Note: All
RegExpstatic properties that expose the last match state globally are deprecated. See deprecated RegExp features for more information.
The RegExp.input static accessor property returns the string against which a regular expression is matched. RegExp.$_ is an alias for this property.
Description
Because input is a static property of RegExp, you always use it as RegExp.input or RegExp.$_, rather than as a property of a RegExp object you created.
The value of input updates whenever a RegExp (but not a RegExp subclass) instance makes a successful match. If no matches have been made, input is an empty string. You can set the value of input, but this does not affect other behaviors of the regex, and the value will be overwritten again when the next successful match is made.
Examples
Using input and $_
const re = /hi/g;
re.test("hi there!");
RegExp.input; // "hi there!"
re.test("foo"); // new test, non-matching
RegExp.$_; // "hi there!"
re.test("hi world!"); // new test, matching
RegExp.$_; // "hi world!"