PHP Regex to Remove http:// from string

$str = 'http://www.google.com';
$str = preg_replace('#^https?://#', '', $str);
echo $str; // www.google.com

That will work for both http:// and https://

You don't need regular expression at all. Use str_replace instead.

str_replace('http://', '', $subject);
str_replace('https://', '', $subject);

Combined into a single operation as follows:

str_replace(array('http://','https://'), '', $urlString);