Python Capture Regular Expressions
什么是捕获呢？使用小括号指定一个子表达式后，匹配这个子表达式的文本（即匹配的内容）可以在表达式或者其他过程中接着用，下面这篇文章就主要介绍了Python正则表达式中关于捕获的相关资料，需要的朋友可以参考下。
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'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')]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']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")]
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'(?P<month>\d{2})/(?P<day>\d{2})/(?P<year>\d{4})', r'\g<year>-\g<month>-\g<day>', sentence)
'from 1629-12-22 to 1643-11-14'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't know"."""
>>> re.findall(r'(?P<quote>["\'])(.*?)\g<quote>', sentence)
[]
>>> re.search(r'(?P<quote>["\'])(.*?)\g<quote>', sentence)
>>> re.search(r'(?P<quote>["\'])(.*?)\1', sentence)
<_sre.SRE_Match object; span=(9, 15), match='"why?"'>
>>> re.search(r'(?P<quote>["\'])(.*?)\1', sentence).groupdict()
{'quote': '"'}【相关推荐】
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