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Python Regular Expressions: Capture

高洛峰
Release: 2017-03-03 11:48:44
Original
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前言

在上一篇文中,我们介绍了关于Python正则表达式的基础,那么在这一篇文章里,我们将总结一下正则表达式关于捕获的用法。下面话不多说,来看看详细的介绍吧。   

捕获

捕获和分组在正则表达式中有着密切的联系,一般情况下,分组即捕获,都用小括号完成(因此,小括号在正则表达式中也属于特殊字符,表达原含义时需要转义): 

      (…)     正常分组,并捕获

      (?:…)   分组,但是不捕获 

举个例子,假设我们需要匹配一个座机号码:

>>> m = re.search(r'^(\d{3,4}-)?(\d{7,8})$','020-82228888')
>>> m.group(0)
'020-82228888'
>>> m.group(1)
'020-'
>>> m.group(2)
'82228888'
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这里,默认分组(0)是完整的匹配,之后的分组则按出现顺序排列。

接下来,我们想在一整段文本中,找出所有的座机号码,这里需要用到re.findall:

>>> re.findall(r'(\d{3,4}-)?(\d{7,8})','020-82228888\n0357-4227865') 
[('020-', '82228888'), ('0357-', '4227865')]
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findall有一个特性,就是如果结果中有捕获的分组,则将捕获的分组组成tuple返回。利用这个特点,和上面提到的分组,但是不捕获的语法,可以得到我们想要的结果:

>>> re.findall(r'(?:\d{3,4}-)?\d{7,8}','020-82228888\n0357-4227865') 
['020-82228888', '0357-4227865']
>>> re.findall(r'(?:\d{3,4}-)?\d{7,8}','020-82228888\n4227865')  
['020-82228888', '4227865']
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在正则表达式中,也可以通过\1,\2等来指代之前捕获的字符串组合。这个经常用于单双引号的正确匹配:

>>> sentence = """You said "why?" and I say "I don't know"."""
>>> re.findall(r'["\'](.*?)["\']', sentence)
['why?', 'I don']
>>> re.findall(r'(["\'])(.*?)\1', sentence)
[('"', 'why?'), ('"', "I don't know")]
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此外,如果觉得\1,\2这种表示可读性不好的话,还可以给捕获起一个英文名字。如下例子中,实现了两种不同的日期格式之间的转换:

>>> sentence = "from 12/22/1629 to 11/14/1643"
>>> re.sub(r&#39;(?P<month>\d{2})/(?P<day>\d{2})/(?P<year>\d{4})&#39;, r&#39;\g<year>-\g<month>-\g<day>&#39;, sentence) 
&#39;from 1629-12-22 to 1643-11-14&#39;
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但是,这种命名引用捕获的方式,在findall、search中却是无效的:

>>> sentence = """You said "why?" and I say "I don&#39;t know"."""
>>> re.findall(r&#39;(?P<quote>["\&#39;])(.*?)\g<quote>&#39;, sentence)  
[]
>>> re.search(r&#39;(?P<quote>["\&#39;])(.*?)\g<quote>&#39;, sentence)   
>>> re.search(r&#39;(?P<quote>["\&#39;])(.*?)\1&#39;, sentence)  
<_sre.SRE_Match object; span=(9, 15), match=&#39;"why?"&#39;>
>>> re.search(r&#39;(?P<quote>["\&#39;])(.*?)\1&#39;, sentence).groupdict()
{&#39;quote&#39;: &#39;"&#39;}
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