#!perl
use strict;
use YAPE::Regex::Explain;
my $str = '&gt; milk &amp; honey &lt; < bill&ben >';

sub convert {
  my $input_stream = shift;
  return undef unless $input_stream;
  my %char_swap_hash = (
   1 => {OLD => '&[^(amp;)|(lt;)|(gt;)]', NEW => '&amp;'}, # don't match legit '&' codes
   2 => {OLD => '<', NEW => '&lt;'},
   3 => {OLD => '>', NEW => '&gt;'}
  );
  $input_stream =~ s/$char_swap_hash{$_}{OLD}/$char_swap_hash{$_}{NEW}/g for keys %char_swap_hash;
  return $input_stream;
}
print YAPE::Regex::Explain->new('&[^(amp;)|(lt;)|(gt;)]')->explain;
print convert($str),"\n";