Separate number by commas, format phone

Separate number by comma with using `gsub`:

def separate_by_comma(number)
  number.to_s.reverse.gsub(/(\d{3})/, '\1,').reverse
end

without:

def spearate_by_comma(number)
  number.to_s.chars.reverse.each_slice(3).map(&:join).join(",").reverse
end

Format phone number with `gsub`:

def formatPhone(number)
  numbers = number.to_s.chars
  "(###) ###-####".gsub(/#/) { numbers.shift } 
end

without:

def formatPhone(number)
  '(%d%d%d) %d%d%d-%d%d%d%d' % number.to_s.chars
end