using System;

using System.Text.RegularExpressions;                   

​

public class Program

{

    public static void Main()

    {

        string input = "E-mail: \r\nJohn Doe (10)\r\n123";

        string pattern = @"(?<=E-mail: \r\n)\s*([^\r\n]*)";

        var match = Regex.Match(input, pattern, RegexOptions.Multiline);

        if (match.Success && match.Groups.Count > 0) 

            Console.WriteLine("Match found: " + match.Groups[1].Value);

        else

            Console.WriteLine("No match found");

    }

}