User Registration Password field - limiting allowed characters?

I've got my user registration script up & running but what was wondering how I could restrict the user password creation field to something like "please enter 5 to 8 characters, 3 of which must be numbers and the rest alphabetical" with no other characters such as $, @, etc.
Any help would be appreciated.

 

You can use javascript for this purpose.

 

$pass = $_POST['pass'];

$err = '';
$min = 5;
$max = 8;

if (strlen($pass) < $min) {
   $err = 'too short';
} else if (strlen($pass) > $max) {
   $err = 'too long';
} else if (!preg_match('/^([a-z0-9]+)+$/i', $pass)) {
   $err = 'invalid char';
}

PHP: