为什么连续的前瞻并不总是有效 [英] Why consecutive lookaheads do not always work

问题描述

我想要一个正则表达式，当至少有5个字符和2个数字时返回true。为此，我使用了前瞻（即（？= ...））。

I want a regex which returns true when there is at least 5 characters et 2 digits. For that, I use a the lookahead (i. e. (?=...)).

// this one works
let pwRegex = /(?=.{5,})(?=\D*\d{2})/;
let result = pwRegex.test("bana12");

console.log("result", result) // true

// this one won't
pwRegex = /(?=.{5,})(?=\d{2})/;
result = pwRegex.test("bana12");

console.log("result", result) // false

为什么我们需要添加 \ D * 才能使其正常工作？

Why we need to add \D* to make it work ?

对我来说， \d {2} 比 \ D * \d {2} <宽松/ code>所以它不应该允许接受测试？

For me, \d{2} is looser than \D*\d{2} so it should not allow an acceptance of the test?

推荐答案

你的前瞻只测试当前的匹配位置。由于你不匹配任何东西，这意味着从一开始。由于 bana12 不以两位数字开头，因此 \d {2} 失败。它就这么简单;）

Your lookaheads only test from the current match position. Since you don't match anything, this means from the start. Since bana12 doesn't start with two digits, \d{2} fails. Its as simple as that ;)

另外，请注意， \d {2} 表示你的数字有相邻。这是你的意图吗？

Also, note that having \d{2} means your digits has to be adjacent. Is that your intention?

要简单地要求2位数，不需要相邻，请尝试

To simply require 2 digits, that doesn't need to be adjacent, try

/(?=.{5,})(?=\D*\d\D*\d)/

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