$re = '/[^\sGWDF](?!.?4)[0-9DTSRF]+/';
$str = 'G4 G4 G7 G9 G9 R3 B4 RD2 R3 YD2 R4 R5 RR W BDT Y0 YS G8 WDF
R3 RD2 R3 YD2 R5 RR BDT Y0 YS';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php