using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var input = new []{"John", "John_Smith_a", "John Smith_", "Cpl\\3 John Smith", "Mr. John Smith"};

​

        foreach(string test in input)

        {

            Console.WriteLine("{0} : {1}",test, Regex.IsMatch(test,@"^(\s+)?[\w.\\]+\s+\w+\s+\w+(\s+)?$"));

        }

    }

}