Validating and saving a MAC address using only one format

Hi people,

I'm developing my first rails app, and I'm having a little issue with
model validations:

When tried standalone (outside rails), it works like a charm :_/ What
i'm doing wrong with exception management ?

irb(main):001:0> require_gem 'netaddr'
=> true
irb(main):002:0>
NetAddr::EUI.create('00:00:11:11:22:22').address(:Delimiter=>'.')
=> "0000.1111.2222"
irb(main):003:0>
NetAddr::EUI.create('00:00:11:11:22:22').address(:Delimiter=>'-')
=> "00-00-11-11-22-22"
irb(main):004:0>
NetAddr::EUI.create('00:00:11:11:22:22').address(:Delimiter=>':')
=> "00:00:11:11:22:22"
irb(main):005:0>

Also tried with ActiveRecord callbacks used in the model:

def after_validation
  self.mac = NetAddr::EUI.create(mac).address(:Delimiter=>'.')
  rescue NetAddr::ValidationError
    errors.add(mac,"Incorrect MAC address format")
end

But the ValidationError never raises :-/ Any bet ?

Thanks