确保正则表达式不匹配空字符串 - 但有一些警告 [英] Make sure regex does not match empty string - but with a few caveats

问题描述

有一个问题我需要做，但有一些注意事项使它变得困难.

There is a problem that I need to do, but there are some caveats that make it hard.

问题:匹配字母表 {abc} 上最多包含一个 a 的所有非空字符串.

Problem: Match on all non-empty strings over the alphabet {abc} that contain at most one a.

示例

一个
ABC
英国广播公司
bbcabb

a
abc
bbca
bbcabb

非示例

aa
呜呜

aa
bbaa

警告:您不能使用前瞻/后视.

Caveats: You cannot use a lookahead/lookbehind.

我拥有的是这个:

^[bc]*a?[bc]*$

但它匹配空字符串.也许是一个提示?我知道任何事情都会有帮助

but it matches empty strings. Maybe a hint? Idk anything would help

(如果重要的话，我正在使用 python).

(And if it matters, I'm using python).

推荐答案

据我了解您的问题，唯一的问题是您当前的模式匹配空字符串.为了防止这种情况，您可以使用 字边界 \b至少需要一个单词字符.

As I understand your question, the only problem is, that your current pattern matches empty strings. To prevent this you can use a word boundary \b to require at least one word character.

^\b[bc]*a?[bc]*$

查看 regex101 上的演示

另一种选择是 alternate 在 组.匹配由任意数量的 [bc] 或一个或多个 [bc] 从头到尾包围的 a ，它可能如下所示:^(?:[bc]*a[bc]*|[bc]+)$

Another option would be to alternate in a group. Match an a surrounded by any amount of [bc] or one or more [bc] from start to end which could look like: ^(?:[bc]*a[bc]*|[bc]+)$

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