python - 后视模式无效

Question

为什么这个正则表达式在 Python 中有效但在 Ruby 中无效:

/(?<!([0-1\b][0-9]|[2][0-3]))/

很高兴听到解释以及如何在 Ruby 中绕过它

编辑整行代码:

re.sub(r'(?<!([0-1\b][0-9]|[2][0-3])):(?!([0-5][0-9])((?i)(am)|(pm)|(a\.m)|(p\.m)|(a\.m\.)|(p\.m\.))?\b)' , ':\n' , s)

基本上，当有冒号而不是时间时，我会尝试添加 '\n'。

Answer 1

Ruby 正则表达式引擎不允许在 look behinds 中捕获组。如果需要分组，可以使用非捕获组 (?:):

[8] pry(main)> /(?<!(:?[0-1\b][0-9]|[2][0-3]))/
SyntaxError: (eval):2: invalid pattern in look-behind: /(?<!(:?[0-1\b][0-9]|[2][0-3]))/
[8] pry(main)> /(?<!(?:[0-1\b][0-9]|[2][0-3]))/
=> /(?<!(?:[0-1\b][0-9]|[2][0-3]))/

Docs:

 (?<!subexp)        negative look-behind

                     Subexp of look-behind must be fixed-width.
                     But top-level alternatives can be of various lengths.
                     ex. (?<=a|bc) is OK. (?<=aaa(?:b|cd)) is not allowed.

                     In negative look-behind, capturing group isn't allowed,
                     but non-capturing group (?:) is allowed.

学习自this answer .