量词 {0} 在某些情况下有意义吗? [英] Does the quantifier {0} make sense in some scenarios?
问题描述
示例:
/(?:Foo){0}bar/
/(?:Foo){0}bar/
我在另一个答案中看到了类似的内容.起初我想那应该是什么"，但后来，好吧，有点消极的看法"，所以在 bar 之前不允许 Foo，但这不起作用.
I saw something like this in another answer. At first I thought "what should that be", but then, "OK could make sense, kind of a negative look behind", so that Foo is not allowed before bar, but this is not working.
你可以看到这个 这里在 Regexr:它只匹配 bar 但它还匹配 Foobar 中的 bar.
当我为行的开头添加一个锚点时:
You can see this here on Regexr: It matches only bar but it matches also the bar in Foobar.
When I add an anchor for the start of the row:
/^(?:Foo){0}bar/
它的行为就像我期望的那样.它只匹配 bar 而不是 Foobar 中的 bar.
it behaves like I expect. It matches only the bar and not the bar in Foobar.
但这与我只使用 /bar/ 或 /^bar/ 时的行为完全相同.
But that's exactly the same behaviour as if I used only /bar/ or /^bar/.
量词 {0} 只是一个无用的副作用，还是真的有一个有用的行为?
Is the quantifier {0} only a useless side effect, or is there really a useful behaviour for that?
推荐答案
{0} 有好的用途.它允许您定义您目前不打算捕获的组.这在某些情况下很有用:
There are good uses of {0}. It allows you to define groups that you don't intend to capture at the moment. This can be useful in some cases:
最好的一个 - 在递归正则表达式(或其他奇怪的结构)中使用组.例如，Perl 具有
(?(DEFINE) )用于相同的用途.
一个快速示例 - 在 PHP 中，这将匹配barFoo(工作示例):
preg_match("/(?:(Foo)){0}bar(?1)/", "barFoo", $matches);
将失败的捕获组(命名或编号)添加到结果匹配中.
Adding a failed captured group (named or numbered) to the result matches.
不太好的用途，正如彼得建议的那样:
Less good uses, as Peter suggested are useful in:
- 生成的模式.
- 可读性 - 具有一定重复性的模式，其中
{0}和{1}可能会引导人们朝着正确的方向思考.(好吧，不是最好的一点)
- Generated patterns.
- Readability - Patterns with certain duplication, where
{0}and{1}may lead thinking in the right direction. (OK, not the best point)
这些都是罕见的情况，在大多数情况下都是错误的.
These are all rare cases, and in most patterns it is a mistake.
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