WebAPI CORS Regex support for origin.

customEnableCorsAttribute

    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Class, AllowMultiple = false)]
    public class SubdomainWildcard : Attribute, ICorsPolicyProvider
    {
        private static List<string> AcceptableDomainPatterns = new List<string>()
        {
            @"mattmazzola([-\w]+)\.sharepoint\.com"
        };

        
        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 isOriginAcceptable = false;

            // Iterate through each pattern available
            // If the origin is matched by one of the regex patterns
            // return true, else return false;

            foreach (var pattern in AcceptableDomainPatterns)
            {
                var regex = new Regex(pattern, RegexOptions.IgnoreCase);
                if (regex.IsMatch(origin))
                {
                    isOriginAcceptable = true;
                    break;
                }
            }
            return isOriginAcceptable;
        }
    }