$re = qr[\d+\D+Y\D(\d|$)];;

print '\\files\builds\data\M9998SBQCACSYD30401S' =~ $re ? 'yes' : 'no';;
yes

print '\\files\builds\data\M9998SBQCACSXD30401S' =~ $re ? 'yes' : 'no';;
no