$input=':<exp>2aF+-b&amp;2z4676</exp><br>';

$input =~ s/(<exp>)(.*?)(<\/exp>)/$1.&exp($2).$3/egsi;

print $input;

sub exp
{
my ($a)=@_;

$a =~ s/[a-z]+/<it>$&<\/it>/gsi;

$a =~ s/(&)(.*?)(;)/$1.&amp($2).$3/egsi;

sub amp
{
my ($m)=@_;

$m =~ s/<\/?it>//gsi;
return($m);

}

return($a);
}

output:
:<exp>2<it>aF</it>+-<it>b</it>&amp;2<it>z</it>4676</exp><br>