后视模式中的无效模式 [英] Invalid pattern in look-behind

问题描述

为什么这个正则表达式在 Python 中有效，而在 Ruby 中无效:

Why does this regex work in Python but not in Ruby:

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

很高兴听到解释以及如何在 Ruby 中解决它

Would be great to hear an explanation and also how to get around it in Ruby

使用整行代码进行

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，当有冒号但不是时间时，我会尝试添加 '\n'.

Basically, I'm trying to add '\n' when there is a colon and it is not a time.

推荐答案

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

Ruby regex engine doesn't allow capturing groups in look behinds. If you need grouping, you can use a non-capturing group (?:):

[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]))/

文档:

 (?<!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.

从这个答案中学习.

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