Removing characters before form_validation

[eluser]roblarter[/eluser]
I have tried a very crude way of doing this to test that earlier, but this simply breaks CodeIgniter, I may need some guidance on how it is possible to for example str_replace strings within CodeIgniter and its callback routine

Code:

$this->form_validation->set_rules('addPostcode[]', 'Postcode', 'callback_clean_postcode');

Code:

function clean_postcode($str) {
    $str = str_replace(" ","",$str);
    return $str;
}

Cheers