The initial value of RegExp.input is the empty string. This property shall have the attributes { [[Enumerable]]: false, [[Configurable]]: false, [[Writable]]: true }. 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 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.