Regular expression that matches the third instance of something? (python)

I'm trying to create a regular expression that will match the third instance of a / in a url, i.e. so that only the website's name itself will be recorded, nothing else.

So http://www.stackoverflow.com/questions/answers/help/开发者_开发技巧 after being put through the regex will be http://www.stackoverflow.com

I've been playing about with them myself and come up with:

base_url = re.sub(r'[/].*', r'', url)

but all this does is reduce a link to http: - so it's obvious I need to match the third instance of / - can anyone explain how I would do this?

Thanks!

I suggest you use urlparse for parsing URLs:

In [1]: from urlparse import urlparse

In [2]: urlparse('http://www.stackoverflow.com/questions/answers/help/').netloc
Out[2]: 'www.stackoverflow.com'

.netloc includes the port number if present (e.g. www.stackoverflow.com:80); if you don't want the port number, use .hostname instead.

URLParse would work, but since you originally asked about Regexes, try a positive match instead of a negative substitution:

match = re.match(r'.+://[^/]+', url);
baseUrl = match.group();

This will grab the http:// (or https://, or ftp://), and everything after it until the first /.

http://www.tutorialspoint.com/python/python_reg_expressions.htm