Vanity phone number converter

vanity_phone_number_converter.rb

# Converts a phone number with letters to a phone number with only numbers.
# Leaves any other characters untouched.
class VanityPhoneNumberConverter
  LETTER_TO_NUMBER = {
    'a' => '2', 'b' => '2', 'c' => '2',
    'd' => '3', 'e' => '3', 'f' => '3',
    'g' => '4', 'h' => '4', 'i' => '4',
    'j' => '5', 'k' => '5', 'l' => '5',
    'm' => '6', 'n' => '6', 'o' => '6',
    'p' => '7', 'q' => '7', 'r' => '7', 's' => '7',
    't' => '8', 'u' => '8', 'v' => '8',
    'w' => '9', 'x' => '9', 'y' => '9', 'z' => '9'
  }
  
  attr_accessor :phone_number
  
  def initialize(phone_number)
    @phone_number = phone_number
  end
  
  def convert
    result = ""
    @phone_number.each_char do |c|
      result << (LETTER_TO_NUMBER[c.downcase] || c)
    end
    result
  end
  
  def self.convert(markdown)
    self.new(markdown).convert
  end
end