Devise_login_hack.rb

# Original find_for_authentication from Devise authenticatable.rb

     def find_for_authentication(conditions)
        find(:first, :conditions => conditions)
     end

# Overwrite find_for_authentication and put it in your User model

  def self.find_for_authentication(conditions={})
    unless conditions[:email] =~ /^([\w\.%\+\-]+)@([\w\-]+\.)+([\w]{2,})$/i # email regex
      conditions[:nickname] = conditions.delete("email")
    end
    super
  end