my %htm = (
 '&' => '&',
 '>' => '>',
 '<' => '&lt;',
);

my $REx = qr'(&(?!(?:amp|lt|gt);)|[><])'; 
$str =~ s/$REx/$htm{$1}/g;
print $str."\n\n";
print YAPE::Regex::Explain->new($REx)->explain;