首先我們看下官方文檔里關于的compile的說明：

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的說明：

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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下面是關于findall的說明：

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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下面舉個栗子進行講解：

>>> 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']

代碼參考下圖進行理解：

對于上面的代碼，我們可以看到：

findall函數返回的總是正則表達式在字符串中所有匹配結果的列表list，此處主要討論列表中“結果”的展現方式，即findall中返回列表中每個元素包含的信息。

1.當給出的正則表達式中帶有多個括號時，列表的元素為多個字符串組成的tuple，tuple中字符串個數與括號對數相同，字符串內容與每個括號內的正則表達式相對應，并且排放順序是按括號出現的順序。

2.當給出的正則表達式中帶有一個括號時，列表的元素為字符串，此字符串的內容與括號中的正則表達式相對應（不是整個正則表達式的匹配內容）。

3.當給出的正則表達式中不帶括號時，列表的元素為字符串，此字符串為整個正則表達式匹配的內容。

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對于.re.compile.findall(data)之后的數據，我們可以通過list的offset索引或者str.join()函數來使之變成str字符串，從而進行方便的處理，下面是python3.5中str.join()的文檔：

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.

經過上面的介紹，相信對crawler里的正則有很大的幫助

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