using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
        string patternMatch = @"(?:\([A-Z]+\))?FS[A-Z ]*-\d(?:[^\n,()-]*\w)?";
        string patternReplace = @"(?<=^FS-[a-zA-Z0-9]*)[a-z]|[\p{Zs}\ta-z](?=[a-zA-Z0-9\p{Zs}\t]*$)";
        string input = @"Fakturasadas : (S)FSK-69/23/GFK/12hjkhjkddddd
(S)FSK-69/23/GFK/ 1 2 
(S)FSK-69/23/GFK/12";
    
        var strings = Regex
            .Matches(input, patternMatch)
            .Cast<Match>().Select(match => {                    
                return Regex.Replace(
                    match.Value,
                    patternReplace,
                    ""
                );
            }
        );
        
        foreach (var s in strings) {
            Console.WriteLine(s);
        }
}
}