$re = '/^
(?=.{2,}) # assert length of two or more?
(?:a(?=.*))*
(?:b(?=.*[^a].*))*
(?:c(?=.*[^a-b].*))*
(?:d(?=.*[^a-c].*))*
# add in the rest...
$/mx';
$str = 'abbbcccd
baab
abc
abd
adb
ab
a
';
preg_match_all($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