using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		string[] strings = {"fx_2019.csv", "fx_201902.csv"};
		foreach (string s in strings)
        {
            string myYear = Regex.Replace(s, @".*_(20[0-9]{2})[0-9]*\.\w+$", "$1");
			Console.WriteLine(myYear);
        }
	}
}