function to generate sequence from number like 992345x8xx, x being replaced by 0..9

number_series_helper.rb

  def generate_sequence(str)
    list = [str.downcase]
    while (num = list.find{|s| s.index 'x'})
      list = list + get_list(list.delete num)
    end
    list
  end

  def get_list(str)
    loc_x = str.index 'x'
    list = []
    (0..9).each {|i| list << (str[loc_x] = i.to_s and str.dup)}
    list
  end