Наложение маски на строку произвольной длины

gistfile1.rb

#!ruby

module Masked
  PLACEHOLDER = '@'
  SPLITTERS = '-'

  def self.render(mask, string)
    str = string.chars
    result = mask.chars.map do |c|
      case
        when str.size == 0 && (c == PLACEHOLDER) then nil
        when c == PLACEHOLDER then str.slice!(0)
        else c
      end
    end
    result.push(str).join.gsub(/([#{ SPLITTERS }])+$/, '')
  end
end

puts Masked.render('@@@', '5432') == '5432'
puts Masked.render('@-@@', '5432') == '5-432'
puts Masked.render('@-@@', '@432') == '@-432'
puts Masked.render('РОС@@-@@-@@@', '5432') == 'РОС54-32'
puts Masked.render('РОС@@-@@-@@@', '42') == 'РОС42'
puts Masked.render('@-@@-TAIL', '5432') == '5-43-TAIL2'
puts Masked.render('@-@@-@@@@@@@@@-TAIL', '5432') == '5-43-2-TAIL'

gistfile2.txt

 ➜  masked_test rvm:(2.1.5@rails)  ruby ./masked.rb
true
true
true
true
true
true
true