Regular Expressions for URL

Is there a guide that shows what regular expressions are exactly and which ones you can use?

I am hoping I can use this for a profile field which the member enters their YouTube channel URL and it checks to verify that a YouTube URL was entered. Another thing that would be nice is if I could just have them enter their channel name and then the rest of the URL is entered for them. So, they would enter "PermaEthos" and the field would have the full URL of "http://www.youtube.com/c/permaethos".

I have other examples of what I want to do with URL's but if I can figure this out I think all of those will be answered as well.

Thank you in advance for any guidance.