print chg("ctag\n", qr/t(g)/);   # print "ctag"
"g" =~ /(c)/;
print chg("ctag\n", qr/t(g)/);   # print "ctag"

print chg("ctag\n", qr/t(g)/);   # print "ctag"
"g" =~ /(g)/;
print chg("ctag\n", qr/t(g)/);   # print "ctga"

sub chg {
	my ($str, $pat) = @_;
	$str =~ /$pat/;
	eval "\$str =~ tr/a$1/$1a/" if $1;
	return $str;
}