$input=~s#(<exp>)(.+?)(</exp>)#$1.Replace($2).$3#eg;

sub Replace
{
	my ($line)=@_;
	$line=~s#([a-zA-Z]+)#<it>$1</it>#g;	
	$line=~s#(&[0-9]*)<it>([^;]+)</it>(;)#$1$2$3#g;
	return $line;
}