SgmlLinkExtractor not extracting the link

Question

I'm new to Python and Scrapy. I'm trying to scrape a list of website which linked from a main site.

The main site is in the format of http://www.example.com/something.aspx, and the sub-sites/children-sites that I would like SgmlLinkExtractor to extract are in the format of http://www.example.com/something.aspx?ac=N123&dc=123 where the value after ac= and dc= changes for different links. Thus, in regex, I write them as http://www\.example\.com/something\.aspx\?ac=[A-Za-z\d]+&dc=\d+

I tried to use SgmlLinkExtractor from scrapy shell. First,

    >>>link = SgmlLinkExtractor()
    >>>link.extract_links(response)

In that way, I manage to get all the links on the page.

If I use

    >>>link = SgmlLinkExtractor(allow=("http://www\.example\.com/something\.aspx\?"))
    >>>link.extract_links(response)

I can still get all links starting with http://www.example.com/something.aspx?. However, If I try with

    >>>link = SgmlLinkExtractor(allow=("http://www\.example\.com/something\.aspx\?ac=[A-Za-z\d]+&dc=\d+"))
    >>>link.extract_links(response)

or even

    >>>link = SgmlLinkExtractor(allow=("http://www\.example\.com/something\.aspx\?ac"))
    >>>link.extract_links(response)

I get nothing. >>>[] is what I get. Any idea on solving this?

EDIT

I tried again by using

    >>>link = SgmlLinkExtractor(allow=(r"ac"))
    >>>link.extract_links(response)

This is working but

    >>>link = SgmlLinkExtractor(allow=(r"ac=[A-Za-z\d]+&dc=\d+"))
    >>>link.extract_links(response)

still not working. I think the problem is probably due to ? and & in the url. Do I need any escape character or anything to make the link extractor work properly when I specify parameter for allow containing ? and &? Essentially I would need &dc... to be included.

Although one workaround is to use restrict_xpath, but I am hoping that it is actually possible for me to include ? and & in the allow parameter.