$re = '/\A(([r]\S+(?!\s+[wbpro]\S+)(\s+|\Z))|([b]\S+(?!\s+[wgo]\S+)(\s+|\Z))|([g]\S+(?!\s+[bprg]\S+)(\s+|\Z))|([w]\S+(?!\s+[wb]\S+)(\s+|\Z))|([op]\S+(?!\s+[wpgo]\S+)(\s+|\Z)))*\Z/m';
$str = 'white
red
green
orange
';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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