$re = '/[1-578]\d\d|(?:0\d[1-9]|0[1-9]\d)|(?:6\d[0-57-9]|6[0-57-9]\d)/';
$str = 'I am supposed to exclude the numbers 000, 666, and all numbers between 900-999.
001
000
002
100
600
660
666
606
900
899
999
';
preg_match_all($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