$a = "<exp>2a+&alpha;-b2+b&beta;2</exp><br>";
$a =~ s#(<exp>)(.*?)(<\/exp>)#$1.&expr($2).$3#egsi;

print $a;

sub expr
 {
 	my($a) = @_;
 	
 	$a =~ s#([A-Z])#<it>$1</it>#gsi; 	
 	
 	$a =~ s#(\&)(.*?)(\;)#$1.&tmp($2).$3#egsi; 	
 	
sub tmp {
 	my($tmp) = @_;
 	$tmp =~ s#(<it>|<\/it>)##gsi; 		
 	return $tmp;
 }
 	
return $a;
 	
}