解析url用的类库:

    python2版本: 

     from urlparse import urlparse
     import urllib

python3版本：

    from urllib.parse import urlparse
    import urllib.request

     研究了不同的url规则发现：只要在搜索关键字是用=嫁接的，查询的关键在解析后的query里

                                                 如果不是用=嫁接，查询的关键在解析后的path里。

    解析的规则都是一样的，正则如下：（6中不同情况的组合）

另外host为‘s.weibo.com’的url编码与其他不同要另做处理。
代码如下：有些网站的规则还不是很清楚，需要花大量时间找规则，规则越清晰，关键字就越清楚，如下规则已适合绝大部分网站，酌情参考。
# -*- coding:utf-8 -*-

from urlparse import urlparse
import urllib
import re

# url
source_txt = "E:\python_Anaconda_code\url.txt"
# 规则
regular = r'(w+(%ww)+w+|(%ww)+w+(%ww)+|w+(%ww)+|(%ww)+w+|(%ww)+|w+)'

# 存放关键字
kw_list = list()

# key为要研究网站的host，value为关键字的嫁接标识符
dict = {
    "www.baidu.com": "wd=",
    "news.baidu.com": "word=",
    "www.sogou.com": "query=",
    "tieba.baidu.com": "kw=",
    "wenku.baidu.com": "word=",
    "music.sina.com.cn": "k=",
    "www.haosou.com": "q=",
    "www.lagou.com": "list_",
    "www.chunyuyisheng.com": "query=",
    "s.weibo.com": "weibo/"
}

def Main():
    with open(source_txt, 'r') as f_source_txt:
        for url in f_source_txt:
            host = url.split("//")[1].split("/")[0]
            if host in dict:
                flag = dict[host]
                if flag.find("=") != -1:
                    query = urlparse(url).query.replace('+', '')
                    kw = re.search(flag + regular, query, re.I)  # .group(0)
                    if kw:
                        kw = urllib.unquote(kw.group(0).split(flag)[1])
                        print(kw)
                else:
                    path = urlparse(url).path.replace('+', '')
                    kw = re.search(flag + regular, path.replace("%25", "%"), re.I)
                    if kw:
                        kw = urllib.unquote(kw.group(0).split(flag)[1])
                        print(kw)
if __name__ == '__main__':
    Main()


url.txt的内容如下：
https://www.baidu.com/s?ie=utf-8&f=8&rsv_bp=0&rsv_idx=1&ch=&tn=baidu&bar=&wd=python&rn=&oq=&rsv_pq=ece0867c0002c793&rsv_t=edeaQq7DDvZnxq%2FZVra5K%2BEUanlTIUXhGIhvuTaqdfOECLuXR25XKDp%2Bi0I&rqlang=cn&rsv_enter=1&inputT=218
https://www.baidu.com/s?ie=utf-8&f=8&rsv_bp=1&rsv_idx=1&ch=&tn=baidu&bar=&wd=python%E9%87%8C%E7%9A%84%E5%AD%97%E5%85%B8dict&oq=python&rsv_pq=96c160e70003f332&rsv_t=0880NkOvMIr3TvOdDP1t8EbloD8qwr4yeP6CfPjQihQNNhdExfuwyOFMrx0&rqlang=cn&rsv_enter=0&inputT=10411 
https://www.baidu.com/s?ie=utf-8&f=8&rsv_bp=1&rsv_idx=1&ch=&tn=baidu&bar=&wd=python%E9%87%8C%E7%9A%84urlprese&oq=python%25E9%2587%258C%25E7%259A%2584re%25E9%2587%258C%25E7%259A%2584%257C%25E6%2580%258E%25E4%25B9%2588%25E7%2594%25A8&rsv_pq=d1d4e7b90003d391&rsv_t=5ff4Vok4EELK1PgJ4oSk8L0VvKAn51%2BL8ns%2FjSubexg7Lb7znKcTvnVtn8M&rqlang=cn&rsv_enter=1&inputT=2797 
https://www.baidu.com/s?ie=utf-8&f=8&rsv_bp=1&rsv_idx=1&ch=&tn=baidu&bar=&wd=python++wo+%E7%88%B1urlprese&oq=python%25E9%2587%258C%25E7%259A%2584urlprese&rsv_pq=eecf45e900033e87&rsv_t=1c70xAYhrvw5JOZA7lpVgt4pw%2BW1TO8hqTejTh67JgEQfqAGyDydd25HAmU&rqlang=cn&rsv_enter=0&inputT=10884 
http://news.baidu.com/ns?word=%E8%B6%B3%E7%90%83&tn=news&from=news&cl=2&rn=20&ct=1  
http://news.baidu.com/ns?ct=1&rn=20&ie=utf-8&bs=%E8%B6%B3%E7%90%83&rsv_bp=1&sr=0&cl=2&f=8&prevct=no&tn=news&word=++++++%E8%B6%B3++%E7%90%83+++++%E4%BD%A0%E5%A5%BD+%E5%98%9B%EF%BC%9F&rsv_sug3=14&rsv_sug4=912&rsv_sug1=4&inputT=8526  
http://tieba.baidu.com/f?ie=utf-8&kw=%E7%BA%A2%E6%B5%B7%E8%A1%8C%E5%8A%A8&fr=search&red_tag=q0224393377  
https://www.sogou.com/web?query=ni+zai+%E6%88%91+%E5%BF%83li&_asf=www.sogou.com&_ast=1520388441&w=01019900&p=40040100&ie=utf8&from=index-nologin&s_from=index&sut=9493&sst0=1520388440692&lkt=8%2C1520388431200%2C1520388436842&sugsuv=1498714959961744&sugtime=1520388440692  
https://www.lagou.com/jobs/list_python%E5%A4%A7%E6%95%B0%E6%8D%AEmr?labelWords=&fromSearch=true&suginput= 
https://www.chunyuyisheng.com/pc/search/?query=%E6%85%A2%E6%80%A7%E4%B9%99%E8%82%9D% 
http://s.weibo.com/weibo/%25E5%2594%2590%25E4%25BA%25BA%25E8%25A1%2597%25E6%258E%25A2%25E6%25A1%25882&Refer=index
http://s.weibo.com/weibo/%25E4%25BD%25A0%25E5%25A5%25BD123mm%2520%25E5%2597%25AF%2520mm11&Refer=STopic_box
结果如下：

如果要研究其他host，可以加到字典dict里。
备注：以上代码和思路仅供参考，如有更好的方法敬请留言！
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