使用正则表达式提取用户名？

Question

Extracting the username from this string?

<title>[FirstName] [SecondName] (@[Username]) on [Site]</title>

I tried using regex but it gives me:

Object reference not found exception.

This is the code I use:

return Regex.Matches(title, @"\(([^)]*)\)").OfType<Match>().LastOrDefault().ToString();

Answer 1

Try following :

            string input = "<title>[FirstName] [SecondName] (@[Username]) on [Site]</title>";

            string pattern = @"\[(?'value'[^\]]+)\]";

            MatchCollection matches = Regex.Matches(input, pattern);

            Console.WriteLine("User Name : '{0}'", matches[2].Groups["value"].Value);
            Console.ReadLine();

Answer 2

If you want to extract Username from <title>[FirstName] [SecondName] (@[Username]) on [Site]</title> you could capture in a group what is between (@[ and ]) by using a negated character class that matches not aa closing square bracket one or more times [^]+ :

\\(@\\[([^\\]]+)\\]\\)

Demo

Or use a positive lookbehind the asserts what is on the left side is (@[ and a positive lookahead that asserts what is on the right side is ]) :

(?<=\\(@\\[)[^]]+(?=\\]\\))

Demo

Answer 3

I just tested the code here and it works:

var match = Regex.Matches("<title>[FirstName] [SecondName] (@[Username]) on [Site]</title>", @"\(([^)]*)\)")
                 .OfType<Match>()
                 .LastOrDefault()
                 .ToString();

The only difference with your code is that I'm not using title variable. So you title variable may be null , or empty or doesn't contains any match so you may end up with LastOrDefault().ToString(); failing because you're calling ToString() on a null reference ( LastOrDefault returns null when you're dealing with an empty collection of reference types).

To fix the error just refactor your code like below:

var match = Regex.Matches("<title>[FirstName] [SecondName] (@[Username]) on [Site]</title>", @"\(([^)]*)\)")
                 .OfType<Match>()?
                 .LastOrDefault()?
                 .ToString();

The code after ? character will be called only when the result is not null .