Javascript 字符串替换了奇怪的东西——$$$ 被折叠为 $$——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，即我们应用程序的用户收到的消息包含无效的 unicode 字符 (0xffff)，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.
为了快速工作，我做了以下工作:
As a quick work aound I did the following:
badStr.replace(/uffff/g, " ");
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，当我在玩这个的时候，我随机尝试了一个字符串替换$$$$"，结果以某种方式折叠了$$".
However, while I was playing around with this, I randomly tried a string replacement of "$$$$" which somehow got collapsed "$$".
你可以自己看看.尝试在您的浏览器网址栏中粘贴以下几行:
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@@@@
但是
javascript: alert(String.fromCharCode(0xffff).replace(/uffff/g, "$$$$"));
结果 $$
这实际上似乎是任何字符串替换的问题，以 $$$$ 作为字符串替换.
This actually seems to be a problem with any string replacement, with $$$$ as the string replacement.
两者:
javascript: alert(String.fromCharCode(0x1234).replace(/u1234/g, "$$$$"));
javascript: alert("hella".replace("h", "$$$$"));
导致 $$ 崩溃.
关于为什么字符串替换会以这种方式进行的任何想法?
Any ideas as to why the string replacement behaves this way?
推荐答案
那是因为替换字符串中的 $ 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 $$ 被解释为 $ 的原因，对于您需要用 $1 实际替换某些内容的情况(字面上，没有组扩展):
That's why $$ 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"));
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