Python 3 re模块3个括号相关的语法

(?aiLmsux)

(One or more letters from the set 'a', 'i', 'L', 'm', 's', 'u', 'x'.) The group matches the empty string; the letters set the corresponding flags: re.A (ASCII-only matching), re.I (ignore case), re.L (locale dependent), re.M (multi-line), re.S (dot matches all), and re.X (verbose), for the entire regular expression. (The flags are described in Module Contents.) This is useful if you wish to include the flags as part of the regular expression, instead of passing a flag argument to the re.compile() function.

Note that the (?x) flag changes how the expression is parsed. It should be used first in the expression string, or after one or more whitespace characters. If there are non-whitespace characters before the flag, the results are undefined.

>>> re.findall(r'(?a)[abcd]','aBcD')
['a', 'c']
>>> re.findall(r'(?i)[abcd]','aBcD')
['a', 'B', 'c', 'D']
>>> re.findall(r'[abcd](?i)','aBcD')
['a', 'B', 'c', 'D']
>>> re.findall(r'[abcd](?a)','aBcD')
['a', 'c']
>>> re.findall(r'[abcd](?a)(?i)','aBcD')
['a', 'B', 'c', 'D']
>>> re.findall(r'(?a)[abcd](?i)','aBcD')
['a', 'B', 'c', 'D']

(?P<name>...)

Similar to regular parentheses, but the substring matched by the group is accessible via the symbolic group name name. Group names must be valid Python identifiers, and each group name must be defined only once within a regular expression. A symbolic group is also a numbered group, just as if the group were not named.

Named groups can be referenced in three contexts. If the pattern is (?P<quote>['"]).*?(?P=quote) (i.e. matching a string quoted with either single or double quotes):

(?P<quote>['"]).*?(?P=quote)就相当于(['"]).*?\1

Context of reference to group “quote”	Ways to reference it
in the same pattern itself	(?P=quote) (as shown) \1
when processing match object m	m.group('quote') m.end('quote') (etc.)
in a string passed to the repl argument of re.sub()	\g<quote> \g<1> \1

这个语法感觉主要适用于正则表达式中括号结构比较繁杂的情况,如下,当你只想要(\w{2,4})提取到的字符串时,你得数左边有多少对括号,嗯,看来是4对,那么我用5就可以访问到它:

>>> m=re.search(r'((\w)(\d(\w)))(\w{2,4})\1','a1atesta1a')
>>> m.group(5)
'test'

如果把它改写成(?P<t>\w{2,4}),你就可以通过m.group('t')达到目的,而不用去数前面有多少对括号.

>>> m=re.search(r'((\w)(\d(\w)))(?P<t>\w{2,4})\1','a1atesta1a')
>>> m.group('t')
'test'

反过来说,如果括号结构比较简单,则没必要用它,直接\number就可以了.比如说:

>>> m=re.search(r'(\w)(\d)','a2b3c')
>>> m.group(1)
'a'
>>> m.group(2)
'2'
>>> 

(?P=name)

A backreference to a named group; it matches whatever text was matched by the earlier group named name.

实战举例

比如你想提取test.py文件中所有字面量字符串里面的内容(即ab,ab,a\nb,a\nb):

正则表达式可以这样写:

r'(?s)(?P<quote>"""|\'\'\'|\'|")(?P<t>.*?)(?P=quote)'

>>> import re
>>> quote_pat=re.compile(r'(?s)(?P<quote>"""|\'\'\'|\'|")(?P<t>.*?)(?P=quote)')
>>> [x.group('t') for x in re.finditer(quote_pat,open('test.py').read())]
['ab', 'ab', 'a\nb', 'a\nb']

如果想对提取到的字符加个'x'前缀和'z'后缀,再写回test.py文件:

>>> s=re.sub(quote_pat,'\g<quote>x\g<t>z\g<quote>',open('test.py').read())
>>> open('test.py','w').write(s)
46

结果:

也可以这样写:

>>> quote_pat=re.compile(r'(?s)("""|\'\'\'|\'|")(.*?)\1')
>>> s=re.sub(quote_pat,r'\1x\2z\1',open('test.py').read())
>>> open('test.py','w').write(s)
54

在括号结构并不复杂的情况下,\number写法要简单清晰.不过要注意加r前缀,否则\1会被认为是ASCII字符,而不是正则表达式中的特殊字符.

最后补个re.sub函数使用经验.对于re.sub(pattern, repl, string, count=0, flags=0),参数pattern和repl的字符串表达式建议一律加r前缀.

因为对于repl参数来说,'\n','\\n'和r'\n'三者是等效的,而'\\1'和r'\1'等效,'\1'和r'\1'却不等效.这真是一种奇特的规则.为了防止意外惊喜,一律加r前缀.