首先我们看下官方文档里关于的compile的说明：

re.compile(pattern, flags=0)
Compile a regular expression pattern into a regular expression object, which can be used for matching using its match() and search() methods, described below.The expression’s behaviour can be modified by specifying a flags value. Values can be any of the following variables, combined using bitwise OR (the | operator).

</pre><pre name="code" class="python">The sequence：
prog = re.compile(pattern)
result = prog.match(string)
<strong><span style="font-size:24px;">is equivalent to</span></strong>
result = re.match(pattern, string)
but using re.compile() and saving the resulting regular expression object for reuse is more efficient when the expression will be used several times in a single program.

Note：The compiled versions of the most recent patterns passed to re.compile() and the module-level matching functions are cached, so programs that use only a few regular expressions at a time needn’t worry about compiling regular expressions.

下面是flag dotall的说明：

re.DOTALL
Make the '.' special character match any character at all, including a newline; without this flag, '.' will match anything except a newline.

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下面是关于findall的说明：

re.findall(pattern, string, flags=0)
Return all non-overlapping matches of pattern in string, as a list of strings. The string is scanned left-to-right, and matches are returned in the order found. If one or more groups are present in the pattern, return a list of groups; this will be a list of tuples if the pattern has more than one group. Empty matches are included in the result unless they touch the beginning of another match.

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下面举个栗子进行讲解：

>>> import re
>>> s = "adfad asdfasdf asdfas asdfawef asd adsfas "

>>> reObj1 = re.compile('((\w+)\s+\w+)')
>>> reObj1.findall(s)
[('adfad asdfasdf', 'adfad'), ('asdfas asdfawef', 'asdfas'), ('asd adsfas', 'asd')]

>>> reObj2 = re.compile('(\w+)\s+\w+')
>>> reObj2.findall(s)
['adfad', 'asdfas', 'asd']

>>> reObj3 = re.compile('\w+\s+\w+')
>>> reObj3.findall(s)
['adfad asdfasdf', 'asdfas asdfawef', 'asd adsfas']

代码参考下图进行理解：

对于上面的代码，我们可以看到：

findall函数返回的总是正则表达式在字符串中所有匹配结果的列表list，此处主要讨论列表中“结果”的展现方式，即findall中返回列表中每个元素包含的信息。

1.当给出的正则表达式中带有多个括号时，列表的元素为多个字符串组成的tuple，tuple中字符串个数与括号对数相同，字符串内容与每个括号内的正则表达式相对应，并且排放顺序是按括号出现的顺序。

2.当给出的正则表达式中带有一个括号时，列表的元素为字符串，此字符串的内容与括号中的正则表达式相对应（不是整个正则表达式的匹配内容）。

3.当给出的正则表达式中不带括号时，列表的元素为字符串，此字符串为整个正则表达式匹配的内容。

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对于.re.compile.findall(data)之后的数据，我们可以通过list的offset索引或者str.join()函数来使之变成str字符串，从而进行方便的处理，下面是python3.5中str.join()的文档：

str.join(iterable)
Return a string which is the concatenation of the strings in the iterable iterable. A TypeError will be raised if there are any non-string values in iterable, including bytes objects.The separator between elements is the string providing this method.

经过上面的介绍，相信对crawler里的正则有很大的帮助