Many search engines track clicked URLs by adding the result's URL to the query string which can take a format like: http://www.example.com/result?track=http://www.stackoverflow.com/questions/ask
In the above example the result URL is part of the query string but in some cases it takes the form http://www.example.com/http://www.stackoverflow.com/questions/ask or URL encoding is used. 
The approach I tried first is to split searchengineurl.split("http://"). Some obvious problems with this:

it would return all parts of the query string that follow the result URL and not just the result URL. This would be a problem with an URL like this: http://www.example.com/result?track=http://www.stackoverflow.com/questions/ask&showauthor=False&display=None
it does not distinguish between any additional parts of the search engine tracking URL's query string and the result URL's query string. This would be a problem with an URL like this: http://www.example.com/result?track=http://www.stackoverflow.com/questions/ask?showauthor=False&display=None
it fails if the "http://" is ommitted in the result URL

What is the most reliable, general and non-hacky way in Python to extract URLs contained in other URLs?
I would try using urlparse.urlparse it will probably get you most of the way there and a little extra work on your end will get what you want.
How to reliably extract URLs contained in URLs with Python?