Removing characters before form_validation

[eluser]roblarter[/eluser]
I am fairly new to CodeIgniter but have a lot of experience with PHP

The problem I have is that I have an postcodes being submitted

Code:

<input name="addPostcode[]">

I want to remove spaces from the submitted field array before running the validation:

Code:

$this->form_validation->set_rules('addPostcode[]', 'Postcode', 'trim|alpha_numeric|xss_clean');

essentially the validation rule will cover anything other than spaces obviously as it only allows alpha_numeric but I want to ensure that it will "allow" spaces from the submitted user but just remove the space without giving an error to the user.

Any help would be greatly appreciated