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);
}
}
}
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