using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var pattern = @"^(?:[^_]+_){2}(\d+)"; 
		var text = "XXXXX_20160207_20180208_XXXXXXX_20190408T160742_xxxxx";
	    var result = Regex.Match(text, pattern)?.Groups[1].Value;
	    Console.WriteLine(result);
	}
}