Regex error with session.cookie_domain in settings.php

row in sites/settings.php:
$domain = '.'. preg_replace('`^www.`', '', $_SERVER['HTTP_HOST']);
should be changed to
$domain = '.'. preg_replace('`^www\.`', '', $_SERVER['HTTP_HOST']);
(screened point symbol in regexp)
my "www1.mydomain.com" converted in "..mydomain.com", that works in last versions of Opera and FF, but don't work in IE6,7