在一段字符串中另一字符串出現的指定的位置插入文字

   string OldFrom=this.neirong.Text.Trim();
    //.Replace("/r/n","<br>");
   OldFrom="<br>"+OldFrom.Replace("/r/n","<br>");
   OldFrom=Regex.Replace(OldFrom,@"<br>[/s]*","<br>",RegexOptions.IgnoreCase);
   OldFrom=Regex.Replace(OldFrom,@"<br><br>","<br>",RegexOptions.IgnoreCase);
   OldFrom=Regex.Replace(OldFrom,@"<br><br>","<br>",RegexOptions.IgnoreCase);
   string[] a=Regex.Split(OldFrom,"<br>");
   if(a.Length>2)
   {
    OldFrom=OldFrom.Replace(a[2],"Second:"+a[2]);
   }   
   
   this.txtResult.Text=OldFrom.Replace("<br>","/r/n　　");