my @list = (
	# Good
	"\\\\files\\builds\\data\\M9998SBQCACSYD30401S",
	# Bad
	"\\\\files\\builds\\data\\M9998SBQCACSAD30401S",
);

for my $x (@list) {
	if ($x =~ /Y[A-Z]\d[^\\]*$/){
		print "$x - matches\n";
	} else {
		print "$x - doesn't match\n";
	}
}