str1="xxxxxxxxxxcsdnnetxxxxxxxxxxxxxx";
str2="xxxxxxccdnnetxxxxxxxx";
str3="xxxxxxxxxxxxxxxxcsdnnatxxxxx";
str4="xxxxxxxxcssnnatxxxxxxxxxxxxxxxx";

tmpstr="csdnnet";

void Main()
{
string    str1="xxxxxxxxxxcsdnnetxxxxxxxxxxxxxx";
string    str2="xxxxxxccdnnetxxxxxxxx";
string    str3="xxxxxxxxxxxxxxxxcsdnnatxxxxx";
string    str4="xxxxxxxxcssnnatxxxxxxxxxxxxxxxx";
Console.WriteLine("{0}:\t{1}",str1,CheckString(str1));
Console.WriteLine("{0}:\t{1}",str2,CheckString(str2));
Console.WriteLine("{0}:\t{1}",str3,CheckString(str3));
Console.WriteLine("{0}:\t{1}",str4,CheckString(str4));
  
  /*
  xxxxxxxxxxcsdnnetxxxxxxxxxxxxxx:    OK
xxxxxxccdnnetxxxxxxxx:    OK
xxxxxxxxxxxxxxxxcsdnnatxxxxx:    OK
xxxxxxxxcssnnatxxxxxxxxxxxxxxxx:    ERROR

  */
   
}
 string[] temp=new string[]{"csdnnet","[a-z]sdnnet","c[a-z]dnnet","cs[a-z]nnet"
   ,"csd[a-z]net","csdn[a-z]et","csdnn[a-z]t","csdnne[a-z]"};
   
   
string CheckString(string str)
{
   string result="ERROR";
  Regex reg;
    foreach(string s in temp)
    {
      reg=new Regex(s);
      if(reg.IsMatch(str))
        {
            result="OK"; 
            break;
        }
    }
  
  return result;
}