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.$` is an alias for this property.
leftContext property is static, it is not a property of an individual regular expression object. Instead, you always use it as
The value of the
leftContext property is read-only and modified whenever a successful match is made.
You can not use the shorthand alias with the dot property accessor (
RegExp.$`), because the parser expects a starting template string in that case and a
SyntaxError is thrown. Use the bracket notation for property access.
var re = /world/g; re.test('hello world!'); RegExp.leftContext; // "hello " RegExp['$`']; // "hello "
Non-standard. Not part of any current specification.