regex - Is it possible for a Regular Expression to Lookahead OR Lookbehind?

Question

I'm struggling to create some kind of RegEx that can either look ahead or look behind.

Something ideal would be to match the a in both ab and ba, triggered by the presence of the b.

Obviously I could repeat the a, such as:

((?<=b)a)|a(?=b) 

But can it be done without the repetition of the matching set?

Answer 1

免责声明：此答案使用PCRE语法/风格。语法可能与其他风格不同，其中一些根本不支持它。

首先，我会警告您，您应该使用以下内容：(?<=b)a|a(?=b). 它简单易懂。

现在，如果您想查看另一种解决方案，我想出了以下a(?(?=b)|(?<=b.)). 那么这是什么意思 ？

a               # match a
(?              # if
    (?=b)       # there is a "b" ahead
    |           # else
    (?<=b.)     # there is a "b" 2 steps behind
)               # end if

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Answer 2

匹配集对我来说似乎是最简单和最清晰的方法。也就是说，您始终可以构建一个仅向前看的正则表达式，然后将其应用于目标字符串及其反向字符串。