ain is valid or not.
	 */
	private function isValidDomain( $domain ) {
		// In case there are unicode characters, convert it into
		// IDNA ASCII URLs
		if ( function_exists( 'idn_to_ascii' ) ) {
			$domain = idn_to_ascii( $domain, IDNA_DEFAULT, INTL_IDNA_VARIANT_UTS46 );
		}

		if ( ! $domain ) {
			return false;
		}

		$domain = preg_replace( '/^\*\.+/', '', $domain );

		return preg_match( '/^(?!\-)(?:[a-z\d\-]{0,62}[a-z\d]\.){1,126}(?!\d+)[a-z\d]{1,63}$/i', $domain );
	}

	/**
	 * Returns a single string of all subdomains associated with this domain.
	 * Example 1: www
	 * Example 2: ww2.www
	 *
	 * @since 4.5.9
	 *
	 * @return array The subdomains associated with this domain.
	 */
	public function getSubdomains( $domain ) {
		// If we can't find a TLD, we won't be able to parse a subdomain.
		if ( empty( $this->getTld( $domain ) ) ) {
			return [];
		}

		// Return any subdomains as an array.
		return array_filter( explode( '.', rtrim( strstr( $domain, $this->getTld( $domain ), true ), '.' ) ) );
	}

	/**
	 * Returns the TLD associated with the given domain.
	 *
	 * @since 4.5.9
	 *
	 * @param  string $domain The domain.
	 * @return string         The TLD.
	 */
	public function getTld( $domain ) {
		if ( preg_match( '/(?P<tld>[a-z0-9][a-z0-9\-]{1,63}\.[a-z\.]{2,6})$/i', $domain, $matches ) ) {
			return $matches['tld'];
		}

		return $domain;
	}
}