《Python Cookbook 3rd》笔记(2.5): 字符串搜索和替换

巨輪 2020-11-12 22:31:58
Python yep


字符串搜索和替换

问题

你想在字符串中搜索和匹配指定的文本模式

解法

对于简单的字面模式,直接使用 str.repalce() 方法即可,比如:

>>> text = 'yeah, but no, but yeah, but no, but yeah'
>>> text.replace('yeah', 'yep')
'yep, but no, but yep, but no, but yep'
>>>

对于复杂的模式,请使用 re 模块中的 sub() 函数。为了说明这个,假设你想将形式为 11/27/2012 的日期字符串改成 2012-11-27 。示例如下:

>>> text = 'Today is 11/27/2012. PyCon starts 3/13/2013.'
>>> import re
>>> re.sub(r'(\d+)/(\d+)/(\d+)', r'\3-\1-\2', text)
'Today is 2012-11-27. PyCon starts 2013-3-13.'
>>>

sub() 函数中的第一个参数是被匹配的模式,第二个参数是替换模式。反斜杠数字比如 n3 指向前面模式的捕获组号。如果你打算用相同的模式做多次替换,考虑先编译它来提升性能。比如:

>>> import re
>>> datepat = re.compile(r'(\d+)/(\d+)/(\d+)')
>>> datepat.sub(r'\3-\1-\2', text)
'Today is 2012-11-27. PyCon starts 2013-3-13.'
>>>

对于更加复杂的替换,可以传递一个替换回调函数来代替,比如:

>>> from calendar import month_abbr
>>> def change_date(m):
... mon_name = month_abbr[int(m.group(1))]
... return '{} {} {}'.format(m.group(2), mon_name, m.group(3))
...
>>> datepat.sub(change_date, text)
'Today is 27 Nov 2012. PyCon starts 13 Mar 2013.'
>>>

一个替换回调函数的参数是一个 match 对象,也就是 match() 或者 find() 返回的对象。使用 group() 方法来提取特定的匹配部分。回调函数最后返回替换字符串。

如果除了替换后的结果外,你还想知道有多少替换发生了,可以使用 re.subn() 来代替。比如:

>>> newtext, n = datepat.subn(r'\3-\1-\2', text)
>>> newtext
'Today is 2012-11-27. PyCon starts 2013-3-13.'
>>> n
2
>>>

讨论

关于正则表达式搜索和替换,上面演示的 sub() 方法基本已经涵盖了所有。其实最难的部分就是编写正则表达式模式,

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/4715051

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