email_backend.rb

require 'email_re'

#I assume this is monkey patching a BasicBackend class 
#that is already defined in Django?
#Or else you would just define both methods in EmailBackend?
class BasicBackend
  def self.get_user(id)
    User.find(id)
  rescue ActiveRecord::RecordNotFound
    nil
  end
end

class EmailBackend < BasicBackend
  def self.authenticate(username, password)
  
    if EMAIL_RE.match(username)
      user = find_by_email(username)
      return nil unless user
    else
      user = find_by_username(username)
      return nil unless user
    end
    user if user.check_password(password)  
  end
end

email_re.rb

EMAIL_RE = /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i

user.rb

require 'email_re'
class User < ActiveRecord::Base
  def self.authenticate(login, password)
    user = if EMAIL_RE.match(login)
      find_by_email(login)
    else 
      find_by_username(login)
    end
    return user if user and user.check_password(password)
  end
end