在Python前瞻正则表达式中,*的目的是什么?

前瞻意味着 展望未来。所以如果你写:

(?=a)

这意味着 a 。有时,例如在检查密码时,您不希望这样做。你想表达的是,在某个地方应该有一个 .因此:

(?=.*a)

b 8 或 @ 一 在某处

^               # start a match at the beginning of the string
(?=.*[a-z])     # should contain at least one a-z character
(?=.*[A-Z])     # should contain at least one A-Z character
(?=.*\d)        # should contain at least one digit
[a-zA-Z\d]{8,}  # consists out of 8 or more characters and only A-Za-z0-9
$               # end the match at the end of the string

没有 .* ,永远不会有匹配的

 "^(?=[a-z])(?=[A-Z])(?=\d)[a-zA-Z\d]{8,}$"

指:

^               # start a match at the beginning of the string
(?=[a-z])       # first character should be an a-z character
(?=[A-Z])       # first character should be an A-Z character
(?=\d)          # first character should be a digit
[a-zA-Z\d]{8,}  # consists out of 8 or more characters and only A-Za-z0-9
$               # end the match at the end of the string

因为没有同时是A-Z字符和数字的字符。这永远不会满足。

1. 我们有 不
2. 圆点 . 默认情况下是这样的 not match the new line character ;
3. ^[A-Za-z0-9]{8,}$ 这意味着您只需验证输入,而不需要新行。