using System;

using System.Text;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var ssn = "757030124";

        Console.WriteLine(Validate.ValidateSSN(ssn));

    }

}

​

public class Validate

{

    public static bool ValidateSSN(string ssn)

    {

        bool @return = false;

        if(ssn.Length == 9)

          ssn = AddMaskSSN(ssn);

        Regex regex = new Regex(@"^\d{3}-\d{2}-\d{4}$");

        @return = regex.IsMatch(ssn);

        return @return;

    }

    public static string AddMaskSSN(string ssn)

    {

        StringBuilder ssnMask = new StringBuilder();

        ssnMask.Append(ssn.Substring(0,3) + "-");

        ssnMask.Append(ssn.Substring(3,2) + "-");

        ssnMask.Append(ssn.Substring(5,4));

        return ssnMask.ToString();

    }

}