REGEX Pattern For URL Validation

I'm using this PHP function for validate an URL.

$url = preg_replace("/[^A-Za-z0-9-\/\.\:]/", "", trim($url));
$url = preg_replace('%^(?!https?://www\.).*%', 'http://www\.$0', $url);

But, this isn't perfect for me. I want to this controls :

1 - Is it starting with http:// or https:// ? If not, add http (or https)

2 - If it hasn't subdomain and if it hasn't www , add . But if it has subdom开发者_开发问答ain don't add www.

Examples :

example.com
CONVERT -> http://www.example.com

www.example.com
CONVERT -> http://www.example.com

subdomain.example.com
CONVERT -> http://subdomain.example.com

http://subdomain.example.com
CONVERT -> http://subdomain.example.com

So, I need improve my REGEX pattern. But I can't. How can I use this?

Remove the www\. from the second regex. This would make it work on the subdomain examples.

If you want to make it more intelligent, use an if and a second regex to check for ^(\w+://)?\w+\.\w+$ prior prefixing it with http:// or an extra www.