OOP way of prototype extension

/**
 * Copyright 9re ( http://wonderfl.net/user/9re )
 * MIT License ( http://www.opensource.org/licenses/mit-license.php )
 * Downloaded from: http://wonderfl.net/c/uILO
 */

package {
    import flash.text.TextField;
    import flash.display.Sprite;
    public class FlashTest extends Sprite {
        public function FlashTest() {
            var tf:TextField = new TextField;
            var str:String ="abbbcccaa";
            tf.text = 'result : ' + new StringEx(str)
                                         .replaceAll('b', 'c')
                                         .replaceAll('c', 'a');
            addChild(tf);
        }
    }
}

class StringEx {
    private var string:String;
    public function StringEx(string:String) {
        this.string = string;
    }
    public function replaceAll(replace:String, replaceWith:String):StringEx {
        string = string.replace(new RegExp(replace, 'g'), replaceWith);
        return this;
    }
    public function toString():String {
        return string;
    }
}