JSFiddle - React, Tailwind, and code Playground

JavaScript

var txt = "I expect five hundred dollars {{$500}}. and new brackets {{$600}}";

var regExp = /\{([^)]+)\}/g;
var matches = txt.match(regExp);
for (var i = 0; i < matches.length; i++) {
    var str = matches[i];
    alert(str.substring(1, str.length - 1));
}