using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var input = @"<div><strong> hello world </strong><p>hello </p> <i /><i class='fab fa-stack-overflow' /></div>";

        var regex = new Regex("(</?([^>/]*)/?>)");

        var matches = regex.Matches(input);

        var tags = matches.OfType<Match>().Select(m => m.Groups[2].Value);

        tags.Dump();

        List<string> whitelistTags = new List<string>() { "strong", "p", "i" };

        tags.All(t => whitelistTags.Contains(t.Trim())).Dump();

    }

}