2.8.6.8 RegExp.$_

The initial value of each of the properties  RegExp.$1, RegExp.$2, RegExp.$3, RegExp.$4, RegExp.$5, RegExp.$6, RegExp.$7, RegExp.$8, and RegExp.$9 is the empty string. These properties shall have the attributes { [[Enumerable]]: false, [[Configurable]]: false, [[Writable]]: false }. The value of this property may be modified by calls to RegExp.prototype.exec. The properties RegExp.input and RegExp.$_ always have the same value. When one of these properties is set to some value, the other is automatically also set to that same value. Unlike most other RegExp constructor properties, this property is writable.

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