controller_example.rb

  def create
    @user = User.new(params[:user])
    pw_match!
    if @user.save
      flash[:notice] = "User created"
      redirect_to admin_url
    else
      render :action => "new"
    end
  end

  def pw_match!
    matched = params[:user][:password] == params[:password_confirmation]
    @user.errors.add(:password, "doesn't match conformation") unless matched
  end