Lack of essential detail about regexp lookbehind syntax in "Lookahead and lookbehind" article

[Part 3 (Additional articles)](https://javascript.info/#tab-3)
[Chapter 7 (Regular expressions)](https://javascript.info/regular-expressions)
[Article 14 (Lookahead and lookbehind)](https://javascript.info/regexp-lookahead-lookbehind)

Why you don't mention that in JS non fixed length regexps (including quantifiers, backreferences etc.) are not allowed inside lookbehind expression ?

First you talk about [lookahead](https://javascript.info/regexp-lookahead-lookbehind#lookahead), and using regexp with quantifier as lookahead condition in your example.

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For example, `\d+(?=\s)`**`(?=.*30)`** looks for `\d+` that is followed by a space `(?=\s)`, and there’s `30` somewhere after it **`(?=.\*30)`**

Right after this you briefly talk about [lookbehind](https://javascript.info/regexp-lookahead-lookbehind#lookbehind) and say it is the same thing but looks behind, without mentioning that it actually cannot do the same thing with non fixed lenght regular expressions (including quantifiers like **\***).

从 issue 中链接的“Lookahead and lookbehind”文章开始，尤其阅读其中关于 lookahead 和 lookbehind 的部分。将文档中的示例与 JavaScript 对 lookbehind 表达式中非固定长度模式的限制进行比较。完成的标准是文章清楚地解释这一限制，并区分 lookahead 的行为与 lookbehind 的行为。

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