Environment::getCookieDomain() 

public function getCookieDomain ( $loginType = 'FE' ) {
	$cookieDomain = $this->getLocalConf( $loginType . '.cookieDomain' )
		?: $this->getLocalConf( 'SYS.cookieDomain' );
	if (!$cookieDomain) {
		return '';
	}
	// Check if cookieDomain is a regex pattern (starts and ends with /)
	if ($cookieDomain[0] === '/') {
		$host = GeneralUtility::getIndpEnv('TYPO3_HOST_ONLY');
		if (@preg_match($cookieDomain, $host, $match)) {
			$cookieDomain = $match[0];
		} else {
			$cookieDomain = '';
		}
	}
	return $cookieDomain;
}