c#任意指定字符串中的字符进行替换

Question

string str="sdfsdafdgfdgahgyt3ads";
Regex reg=new Regex("a");
 
str=reg.Replace(str,"6",1,str.IndexOf("a")+1);
 
str=reg.Replace(str,"5",1,str.IndexOf("a")+1);

Answer 1

    string ss = "asardriiosar";
          int i = 0;
          string sqs =new string( ss.Select(item =>
          {              if (item == 'r')
              {
                  i++;
                  if (i == 2)
                  {
                      item = 'E';
                  }
                 else if(i==3)
                  {
                       item='p';
                   }
              }
              return item;
          }).ToArray());

Answer 2

Regex不能在WebService里启用，只好用循环+IndexOf来处理！