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, @ "(? 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?)" )) { in above expression ? what is the meaning of this and ? whay it is first used...? Any one who can help me....

Vulpes · Answer

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 (
, 	,  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.

Vulpes · Answer

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 }

Suthish Nair · Answer

If the query got resolved, then mark the post that helped you as Answer so will be helpful for others. Otherwise if you got the solution please share with us.

Raja Krishnamurthy · Answer

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

Raja Krishnamurthy · Answer

@Vulpes - Awesome Explanation :-) +1 Regards, Raja

Smart Lucky · Answer

Aslam-o-Alaikum Sorry still not clear and how match the number in regular expression....

hamida khanam · Answer

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

Regular Expression

Answers (7)