Web/JavaScript/Reference/Global objects/RegExp/leftContext

Non-standard This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.

The non-standard leftContext property is a static and read-only property of regular expressions that contains the substring preceding the most recent match. RegExp.$` is an alias for this property.

Description

The leftContext property is static, it is not a property of an individual regular expression object. Instead, you always use it as RegExp.leftContext or RegExp['$`'].

The value of the leftContext property is read-only and modified whenever a successful match is made.

You can not use the shorthand alias with the dot property accessor (RegExp.$`), because the parser expects a starting template string in that case and a SyntaxError is thrown. Use the bracket notation for property access.

Examples

Using leftContext and $`

var re = /world/g;
re.test('hello world!');
RegExp.leftContext; // "hello "
RegExp['$`'];       // "hello "

Specifications

Specification
Legacy RegExp features in JavaScript

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`RegExp.leftContext` ($`) Non-standard'	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 5.5	Opera Full support 8	Safari Full support 3	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support Yes

Legend

Full support: Full support
Non-standard. Expect poor cross-browser support.': Non-standard. Expect poor cross-browser support.