$re = '/\/dev\/sd[a-z][a-z]?(?<!\/dev\/sda)/';
$str = '/dev/sda
/dev/sdb
/dev/sdc
/dev/sdd
/dev/sde
/dev/sdaa
/dev/sdab
';
$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