Rails three has made it very easy to write custom validators. Ryan Bates has a beautiful example of how to use these validators here (make sure to watch this or read the ascii cast). He uses the following example to illustrate how to create a custom validator for checking the format of an email address:

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class EmailFormatValidator < ActiveModel::EachValidator
  def validate_each(object, attribute, value)
    unless value =~ /^([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})$/i
      object.errors[attribute] << (options[:message] || "is not formatted properly")
    end
  end
end

This does a good job but is not very useful for an international site. The above code can be rewritten as follows:

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class EmailFormatValidator < ActiveModel::EachValidator
  def validate_each(object, attribute, value)
    unless value =~ /^([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})$/i
      object.errors.add(attribute, (options[:message] || :email_format))
    end
  end
end

Essentially, we replace the string ‘is not formatted properly’ with the symbol :email_format. This symbol matches the type of validation we are performing.

In addition to this, add the following to the en.yml file of your rails app:

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en:
  errors:
    messages:
      email_format: "is not formatted properly"

This way you can customize translations not only for the built in rails validators but also custom ones you define yourself.

Feel free to use the code above subject to the following license