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

public class Test
{
	public static void Main()
	{
		List<string> _separator = new List<string>(){"existing item"};
		Regex containsDelimitersInBrackets = new Regex(@"\[([^][]*)]");
		
		List<string> _sequence = new List<string>(){"test[1] test[2]", "test[123]"}
		.SelectMany(s => 
		    containsDelimitersInBrackets.Matches(s)
		    .Select(m => 
		        m.Groups[1].Value
		    )
		).ToList();
		
		_separator.AddRange(_sequence);
		_separator.ForEach(s => Console.WriteLine(s));
	}
}