Console.WriteLine(s.ToUpper());//转换成大写
Console.WriteLine(s.ToLower());//转换成小写

public static void fanxiang(string s)
  {
   char[] arrey = s.ToCharArray();
   StringBuilder s1 = new StringBuilder("");
   for (int i = arrey.Length - 1; i >= 0; i--)
   {
    s1.Append(Convert.ToString(arrey[i]));
   }
   Console.WriteLine("反向字符串为{0}",s1);
  }

public static void pipei(string s)
  {
   int count = 0;
   int i;
   Console.WriteLine("请输入短字符串");
   string s2 = Console.ReadLine();
   while ((i=s.IndexOf(s2)) >= 0)
   {
    count++;
    s = s.Substring(i + s2.Length);
   }
   Console.WriteLine("字符串中出现了{0}次{1}", count, s2);
  }

public static void zzpipei(string s)
  {
   Console.WriteLine("请输入正则表达式");
   string zz = Console.ReadLine();
   Regex re = new Regex(zz);
   string s2 = "";
   if (re.IsMatch(s))
   {
    Console.WriteLine("匹配成功");
    MatchCollection mc = re.Matches(s);
    foreach (Match ma in mc)
    {
     s2 += ma.Value;
     s2 += ("\r\n");
    }
    Console.WriteLine("一行为一个匹配结果");
    Console.WriteLine(s2);
   }
   else
   { Console.WriteLine("无匹配结果"); }
  }

 public static void basejiami(string s)
  {
   byte[] bytes = Encoding.Default.GetBytes(s);
    Console.WriteLine("字符串base64加密为{0}", Convert.ToBase64String(bytes));
  }

 public static void basejiemi(string s)
  {
   byte[] bytes = Convert.FromBase64String(s);
    Console.WriteLine("字符串base64解密为{0}", Encoding.Default.GetString(bytes));
  }

public static void rotjm(string s)
  {
   string jmzf = "";//解密加密后的字符串
   char[] arrey = s.ToCharArray();
   Console.WriteLine("字符串长度为{0}", arrey.Length);
   for (int i = 0; i < arrey.Length; i++)
   {
    int zfcode = (int)arrey[i];
    if (zfcode >= 97 && zfcode <= 109)
     zfcode = zfcode + 13;
    else if (zfcode >= 110 && zfcode <= 122)
     zfcode = zfcode - 13;
    else if (zfcode >= 65 && zfcode <= 77)
     zfcode = zfcode + 13;
    else if (zfcode >= 78 && zfcode <= 90)
     zfcode = zfcode - 13;
    jmzf = jmzf + (char)zfcode;
   }
   Console.WriteLine("结果为{0}", jmzf);
  }

public static void thzf(string s)
  {
   Console.WriteLine("请输入想要被替换的字符串");
   string str1 = Console.ReadLine();
   Console.WriteLine("请输入想要替换成的字符串");
   string str2 = Console.ReadLine();
   Console.WriteLine(s.Replace(str1, str2));
  }

public static void md5jm(string s)
  {
   MD5 md5 = new MD5CryptoServiceProvider();
   //将字符编码为字节序列
   byte[] data = System.Text.Encoding.Default.GetBytes(s);
   byte[] md5data = md5.ComputeHash(data);
   md5.Clear();
   //遍历加密数组,加密字节，该方法为32位加密
   string str = "";
   for (int i = 0; i < md5data.Length; i++)
   {
    str += md5data[i].ToString("x").PadLeft(2, '0');
   }
   Console.WriteLine("加密结果为{0}",str);
  }