using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var result = Regex.Match(@"D:\DataFiles\Files_81\aars2016FAKH1800010.pdf", @"\\\w*(FAKH)\w*\.pdf");

        var FAKH = result.Groups[1].Value;

        Console.WriteLine(FAKH);

    }

}