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

public class Test
{
	public static void Main()
	{
		IEnumerable<string> titles = new string[]
	    {
	        "abc [FEAT one two] 123",
	        "def(FEAT. three'four) 456",
	        "ghi Featuring five",
	        "jkl"
	    };

		var keys = new List<string> { "FEAT", "FEAT.", "Featuring" };
		keys = keys.OrderByDescending(x => x.Length).ToList();
		var pattern = $@"(?:{string.Join("|", keys.Select(z => Regex.Escape(z)))})\s*([^])]*)";
		Console.WriteLine(pattern);
		
		var result = new List<string>();
		foreach(string title in titles)
	    {
	        var _ = Regex.Match(title, pattern);
    		if (_.Success)
        		result.Add(_.Groups[1].Value);
	    }
	
	    Console.WriteLine( result.Count()); // Assert.AreEqual(3, result.Count());
	    Console.WriteLine( result.Contains("one two") ); //Assert.IsTrue(result.Contains("one two"));
	    Console.WriteLine( result.Contains("three'four") ); //Assert.IsTrue(result.Contains("three'four"));
    	Console.WriteLine( result.Contains("five") ); // Assert.IsTrue(result.Contains("five"));
	}
}