Regular Expression

Question

Aslam-o-Alaikum


if (Regex.IsMatch("testtest", @"^\w+$")) { // Do something here }

Why @ sign is used here....?

string pattern = "^a+$";
       if (Regex.IsMatch(content, pattern))
           Console.WriteLine("Pattern Found");
       else
           Console.WriteLine("Pattern Not Found");

and in this  @ sign is not used why......?



if(Regex.IsMatch(inputString, @"(?<First>2[0-4]\d|25[0-5]|[01]?\d\d?)\.(?<Second>2[0-4]\d|25[0-5]|[01]?\d\d?)\.(?<Third>2[0-4]\d|25[0-5]|[01]?\d\d?)\.(?<Fourth>2[0-4]\d|25[0-5]|[01]?\d\d?)"))
            {
in above expression ?<First> what is the meaning of this and ? whay it is first used...?

Any one who can help me....