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RegExp.n

The legacy RegExp $1, $2, $3, $4, $5, $6, $7, $8, $9 properties are static and read-only properties of regular expressions that contain parenthesized substring matches.

Description

The $1, ..., $9 properties are static, they are not a property of an individual regular expression object. Instead, you always use them as RegExp.$1, ..., RegExp.$9.

The values of these properties are read-only and modified whenever successful matches are made.

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.

These properties can be used in the replacement text for the String.replace method. When used this way, do not prepend them with RegExp. The example below illustrates this. When parentheses are not included in the regular expression, the script interprets $n's literally (where n is a positive integer).

Examples

Using $n with String.replace

The following script uses the replace() method of the String instance to match a name in the format first last and output it in the format last, first. In the replacement text, the script uses $1 and $2 to indicate the results of the corresponding matching parentheses in the regular expression pattern.

var re = /(\w+)\s(\w+)/;
var str = 'John Smith';
str.replace(re, '$2, $1'); // "Smith, John"
RegExp.$1; // "John"
RegExp.$2; // "Smith"

Using $n with RegExp.test

The following script uses the test() method of the RegExp instance to grab a number in a generic string.

var str = 'Test 24';
var 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.

Specifications

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
RegExp.$1-$9 1 12 1 4 5 1
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
RegExp.$1-$9 1 18 4 10.1 1 1.0
Server
Node.js
RegExp.$1-$9 Yes

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/n