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

public class Test
{
	public static void Main()
	{
		string pattern = @"\b[fF]iles\b[^*()]*(?:\([^()]*\))?\|\*(?<ext>\.[\w*]+)(?:[,;]\*(?<ext>\.[\w*]+))*";
        string input = @"txt files `(*.txt)|*.txt|All files (*.*)|*.*`
Image Files`|*.jpg;*.jpeg;*.png;`
Excel Files `(*.xls, *.xlsx)|*.xls;*.xlsx|CSV Files (*.csv)|*.csv`";

        var strings = Regex.Matches(input, pattern)

        .SelectMany(m => m.Groups["ext"].Captures.Select(c => c.Value))
        .ToArray()
        .Distinct();
        
        foreach (var s in strings)
        {
            Console.WriteLine(s);
        }
	}
}