best way to add validation functions

[eluser]ontguy[/eluser]
I think extending the validation class is the best way to centralize callbacks:

So I could have a username_duplicate_check callback function included in MY_Validation.php, then use it in the validation rules:

Code:

$rules['username'] = 'trim|required|username_duplicate_check';

I have been giving some thought about where to centralize rules for fields used in multiple controllers; I think a config file maybe the place to do it or maybe even putting it MY_Validation and overriding it if needed.