How to clean an URL with Regex

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A regex pattern to fix 1st level domains. Useful for when data is submitted from users:

Turns this:

google.com
www.google.com
google.com/
http://google.com
http://www.google.com
http://www.google.com/

Into This:

http://www.google.com
http://www.google.com
http://www.google.com
http://www.google.com
http://www.google.com
http://www.google.com

Expression:

Search: 
((http://)?(www.)?([wd-]*?.)(w{2,4})/?)
Replace:
http://www.$4$5

PHP Usage:

	# Fix the URL:
	$fixedURL = preg_replace('/((http://)?(www.)?([wd-]*?.)(w{2,4})/?)/i', 'http://www.$4$5', $previousURL);