What is the Regular Expression Validation for a URL

I'm giving Directory Plugin a try again and I'm Stuck on setting up a Custom Field
the field is URL
I think I asked this before but can't find it in my old post but here goes again
I need the Regular Expression Validation for a URL
* it can be any case.
* it can have a WWW or no WWW before the domain and can have any extension.

I tried regex and regbuddy but can't figure it out
thank you in advance for any help