using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var text = "Statement_10125229_20170807.pdf";
		// No regex
    	Console.WriteLine(Path.GetFileNameWithoutExtension(text).Split('_').LastOrDefault());
    	// Regex
    	Console.WriteLine(Regex.Match(text, @"_(\d+)\.pdf$")?.Groups[1].Value);
	}
}