Regular Expression

@Vulpes - Buddy you got to write an article on Regular expressions :-)..never knew "" meant that...+1

Thanks Raja.

I've seen this question come up a few times on C# forums over the years so, hopefully, my explanation is improving :)

Incidentally, we don't seem to have answered the second part of the OP's question.

When you have something like this:  (? )

it means that everything else within the enclosing brackets (...) - except ? -  is a 'named' group. Here the group is called 'first' and, if the code were re-expressed like this, then you could recover what's in the 'first' group as follows:

string inputString = "239.240.241.242";
Match m = Regex.Match(inputString, @"(?2[0-4]\d|25[0-5]|[01]?\d\d?)\.(?2[0-4]\d|25[0-5]|[01]?\d\d?)\.(?2[0-4]\d|25[0-5]|[01]?\d\d?)\.(?2[0-4]\d|25[0-5]|[01]?\d\d?)");
if (m.Success)
{
   MessageBox.Show(m.Groups["First"].Value); // 239
}

In regular expressions, \w represents any word character i.e. a-z, A-Z, 0-9 or an underscore.


Now, if you used this in an ordinary C# string, then the compiler would think it was an escape character (\n, \t, \r etc) and give an error because there is no such escape character as \w.


To avoid this, you need to prefix the string with @ which tells the compiler to treat everything in it literally i.e. read \w as being a backslash followed by a 'w'. Such strings are technically called 'verbatim string literals'.


To match numbers, replace \w in your regular expression with \d which repesents any digit 0-9.

Wa-AlakumSalam
this is verbose strings feature of c# and here @ you dont need escape sequence to for escaping special characters.