如何用正则表达式表示epsilon?

(ε) 符号,但如何将该符号直接转换为代码而不必完全重写正则表达式?

a (或两者兼有)。

不是100%确定这是正确的,但是。。。

((a)| epsilon * a) |(a[a-z] (一)| ε

因此,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"

我只想交换什么 ε

这样的符号存在吗?

我想要的可能吗?