正则表达式中的问号 [英] Question marks in regular expressions

问题描述

我正在阅读正则表达式参考，我在想?和 ??人物.你能用一些例子向我解释它们的用处吗?我不太了解它们.

I'm reading the regular expressions reference and I'm thinking about ? and ?? characters. Could you explain me with some examples their usefulness? I don't understand them enough.

谢谢

推荐答案

? 和 ?? 之间的主要区别在于他们的懒惰.?? 是懒惰的，? 不是.

The key difference between ? and ?? concerns their laziness. ?? is lazy, ? is not.

假设您想在文本正文中搜索单词car"，但您不想仅限于单数的car"；您还想匹配复数汽车".

Let's say you want to search for the word "car" in a body of text, but you don't want to be restricted to just the singular "car"; you also want to match against the plural "cars".

这是一个例句:

我拥有三辆车.

现在，如果我想匹配单词car"而我只想得到字符串car"作为回报，我会使用懒惰的??像这样:

Now, if I wanted to match the word "car" and I only wanted to get the string "car" in return, I would use the lazy ?? like so:

汽车??

这表示，寻找汽车或汽车这个词；如果找到，则返回car，仅此而已".

This says, "look for the word car or cars; if you find either, return car and nothing more".

现在，如果我想匹配相同的词(car"或cars")并且我想得到整个匹配的回报，我会使用非懒惰的 <代码>? 像这样:

Now, if I wanted to match against the same words ("car" or "cars") and I wanted to get the whole match in return, I'd use the non-lazy ? like so:

汽车?

这表示，查找汽车或汽车这个词，然后返回汽车或汽车，无论您找到什么".

This says, "look for the word car or cars, and return either car or cars, whatever you find".

在计算机编程的世界中，懒惰通常意味着仅根据需要进行评估".所以懒惰的 ?? 只返回匹配所需的数量；由于cars"中的s"是可选的，所以不要返回它.另一方面，非惰性(有时称为 greedy)操作会尽可能多地求值，因此 ? 返回所有匹配项，包括可选的s".

In the world of computer programming, lazy generally means "evaluating only as much as is needed". So the lazy ?? only returns as much as is needed to make a match; since the "s" in "cars" is optional, don't return it. On the flip side, non-lazy (sometimes called greedy) operations evaluate as much as possible, hence the ? returns all of the match, including the optional "s".

就我个人而言，我发现自己使用 ? 作为一种使其他正则表达式运算符变得懒惰的方式(例如 * 和 + 运算符)更多通常比我将它用于简单的字符可选性，但是 YMMV.

Personally, I find myself using ? as a way of making other regular expression operators lazy (like the * and + operators) more often than I use it for simple character optionality, but YMMV.

以上以Clojure实现为例:

Here's the above implemented in Clojure as an example:

(re-find #"cars??" "I own three cars.")
;=> "car"

(re-find #"cars?" "I own three cars.")
;=> "cars"

项目 re-find 是一个函数，它将其第一个参数作为正则表达式 #"cars??" 并返回它在第二个中找到的第一个匹配项参数 我拥有三辆车."

The item re-find is a function that takes its first argument as a regular expression #"cars??" and returns the first match it finds in the second argument "I own three cars."

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