Pretty printing a phone number from a string

pretty_phone_number.rb

  def pretty_phone_number(str_num)
    # I know how to handle 9 digit numbers
    # I know how to handle 10 digit numbers
    # for above that I only know how to divide into 3s

    result = ""
    case
      when str_num.length==10
        result << '('+str_num[0..2]+') '+str_num[3..5]+'-'+str_num[6..9]
      when str_num.length == 11 && str_num =~ /^1/
        # assume north america and ignore the 1
        result << '('+str_num[1..3]+') '+str_num[4..6]+'-'+str_num[7..10]
      when str_num.length == 11 && str_num =~ /^[^1]/
        result << '+'+str_num[0, 1]+' ('+str_num[1..3]+') '+str_num[4..6]+'-'+str_num[7..10]
      else
        str_num.chars.each_with_index do |c, i|
          result << c
          result << '-' if i%3==2 &&(i+3<str_num.length)
        end
    end
    return result
  end