`?` 在这个 Perl 正则表达式中是什么意思? [英] What does `?` mean in this Perl regex?

问题描述

我有一个 Perl 正则表达式.但我不确定是什么?"在这种情况下意味着.

I have a Perl regex. But I'm not sure what "?" means in this context.

m#(?:\w+)#

? 在这里是什么意思?

推荐答案

在这种情况下，? 实际上是与 : 结合使用的.放在一起， ?: 在分组的开头意味着分组但不捕获括号内的文本/模式(例如，它不会存储在任何反向引用中，例如 \1 或 $1，因此您将无法直接访问分组文本).

In this case, the ? is actually being used in connection with the :. Put together, ?: at the beginning of a grouping means to group but not capture the text/pattern within the parentheses (as in, it will not be stored in any backreferences like \1 or $1, so you will not be able to access the grouped text directly).

更具体地说，? 在正则表达式中具有三个不同的含义:

More specifically, a ? has three distinct meanings in regex:

1. ? 量词表示表达式的零次或一次重复".我见过的规范示例之一是 s?he 它将匹配 she 和 he 因为 ?> 使 s 成为可选"

1. The ? quantifier signifies "zero or one repetitions" of an expression. One of the canonical examples I've seen is s?he which will match both she and he since the ? makes the s "optional"

当一个量词(+、*、?，或者一般的{n,m}code>) 后跟一个 ? 那么匹配是非贪婪的(即它将匹配从允许匹配继续的那个位置开始的最短字符串)

When a quantifier (+, *, ?, or the general {n,m}) is followed by a ? then the match is non-greedy (i.e. it will match the shortest string starting from that position that allows the match to proceed)

带括号的组开头的 ? 表示您要执行特殊操作.在这种情况下，: 表示分组但不捕获.可用操作的确切列表会因正则表达式引擎而有所不同，但这里是其中一些的列表(不一定包含所有内容):

A ? at the beginning of a parenthesized group signifies that you want to perform a special action. As in this case, : means to group but not capture. The exact list of actions available will vary somewhat from one regex engine to another, but here's a list (not necessarily all-inclusive) of some of them:

A.非捕获组:(?:text)
B. 环视:<代码>(?= A)对于一个先行, ?! 用于负向前瞻，或 ?<= 和 ?<! 用于后向(分别为正和负).
C. 条件匹配:(?(condition)then|else).
D. 原子分组:a(?>bc|b)c(匹配 abcc 但不 abc；见链接)
E. 内联启用/禁用正则表达式匹配修饰符:?i 启用模式，?-i 禁用.您还可以通过简单地连接它们一次启用/禁用多个修饰符，例如 ?im(i 不区分大小写，m是多行的).
F. 命名捕获组:(?Ppattern)，稍后可以使用 (?P=name) 引用..NET 正则表达式引擎使用语法 (?pattern) 代替.
G. 评论:(?#Comment text).我个人认为这只会增加混乱，但我想它可以起到一些作用...自由间距模式 可能是更好的选择((?x) 修饰符).

A. Non-capturing group: (?:text)
B. Lookaround: (?=a) for a lookahead, ?! for negative lookahead, or ?<= and ?<! for lookbehinds (positive and negative, respectively).
C. Conditional Matches: (?(condition)then|else).
D. Atomic Grouping: a(?>bc|b)c (matches abcc but not abc; see the link)
E. Inline enabling/disabling of regex matching modifiers: ?i to enable a mode, ?-i to disable. You can also enable/disable more than one modifier at a time by simply concatenating them, such as ?im (i is case insensitive and m is multiline).
F. Named capture groups: (?P<name>pattern), which can later be referenced using (?P=name). The .NET regex engine uses the syntax (?<name>pattern) instead.
G. Comments: (?#Comment text). I personally think this just adds clutter, but I guess it could serve some use...free-spacing mode might be a better option (the (?x) modifier).

所以基本上，在 的目的仅仅是上下文.如果您想要零次或多次重复文字 ( 字符，您必须使用 \(? 来转义括号.

So essentially, the purpose of the ? is just contextual. If you wanted zero or more repetitions of a literal ( character you'd have to use \(? to escape the paren.

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