《Python Cookbook 3rd》笔记(2.1):使用多个界定符分割字符串

巨輪 2020-11-09 23:53:23
Python


使用多个界定符分割字符串

问题

你需要将一个字符串分割为多个字段,但是分隔符 (还有周围的空格) 并不是固定的。

解法

string 对象的 split() 方法只适应于非常简单的字符串分割情形,它并不允许有多个分隔符或者是分隔符周围不确定的空格。当你需要更加灵活的切割字符串的时候,最好使用 re.split() 方法:

>>> line = 'asdf fjdk; afed, fjek,asdf, foo'
>>> import re
>>> re.split(r'[;,\s]\s*', line)
['asdf', 'fjdk', 'afed', 'fjek', 'asdf', 'foo']

讨论

函数 re.split() 是非常实用的,因为它允许你为分隔符指定多个正则模式。比如,在上面的例子中,分隔符可以是逗号,分号或者是空格,并且后面紧跟着任意个的空格。只要这个模式被找到,那么匹配的分隔符两边的实体都会被当成是结果中的元素返回。返回结果为一个字段列表,这个跟 str.split() 返回值类型是一样的。

当你使用 re.split() 函数时候,需要特别注意的是正则表达式中是否包含一个括号捕获分组。如果使用了捕获分组,那么被匹配的文本也将出现在结果列表中。比如,观察一下这段代码运行后的结果:

>>> fields = re.split(r'(;j,j\s)\s*', line)
>>> fields
['asdf', ' ', 'fjdk', ';', 'afed', ',', 'fjek', ',', 'asdf', ',', 'foo']
>>>

获取分割字符在某些情况下也是有用的。比如,你可能想保留分割字符串,用来在后面重新构造一个新的输出字符串:

>>> values = fields[::2]
>>> delimiters = fields[1::2] + ['']
>>> values
['asdf', 'fjdk', 'afed', 'fjek', 'asdf', 'foo']
>>> delimiters
[' ', ';', ',', ',', ',', '']
>>> # Reform the line using the same delimiters
>>> ''.join(v+d for v,d in zip(values, delimiters))
'asdf fjdk;afed,fjek,asdf,foo'
>>>

如果你不想保留分割字符串到结果列表中去,但仍然需要使用到括号来分组正则表达式的话,确保你的分组是非捕获分组,形如 (?:...) 。比如:

>>> re.split(r'(?:,j;j\s)\s*', line)
['asdf', 'fjdk', 'afed', 'fjek', 'asdf', 'foo']
>>>
版权声明
本文为[巨輪]所创,转载请带上原文链接,感谢
https://my.oschina.net/jallenkwong/blog/4710362

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