via http://sourcefrog.net/projects/natsort/

gistfile1.rb

# coding: utf-8 
                                                                                                                                                                                                          
module NaturalOrderStringComparison 
 
  COMPARE_EXPRESSION = /^(\D*)(\d*)(.*)$/.freeze 
 
  def compare(str1, str2) 
    str1, str2 = str1.dup, str2.dup 
 
    while (str1.length > 0) || (str2.length > 0) do 
      chars1, num1, str1 = str1.match(COMPARE_EXPRESSION).values_at(1, 2, 3) 
      chars2, num2, str2 = str2.match(COMPARE_EXPRESSION).values_at(1, 2, 3) 
 
      result = chars1 <=> chars2 
      return result if result != 0 
 
      if num1[0].ord != 48 || num2[0].ord != 48 
        num1, num2 = num1.to_i, num2.to_i 
      end 
      result = num1 <=> num2 
      return result if result != 0 
    end 
    return 0 
  end 
  module_function :compare 
end