Instead of using your `let` above and `model.should` here (and lines 43, 47), take advantage of the `validating_format` method that's already available (from `spec/shoulda/matchers/active_model/allow_value_matcher_spec.rb`). It'll look something like this:

     validating_format.should(:with => /abc/, :on => :customisable) allow_value("xyz").for(:attr)