解释查找CSS注释的正则表达式 [英] Explain regex that finds CSS comments

问题描述

我发现这个正则表达式代码会在w3.org的 CSS语法中找到注释< a> page。

I found this regex code that finds comments in w3.org's CSS grammar page.

\/\*[^*]*\*+([^/*][^*]*\*+)*\/

这是很长，有点难以理解。我只是把

It is quite long and bit difficult to understand. I'd just put

\/\*.*\*\/

来查找注释，但是当我在 RegexPal 找到单行注释，而不是多行注释，而原始的正则表达式可以找到所有类型的注释。

to find comments, but when I tested it in RegexPal it finds single line comments and not multi-line comments whereas the original regex can find all types of comments.

我不明白

+([^/*][^*]*\*+)*

推荐答案

你发现只有单行注释的原因是，在典型的正则表达式中，。匹配任何（换行除外）;而另一个使用一个否定的字符类，它匹配任何除了指定的字符，因此可以匹配换行符。

The reason yours finds only single line comments is that, in typical regular expressions, . matches anything except newlines; whereas the other one uses a negated character class which matches anything but the specified characters, and so can match newlines.

但是，如果你要解决这个问题选项为多行或仿佛单行匹配），您会发现它将从第一个注释的 / * 匹配到 * / 的最后评论;您必须使用非贪心量词，。*？才能匹配不超过一个注释。

However, if you were to fix that (there's usually an option for multiline or "as if single line" matching), you would find that it would match from the /* of the first comment to the */ of the last comment; you would have to use a non-greedy quantifier, .*?, to match no more than one comment.

然而，你给出的更复杂的正则表达式比这更复杂。基于nikc.org的答案，我相信它是强制执行评论不能嵌套的限制;也就是说，它们中不能包含 / * 。在允许注释 / * like / * this * / （即，内部/ *既不被禁止也不是嵌套注释）的其他语言中，模式 \ / \ *。*？\ * \ / 将适合匹配它们。

However, the more complex regular expression you give is even more complex than that. Based on nikc.org's answer, I believe it is to enforce the restriction that "comments may not be nested"; that is, they must not contain /* within them. In other languages which permit comments /* like /* this */ (that is, an internal /* is neither prohibited nor a nested comment), the pattern \/\*.*?\*\/ would be appropriate to match them.

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