callback not functioning

[eluser]CIfan1000[/eluser]
I experimented and found the solution:

There can be no spaces between validation parameters and the separator | - I did have spaces, when I removed them, the call back worked.

$rules['LoginUsername'] = "trim|required|min_length[5]|max_length[12]|callback_username_check | xss_clean|alpha_dash";

callback_username_check |
Had to be changed to:
callback_username_check|