open(DOC, "<$doc_filename") || die "can't open doc $doc_filename: $!";
    binmode(DOC);
    $document = <DOC>;
    close(DOC);

    while ($document =~ m{< ?(.*?) ?>}g) {
      $tag = $1;
                                        # find tags with links
      $link =
        ($tag =~ m{
                    (background|src|usemap|action|href)\s?=\s?
                    (['"]*)
                    ([^\2 ]+)\2
                 }xi)[2];
      next unless (defined $link);