如何解释"$ 1，$ 2"在JavaScript中使用正则表达式时? [英] How to explain "$1,$2" in JavaScript when using regular expression?

问题描述

一段JavaScript代码如下:

A piece of JavaScript code is as follows:

num = "11222333";
re = /(\d+)(\d{3})/;
re.test(num);
num.replace(re, "$1,$2");

我无法理解"$ 1，$ 2" 的语法.带有此代码的书说 $ 1 表示 RegExp.$ 1 ， $ 2 表示 RegExp.$ 2 .但是这些解释导致了更多的问题:

I could not understand the grammar of "$1,$2". The book from which this code comes says $1 means RegExp.$1, $2 means RegExp.$2. But these explanations lead to more questions:

1. 众所周知，在JavaScript中，变量名称应以字母或_开头，在这里 $ 1 如何成为RegExp成员变量的有效名称?

1. It is known that in JavaScript, the name of variables should begin with letter or _, how can $1 be a valid name of member variable of RegExp here?

如果我输入 $ 1 ，则命令行显示未定义.如果我输入"$ 1" ，则命令行仅回显 $ 1 ，而不是11222.因此，replace方法如何知道 $ 1，$ 2" 是什么意思?

If I input $1, the command line says it is not defined; if I input "$1", the command line only echoes $1, not 11222. So, how does the replace method know what "$1,$2" mean?

谢谢.

推荐答案

它不是变量"，-在 .replace()调用中使用的占位符. $ n 表示正则表达式的 nth 个捕获组.

It's not a "variable" - it's a placeholder that is used in the .replace() call. $n represents the nth capture group of the regular expression.

var num = "11222333";

// This regex captures the last 3 digits as capture group #2
// and all preceding digits as capture group #1
var re = /(\d+)(\d{3})/;

console.log(re.test(num));

// This replace call replaces the match of the regex (which happens
// to match everything) with the first capture group ($1) followed by
// a comma, followed by the second capture group ($2)
console.log(num.replace(re, "$1,$2"));

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