to get the chatterbox ticker.
Line 8
LWP::Simple get the requested URL, and put the content of the page in the $cb_ticker scalar.
Lines 11 to 16

The most interesting part, no doubt. We create here a new XML::Parser object. The Parser can come in different styles, but when you have to deal with simple data, like the CB ticker, the Handlers way is the easiest (see also the Subs style, as it is really close to this one).

For this object, we define four handlers subs, each representing a different state in the parsing process.

  • The 'Start' handler is called whenever a new element (or tag, HTML-wise) is found. The sub given is called with the expat object, the name of the element, and a hash containing all the atrributes of this element.
  • The 'End' is called whenever an element is closed, and is called with the same parameters as the 'Start', minus the attributes.
  • The 'Char' handler is called when the parser finds something which is not mark-up (in our case, the text enclosed in the <message> tag).
  • Finally, the 'Default' handler is called, well, by default, when anything else matching the three other handlers is called.
Line 17
The line that does all the magic, parsing and calling all your subs for you at the right moment.
Lines 20-25: the Start handler

We only want to deal with the <message> elements (those containing what it is being said in the Chatterbox) so we'll happily skip every other element.

We got a hash with the attributes of the element, and we're going to use this hash to store the string that will contain the text to be displayed in the $atts{'_str'}

Lines 27-30: the End handler
Once we've reached the end of a message element, we format all the info we have gathered and prints them via the format_message sub.
Lines 32-35: the Char handler
This sub gets all the strings returned by the parser and appends it to the string to be finally displayed
Line 37: the Default handler
It does nothing, but it doesn't have to figure out what to do with this!
Lines 39-52
This subroutine mangles all the info we got from the XML file, with bad regexes and all, and prints the formatted text in a hopefully readable way. Please note that XML::Parser handled all of the decoding of the &lt; and &gt; entities that were included in the original XML file

We now have a complete and simple parser, ready to analyse, extract, report everything inside the Chatterbox XML ticker!

That's all for now, here are some links you may find useful:

Thanks to mirod, arhuman and danger for the review!