The Ruby Way: Solutions and Techniques in Ruby Programming, Second Edition

5.6. Formatting Numbers with Commas

There may be better ways to do it, but this one works. We reverse the string to make it easier to do global substitution and then reverse it again at the end:

def commas(x)
  str = x.to_s.reverse
  str.gsub!(/([0-9]{3})/,"\\1,")
  str.gsub(/,$/,"").reverse
end

puts commas(123)        # "123"
puts commas(1234)       # "1,234"
puts commas(12345)      # "12,435"
puts commas(123456)     # "123,456"
puts commas(1234567)    # "1,234,567"