RegExp.$_ (Property)

An alias for the input buffer of the regular expression.

Availability:

JavaScript - 1.2
JScript - 3.0
Internet Explorer - 4.0
Netscape - 4.0
Netscape Enterprise Server - 3.0
Opera 5.0
Property/method value type:String primitive
JavaScript syntax:-myRegExp.$_

Refer to:

RegExp.input