using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        const string pattern = @"(?<=\d)[a-zA-Z]+(?=\d{3}[.]\d{3}[.]\d{3}-.+2$)";

        string[] users = new string[5]

        {

            "1Paulo111.111.111-11addaqwe2",

            "2Rambo425.433-628-43ererssd3",

            "1Momba111.111.111-11asdsad4432",

            "1Mauricio111.111.111-22wwcssfd2",

            "1Saulo111.111.111-11qwe1231231"

        };

​

        foreach (string user in users)

        {

            Match m = Regex.Match(user, pattern);

            if(m.Success) Console.WriteLine(m.Value);

        }

    }

}