如何在正则表达式中表示 epsilon？

发布于 2024-09-19 20:08:32 字数 899 浏览 5 评论 0原文

教科书教我们使用 epsilon (ε) 符号编写正则表达式，但是如何将该符号直接转换为代码，而不必完全重新编写正则表达式呢？

例如，我将如何编写这个正则表达式来捕获所有以 a 开头或结尾（或两者）的小写字符串。

不能 100% 确定这是正确的，但是...

((a|epsilon)[az]*a) | (a[az]*(a|epsilon))

所以一些应该匹配的字符串包括：

a //single "a" starts or ends with "a"

aa //starts and ends with "a"

ab //starts with "a"

ba //ends with "a"

aba //starts and ends with "a"

aaaaaaaa //starts and ends with "a"

abbbbbbb //starts with "a"

bbbbbbba //ends with "a"

abbbbbba //starts and ends with "a"

asdfhgdu //starts with "a"

onoineca //ends with "a"

ahnrtyna //starts and ends with "a"

我只用什么来交换 epsilon 为正确的符号，我不想修改表达式其余部分的任何部分。另外我想澄清一下，我实际上并不是在检查 epsilon 符号，我想选择一个字符或什么都没有（当然不是什么都没有...... epsilon）。

这样的符号存在吗？

我想要的可能吗？

原文

The text book teaches us to write regular expressions using the epsilon (ε) symbol, but how can I translate that symbol directly to code without having to completely rework my regular expression?

For instance, how would I write this regex which would catch all lowercase strings that either begin or end in a (or both).

Not 100% sure this is correct but...

((a|epsilon)[a-z]*a) | (a[a-z]*(a|epsilon))

So some strings that should match include:

a //single "a" starts or ends with "a"

aa //starts and ends with "a"

ab //starts with "a"

ba //ends with "a"

aba //starts and ends with "a"

aaaaaaaa //starts and ends with "a"

abbbbbbb //starts with "a"

bbbbbbba //ends with "a"

abbbbbba //starts and ends with "a"

asdfhgdu //starts with "a"

onoineca //ends with "a"

ahnrtyna //starts and ends with "a"

I only what to exchange epsilon for the correct symbol, I do not want to modify any part of the rest of the expression. Also I want to be clear, I am not actually checking for the epsilon symbol, I want to have a choice of a character or nothing (well not nothing... epsilon).

Does such a symbol exist?

Is what I want possible?

分享到QQ

分享到微博