After looking at the following documents, this grouping is basically the same as the regular grouping.

For example/a/foo.jsThe expression above/a/(**.js)

$0:/a/foo.js
$1:foo.js

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update:

Can refer tohttps://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/RegExp

Explanation of Regular Grouping

(x)
 
 Match x and capture matches.  This is called capturing parentheses.
 
 For example, /(foo)/ matches and captures "foo" in "foo bar."  The matched substring can be found in the elements [1], ..., [n] of the result array, or in the attributes $1, ..., $9 of the defined RegExp object.

Here, $1, …, $9 is the matching content in the first, …, ninth brackets.

In brief:

$0Is all the matching content,$1Is the first parenthetical match, …$9Is the ninth bracket match