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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.$` is an alias for this property.
Syntax
RegExp.leftContext RegExp['$`']
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
Non-standard. Not part of any current specification.
Browser compatibility
| Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
|---|---|---|---|---|---|
| Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
| Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
|---|---|---|---|---|---|---|
| Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |