Python正则表达式:如何指定可选匹配(对于可能为空的子表达式)？

Question

我需要匹配以下输入集:

foo_abc_bar  
foo_bar

并得到"abc"或空字符串作为结果.

所以这是我写的正则表达式:

r'foo_(abc|)[_|]bar'

但由于某种原因,这与我给出的第二个字符串不匹配.

在进一步检查时,我发现它[_|]与空字符串不匹配.

那么,我该如何解决这个问题呢？

Answer 1

要进行abc_选择,您可以使用问号运算符:

(abc_)?

因此,整个正则表达式成为:

r'foo_(abc_)?bar'

With this regex, the second underscore (if present) will become part of the capture group. If you don't want that, you could either remove it post-match with .rstrip('_') or use a slightly more complex regex:

r'foo_(?:(abc)_)?bar'

I found that [_|] does not match an empty string.

That's right. Square brackets denote a character group. The [_|] would match exactly one underscore or exactly one vertical bar, and nothing else. In other words, the vertical bar loses its special meaning when it appears inside a character group.