using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        string[] phoneNumbers = new string[]

        {

         "3342015352", "(334) 201-5352", "334-2015352", "334-201-5352", "2015352", "201-5352", "4711", "1", "12", "123", "123456789101112333", "1234-567891-0111-2333", "", " "

        };

​

        foreach (var phone in phoneNumbers)

        {

            Console.WriteLine(FormatCanonicalPhone(phone));

        }

        Console.WriteLine(FormatCanonicalPhone(null));

    }

​

    private static string FormatCanonicalPhone(string phoneNumber)

    {

            if (string.IsNullOrEmpty(phoneNumber))

                return string.Empty;

​

            phoneNumber = new Regex(@"\D").Replace(phoneNumber, string.Empty);

​

            switch (phoneNumber.Length)

            {

                //no area code

                case 7:

                    phoneNumber = string.Format("{0}-{1}", phoneNumber.Substring(0, 3), phoneNumber.Substring(3, 4));

                    return phoneNumber;

                case 10:

                    phoneNumber = string.Format("({0}) {1}-{2}", phoneNumber.Substring(0, 3),

                        phoneNumber.Substring(3, 3), phoneNumber.Substring(6, 4));

                    return phoneNumber;

                default:

                    return phoneNumber;

            }

        }

}