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20.3. Extracting URLs

Problem

You want to extract all URLs from an HTML file.

Solution

Use the HTML::LinkExtor module from CPAN:

use HTML::LinkExtor;

$parser = HTML::LinkExtor->new(undef, $base_url);
$parser->parse_file($filename);
@links = $parser->links;
foreach $linkarray (@links) {
    my @element = @$linkarray;
    my $elt_type = shift @element;                  # element type

    # possibly test whether this is an element we're interested in
    while (@element) {
        # extract the next attribute and its value
        my ($attr_name, $attr_value) = splice(@element, 0, 2);
        # ... do something with them ...
    }
}

Discussion

You can use HTML::LinkExtor in two different ways: either to call links to get a list of all links in the document once it is completely parsed, or to pass a code reference in the first argument to new . The referenced function will be called on each link as the document is parsed.

The links method clears the link list, so you can call it only once per parsed document. It returns a reference to an array of elements. Each element is itself an array reference with an HTML::Element object at the front followed by a list of attribute name and attribute value pairs. For instance, the HTML:

<A HREF="http://www.perl.com/">Home page</A>
<IMG SRC="images/big.gif" LOWSRC="images/big-lowres.gif">

would return a data structure like this:

[
  [ a,   href   => "http://www.perl.com/" ],
  [ img, src    =>"images/big.gif",
         lowsrc => "images/big-lowres.gif" ]
]

Here's an example of how you would use the $elt_type and the $attr_name to print out and anchor an image:

if ($elt_type eq 'a' && $attr_name eq 'href') {
    print "ANCHOR: $attr_value\n" 
        if $attr_value->scheme =~ /http|ftp/;
}
if ($elt_type eq 'img' && $attr_name eq 'src') {
    print "IMAGE:  $attr_value\n";
}

Example 20.2 is a complete program that takes as its arguments a URL, like file:///tmp/testing.html or http://www.ora.com/, and produces on standard output an alphabetically sorted list of unique URLs.

Example 20.2: xurl

#!/usr/bin/perl -w
# xurl - extract unique, sorted list of links from URL
use HTML::LinkExtor;
use LWP::Simple;

$base_url = shift;
$parser = HTML::LinkExtor->new(undef, $base_url);
$parser->parse(get($base_url))->eof;
@links = $parser->links;
foreach $linkarray (@links) {
    my @element  = @$linkarray;
    my $elt_type = shift @element;
    while (@element) {
        my ($attr_name , $attr_value) = splice(@element, 0, 2);
        $seen{$attr_value}++;
    }
}
for (sort keys %seen) { print $_, "\n" }

This program does have a limitation: if the get of $base_url involves a redirection, your links will all be resolved with the original URL instead of the URL at the end of the redirection. To fix this, fetch the document with LWP::UserAgent and examine the response code to find out if a redirection occurred. Once you know the post-redirection URL (if any), construct the HTML::LinkExtor object.

Here's an example of the run:

% xurl http://www.perl.com/CPAN




ftp://ftp@ftp.perl.com/CPAN/CPAN.html








http://language.perl.com/misc/CPAN.cgi








http://language.perl.com/misc/cpan_module








http://language.perl.com/misc/getcpan








http://www.perl.com/index.html








http://www.perl.com/gifs/lcb.xbm



Often in mail or Usenet messages, you'll see URLs written as:

<URL:http://www.perl.com>

This is supposed to make it easy to pick URLs from messages:

@URLs = ($message =~ /<URL:(.*?)>/g);











See Also

The documentation for the CPAN modules LWP::Simple, HTML::LinkExtor, and HTML::Entities; Recipe 20.1











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