How do you access the base class when extending the String class

[lloyd13]

I want to extend the String class so that I can add a replace method.

Currently I have the following

class RegString {

var str:String;

function RegString( string:String ) {
str = string;
}

function replace( matchstr:String, replacestr:String ) {

var s = str;
.
.
.


The replace function works fine but I can't use any RegString instances like String objects, whereas I would like to be able to do assignments etc... with other String objects.

So what I want is

class RegString extends String {

function replace( matchStr:String, replace:String ) {

...but how do I access the base string here?...