Javascript字符串取代怪异 - $$$$被折叠为$$ - 这个结果背后的原因是什么？ [英] Javascript string replace weirdness -- $$$$ gets collapsed to $$ -- what's the reason behind this result?

问题描述

在工作中，我遇到了一个问题，即我们的应用程序的用户正在接收具有无效unicode字符（0xffff）的消息，根据标准，该字符永远不应映射到符号。

At work, I was encountering a problem where users of our application were receiving messages featuring an invalid unicode character (0xffff), which according to the standard, should never be mapped to a symbol.

作为快速工作，我做了以下几点：

As a quick work aound I did the following:

badStr.replace(/\uffff/g, " ");

它按预期工作，并让用户继续使用该应用程序，直到我们找到更好的解决方案。

Which works as expected, and lets the user continue using the application until we find a better solution.

然而，当我正在玩这个时，我随机尝试了一个字符串替换$$$$，它以某种方式折叠了$$。

However, while I was playing around with this, I randomly tried a string replacement of "$$$$" which somehow got collapsed "$$".

你可以亲自看看。尝试在浏览器网址栏中粘贴以下行：

You can see for yourself. Try pasting the following lines in your browser url bar:

javascript: alert(String.fromCharCode(0xffff).replace(/\uffff/g, "@@@@"));

结果为@​​@@@

但

javascript: alert(String.fromCharCode(0xffff).replace(/\uffff/g, "$$$$"));

结果为$$

这实际上是似乎是任何字符串替换的问题，使用$$$$作为字符串替换。

This actually seems to be a problem with any string replacement, with $$$$ as the string replacement.

两者：

javascript: alert(String.fromCharCode(0x1234).replace(/\u1234/g, "$$$$"));
javascript: alert("hella".replace("h", "$$$$")); 

导致$$崩溃。

关于字符串替换为什么会这样做的任何想法？

Any ideas as to why the string replacement behaves this way?

推荐答案

那是因为 $ 具有特殊含义（组扩展）。看看这个例子：

That's because $ in the replace string has special meaning (group expansion). Have a look at this example:

alert("foo".replace(/(.*)/, "a$1b"));

这就是为什么 $$ 被解释为 $ ，对于你需要用 $ 1 实际替换的东西（字面意思是没有组扩展）：

That's why $$ is interpreted as $, for the case where you would need to actually replace something by $1 (literally, without group expansion):

alert("foo".replace(/(.*)/, "a$$1b"));

另见 https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/String/replace#Specifying_a_string_as_a_parameter 。

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