so I am trying to write a regular expression lets just take this case for an example: OR an empty string, how would I add that part?

I tried stuff like: but to no avail, any ideas?

My suggestion would be to avoid putting so much overhead in the regular expression engine for such a simple test. You can accomplish what you want in PHP by doing something like:
[PHP]
# Assuming you are testing $string...
if(preg_match('/^[0-9a-z]{5,9}$/',$string) || strlen($string) < 1) {
# Do things here.
}
[/PHP]
How about that?

^[0-9a-z]{5,9}$|^$