单词单复数转换函数

/// <summary>
        /// 单词单复数转换函数,本函数对2个或2个字母以上单词有效
        /// </summary>
        /// <param name="world">待转换的单词</param>
        /// <returns>返回一个长度为2的字符串数组,第一个值为单数单词,第二个为复数单词</returns>
        public static string[] worldTransform(string world)
        {
            world = world.ToLower();//将单词转成小写形式
            string vowel = "aoeiu";//声明元音字符串
            int length = world.Length;//获取单词字符长度待用
            string[] worlds = new string[2];//声明一个二维字符串数组
            if (length == 1)
            {
                worlds[0] = world;//存储单词单数形式
                worlds[1] = world;//存储单词复数形式
                return worlds;
            }
            else
            {
                char last = world[length - 1];//取出单词最后一个字符待用
                char last2 = world[length - 2];//取出单词倒数第二个字符待用
                char last3 = ' ';
                if (length >= 3)
                    last3 = world[length - 3];//取出单词倒数第三个字符待用
                char last4 = ' ';
                if (length >= 4)
                    last4 = world[length - 4];//取出单词倒数第四个字符待用
                worlds[0] = world;//存储单词单数形式
                worlds[1] = world;//存储单词复数形式


                if (length == 1)
                {
                    return worlds;
                }
                if (last != 's' || length <= 2)
                {
                    //单词最后一个字母不为"s"，则可判断为单数形式
                    switch (last)
                    {
                        case 'y':
                            if (vowel.IndexOf(last2) < 0)
                            {
                                worlds[1] = worlds[1].Substring(0, length - 1) + "i";
                                worlds[1] += "es";
                            }
                            else
                                worlds[1] += "s";
                            break;
                        case 'f':
                            worlds[1] = worlds[1].Substring(0, length - 1) + "v";
                            worlds[1] += "es";
                            break;
                        case 'e':
                            if (last2 == 'f')
                            {
                                worlds[1] = worlds[1].Substring(0, length - 2) + "ves";
                            }
                            else
                                worlds[1] += "s";
                            break;
                        case 'o':
                            worlds[1] += "es";
                            break;
                        case 'z':
                            worlds[1] += "es";
                            break;
                        case 's':
                            worlds[1] += "es";
                            break;
                        case 'x':
                            worlds[1] += "es";
                            break;
                        case 'h':
                            if (last2 == 'c' || last2 == 's')
                                worlds[1] += "es";
                            else
                                worlds[1] += "s";
                            break;
                        default:
                            worlds[1] += "s";
                            break;
                    }
                    return worlds;
                }
                else
                {
                    if (last2 == 'e')
                    {
                        //单词最后两个字母为“es”，则可判断其为复数形式
                        if (last3 == 'z' || last3 == 'x' || last3 == 's' || last3 == 'h' || last3 == 'o')
                            worlds[0] = worlds[0].Remove(length - 2);
                        else if (last3 == 'i')
                        {
                            if (length >= 4)
                            {
                                if (vowel.IndexOf(last4) < 0)
                                {
                                    worlds[0] = worlds[0].Remove(length - 2);
                                    worlds[0] = worlds[0].Substring(0, length - 3) + "y";
                                }
                                else
                                    worlds[0] = worlds[0].Remove(length - 2);
                            }
                        }
                        else if (last3 == 'v')
                        {
                            worlds[0] = worlds[0].Remove(length - 3);
                            worlds[0] += "f";//此步骤会产生误差。因为大多数的“f”、“fe”结尾单词中多以“f”结尾(还没有调查过)，顾忽略“fe”。要避免误差只能用词库了。
                        }
                        else
                        {
                            worlds[0] = worlds[0].Remove(length - 1);
                        }
                    }
                    else
                    {
                        worlds[0] = worlds[0].Remove(length - 1);//此步骤误差最大，当单词最后一个字母为s时，很难判断它是复数还是单数，目前还没有找到判断的规律，估计要用到词库此才行。
                    }
                    return worlds;
                }
            }
        }

http://www.cnblogs.com/smalltree/archive/2009/02/14/1390631.html