using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		string temp = @"\folder_name\file_name.filetype.[somename@somedomain].wallet";
		var m = Regex.Match(temp, @"(?<=\\)[^.]*\.[^.]*(?=\.\[[^\]]*\].wallet)");
		if (m.Success) {
			Console.WriteLine(m.Value);
		} else {
			Console.WriteLine("<no match>");
		}
	}
}