Detect URLs in a string and wrap with "<a href..." tag

I am looking to write something that seems like it should be easy enough, but for whatever reason I'm having a tough time getting my head around it.

I am looking to write a python function that, when passed a string, will pass that string back with HTML encoding around URLs.

unencoded_string = "This is a link - http://google.com"

def encode_string_with_links(unencoded_string):
    # some sort of regex magic occurs
    return encoded_string

print encoded_string

'This is a link - <a href="http://google.com">http://google.com</a>'

Thank you!