re模块

python也支持正则表达式，这节说正则表达式模块re

>>> import re
>>> dir(re)
['A', 'ASCII', 'DEBUG', 'DOTALL', 'I', 'IGNORECASE', 'L', 'LOCALE', 'M', 
4 'MULTILINE', 'S', 'Scanner', 'T', 'TEMPLATE', 'U', 'UNICODE', 'VERBOSE', 'X',
5 '_MAXCACHE', '__all__', '__builtins__', '__cached__', '__doc__', '__file__', '__loader__', 
6 '__name__', '__package__', '__spec__', '__version__', '_alphanum_bytes', '_alphanum_str', '_cache', 
7 '_cache_repl', '_compile', '_compile_repl', '_expand', '_locale', '_pattern_type', '_pickle', '_subx', 
8 'compile', 'copyreg', 'error', 'escape', 'findall', 'finditer', 'fullmatch', 'match', 'purge', 'search', 
9 'split', 'sre_compile', 'sre_parse', 'sub', 'subn', 'sys', 'template']

使用正则表达式模块首先需要导入模块，re模块中比较重要的几个方法 findall / match / search /compile ，下面用例子解释下：

#findall
>>> help(re.findall)
Help on function findall in module re:

findall(pattern, string, flags=0)
    Return a list of all non-overlapping matches in the string.
    
    If one or more capturing 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.
其中：flags定义包括：
re.I：忽略大小写
re.L：表示特殊字符集 w, W, , B, s, S 依赖于当前环境
re.M：多行模式
re.S：' . '并且包括换行符在内的任意字符（注意：' . '不包括换行符）
re.U： 表示特殊字符集 w, W, , B, d, D, s, S 依赖于 Unicode 字符属性数据库

>>> a='this is a test string'
>>> re.findall('a')
>>> re.findall(r'test',a)   #查找匹配‘test'的单词
['test'] 
>>> a='this is a {{test}} string'  
>>> patten=r'{{(.*?)}}'     #结合稍微复杂的模式
>>> re.findall(patten,a)
['test']

>>> pattern=r'a.c'
>>> test_str='abc a1c a*c a|c abd aed a
c'
>>> re.findall(pattern,test_str)
['abc', 'a1c', 'a*c', 'a|c']
>>> re.findall(pattern,test_str,re.S)      
['abc', 'a1c', 'a*c', 'a|c', 'a
c']

#re.search   该方法检索字符串，找到匹配后返回，
>>> help(re.search)
Help on function search in module re:

search(pattern, string, flags=0)
    Scan through string looking for a match to the pattern, returning
    a match object, or None if no match was found.
>>> test_str='this is a test ,is good,is bad'
>>> pattern=r'is'
>>> re.search(pattern,test_str)
<_sre.SRE_Match object; span=(2, 4), match='is'>
>>> dir(b)
['__class__', '__copy__', '__deepcopy__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', 
'__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__', '__lt__', '__ne__', '__new__', 
'__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'end', 'endpos', 
'expand', 'group', 'groupdict', 'groups', 'lastgroup', 'lastindex', 'pos', 're', 'regs', 'span', 'start', 'string']
>>> b.group()
'is'

#re.match  仅从字符串首开始匹配
>>> help(re.match)
Help on function match in module re:

match(pattern, string, flags=0)
    Try to apply the pattern at the start of the string, returning
    a match object, or None if no match was found.
>>> test_str
'this is a test ,is good,is bad'
>>> pattern
'is'
>>> b=re.match(pattern,test_str)
>>> b
>>> print(b)
None           #没有匹配到字符串中的is

re.compile 将正则表达式转换为模式对象，可以实现更有效率的匹配，

>>> help(re.compile)
Help on function compile in module re:

compile(pattern, flags=0)
    Compile a regular expression pattern, returning a pattern object.
>>> pattern=r'{{(.*?)}}'
>>> pat_obj=re.compile(pattern)        #编译成模式对象
>>> pat_obj
re.compile('{{(.*?)}}')
>>> test_str='this is a {{test}} str, {{}}'
>>> pat_obj.findall(test_str)
['test', '']                   #我们看到这同样匹配了找到的所有结果