Note: All
RegExpstatic properties that expose the last match state globally are deprecated. See deprecated RegExp features for more information.
The RegExp.$1, …, RegExp.$9 static accessor properties return parenthesized substring matches.
Description
Because $1–$9 are static properties of RegExp, you always use them as RegExp.$1, RegExp.$2, etc., rather than as properties of a RegExp object you created.
The values of $1, …, $9 update whenever a RegExp (but not a RegExp subclass) instance makes a successful match. If no matches have been made, or if the last match does not have the corresponding capturing group, the respective property is an empty string. The set accessor of each property is undefined, so you cannot change the properties directly.
The number of possible parenthesized substrings is unlimited, but the RegExp object can only hold the first nine. You can access all parenthesized substrings through the returned array's indexes.
$1, …, $9 can also be used in the replacement string of String.prototype.replace, but that's unrelated to the RegExp.$n legacy properties.
Examples
Using $n with RegExp.prototype.test()
The following script uses the RegExp.prototype.test method to grab a number in a generic string.
const str = "Test 24";
const number = /(\d+)/.test(str) ? RegExp.$1 : "0";
number; // "24"
Please note that any operation involving the usage of other regular expressions between a re.test(str) call and the RegExp.$n property, might have side effects, so that accessing these special properties should be done instantly, otherwise the result might be unexpected.