简化正则表达式“ab|a|b" [英] Simplifying the regex "ab|a|b"

问题描述

(如何)可以简化以下正则表达式:

(How) could the following regex be simplified:

ab|a|b

?

我正在寻找一个不那么冗余的，即只有一个 a 和一个 b.可能吗?

I'm looking for a less redundant one, i.e. with only one a and one b. Is it possible?

一些尝试:

a?b?       # matches empty string while shouldn't
ab?|b      # still two b

<小时>

请注意，真正的正则表达式具有更复杂的 a 和 b 部分，即不是单个字符而是内部子正则表达式.

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Note that the real regex has more complicated a and b parts, i.e. not a single char but inner subregexes let's say.

推荐答案

如果您正在使用 Perl 或某些 PCRE 引擎(如 PHP 的 preg_ 函数)，您可以参考模式中的前几组，像这样:

If you are using Perl or some PCRE engine (like PHP's preg_ functions), you can refer to previous groups in the pattern, like this:

/(a)(b)|(?1)|(?2)/

此功能的主要目的是支持递归，但也可用于模式重用.

The main purpose of this feature is to support recursion, but it can be used for pattern reuse as well.

请注意，在这种情况下，您无法避免在第一次交替中捕获 a 和 b，这会导致一些(可能)不必要的开销.为避免这种情况，您可以在永远不会执行的条件中定义组.执行此操作的规范方法是使用 (?(DEFINE)...) 组(检查命名的 DEFINE 组是否匹配任何内容，但该组当然不会'不存在):

Note that in this case you cannot get around capturing a and b in the first alternation, which incurs some (possibly) unnecessary overhead. To avoid this, you can define the groups inside a conditional that is never executed. The canonical way to do this is to use (?(DEFINE)...) group (which checks if a named DEFINE group matched anything, but of course that group doesn't exist):

/(?(DEFINE)(a)(b))(?1)(?2)|(?1)|(?2)/

如果您的引擎不支持该功能(，因为您使用的是 Java，因此不支持此功能)，那么您可以在单一模式中获得的最佳效果确实是

If your engine doesn't support that ( since you are using Java, no this feature is not supported), the best you can get in a single pattern is indeed

ab?|b

或者，您可以通过字符串连接/格式手动构建 ab|a|b 版本，例如:

Alternatively, you can build the ab|a|b version manually by string concatenation/formatting like:

String a = "a";
String b = "b";
String pattern = a + b + "|" + a + "|" + b;

这也避免了重复.或者您可以对主题字符串使用 3 个单独的模式 ab、a 和 b(其中第一个再次是后两个的串联).

This avoids the duplication as well. Or you can use 3 separate patterns ab, a and b against the subject string (where the first one is again a concatenation of the latter two).

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