CORS Attribute that matches based on regular expression from Web.config

RegexCors.cs

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Net.Http;
using System.Text.RegularExpressions;
using System.Threading;
using System.Threading.Tasks;
using System.Web;
using System.Web.Cors;
using System.Web.Http.Cors;

namespace CorsOriginRegexWebAPI.CORS
{
    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Class, AllowMultiple = false)]
    public class RegexCorsAttribute : Attribute, ICorsPolicyProvider
    {
        public async Task<CorsPolicy> GetCorsPolicyAsync(HttpRequestMessage request, CancellationToken token)
        {
            var corsRequestContext = request.GetCorsRequestContext();
            var originRequested = corsRequestContext.Origin;

            if (await IsOriginAcceptable(originRequested))
            {
                // Grant CORS request
                var policy = new CorsPolicy
                {
                    AllowAnyHeader = true,
                    AllowAnyMethod = true,
                };
                policy.Origins.Add(originRequested);
                return policy;
            }
            else
            {
                return null;
            }
        }

        private async Task<Boolean> IsOriginAcceptable(string origin)
        {
            var pattern = ConfigurationManager.AppSettings["CorsRegexPattern"];
            var regex = new Regex(pattern, RegexOptions.IgnoreCase);

            return regex.IsMatch(origin);
        }
    }
}