Replacing valid_email() with filter_var()

Yes you would need to write your own or use a callback:

PHP Code:

if (!filter_var($email, FILTER_VALIDATE_EMAIL) === false) {
  echo("$email is a valid email address");
} else {
  echo("$email is not a valid email address");
} 


I would rather extend the CI_Form_Validation then write a callback. This is the best way to do this because you will have a new rule in all controllers and it's keep clean your controllers