Parse a string into an array of tags (denoted by square brackets) and terms (which may be quoted to preserve whitespace).

Program.cs

using System;
using System.Linq;

public class Program
{
    static readonly char[] _trmDelimiters = { ' ', ',', '.', '!', '/' };
    static readonly char[] _tagDelimiters = { '[', ']' };
	
    public static void Main()
    {
        string s = "This is a [tagged term], a sentence fragment, a [tag with spaces], and a \"quoted term with spaces.\"";
    
        var extract =   from c in s.Split('"', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries).Chunk(2) // "quoted text"
                        from t in c[0].Split(_tagDelimiters, StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries).Chunk(2) // [tagged text]
                        from w in t[0].Split(_trmDelimiters, StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries) // words
                        where w.Length > 2 // ignore "a", "is" etc.
                        select (Word: w, Tag: string.Concat(t[1..]), Quote: string.Concat(c[1..]));

        var result = new 
        {
            Terms = extract.Select(e => e.Word).Union(extract.Select(e => e.Quote)).Distinct().ToArray(),
            Tags = extract.Select(e => e.Tag).Where(t => !string.IsNullOrWhiteSpace(t)).Distinct().ToArray()
        };

        Console.WriteLine($"{result.Terms.Length} terms:");
        Console.WriteLine(string.Join(",", result.Terms));
		
        Console.WriteLine($"\n{result.Tags.Length} tags:");
        Console.WriteLine(string.Join(",", result.Tags));
    }
}