为什么 Python findall() 和 finditer() 在未锚定的 .* 搜索中返回空匹配? [英] Why do Python findall() and finditer() return empty matches on unanchored .* searches?

问题描述

findall() 和 finditer() 的 Python 文档声明:

The Python docs for findall() and finditer() state that:

空匹配项包含在结果中，除非它们触及另一场比赛开始

Empty matches are included in the result unless they touch the beginning of another match

这可以证明如下:

In [20]: [m.span() for m in re.finditer('.*', 'test')]
Out[20]: [(0, 4), (4, 4)]

谁能告诉我，为什么这个模式首先返回一个空匹配?.* 不应该消耗整个字符串并返回单个匹配项吗?此外，如果我将模式锚定到字符串的开头，为什么最后没有空匹配?例如

Can anyone tell me though, why this pattern returns an empty match in the first place? Shouldn't .* consume the entire string and return a single match? And further, why is there no empty match at the end if I anchor the pattern to the beginning of the string? e.g.

In [22]: [m.span() for m in re.finditer('^.*', 'test')]
Out[22]: [(0, 4)]

推荐答案

1. .* 是零个或多个，所以一旦消耗了四个字符，末尾的零长度空字符串(不触及开头的任何匹配)仍然存在；和
2. 末尾的空字符串与模式不匹配 - 它不在字符串的开头.

1. .* is zero or more, so once the four characters are consumed, the zero-length empty string at the end (which doesn't touch the start of any match) still remains; and
2. The empty string at the end doesn't match the pattern - it doesn't start at the start of the string.

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