public static function top_level_domains() {
		if ( empty( self::$top_level_domains_pattern ) ) {
			// Initialize valid top level domains from IANA list.
			self::$top_level_domains_pattern = self::get_top_level_domains_from_file( __DIR__ . '/IANA/tlds-alpha-by-domain.txt' );
		}

		return self::$top_level_domains_pattern;
	}