Example 20-13. hrefsub

  #!/usr/bin/perl -w
  # hrefsub - make substitutions in <A HREF="..."> fields of HTML files
  # from Gisle Aas <gisle@aas.no>
  
  sub usage { die "Usage: $0 <from> <to> <file>...\n" }
  
  my $from = shift or usage;
  my $to   = shift or usage;
  usage unless @ARGV;
  
  # The HTML::Filter subclass to do the substitution.
  
  package MyFilter;
  use HTML::Filter;
  @ISA=qw(HTML::Filter);
  use HTML::Entities qw(encode_entities);
  
  sub start {
     my($self, $tag, $attr, $attrseq, $orig) = @_;
     if ($tag eq 'a' && exists $attr->{href}) {
             if ($attr->{href} =~ s/\Q$from/$to/g) {
                 # must reconstruct the start tag based on $tag and $attr.
                 # wish we instead were told the extent of the 'href' value
                 # in $orig.
                 my $tmp = "<$tag";
                 for (@$attrseq) {
                     my $encoded = encode_entities($attr->{$_});
                     $tmp .= qq( $_="$encoded ");
                 }
                 $tmp .= ">";
                 $self->output($tmp);
                 return;
             }
     }
     $self->output($orig);
  }
  
  # Now use the class.
  
  package main;
  foreach (@ARGV) {
          MyFilter->new->parse_file($_);
  }