JSFiddle - React, Tailwind, and code Playground

JavaScript

var doit = function (s) {
    var res = s.match(/ABCD/g);
    return res ? res.join('') : '';
};

var myStr;

myStr = "ABCA ABCB ABCD BCD ABC";
console.log(1, doit(myStr));

myStr = "ABCA ABCD ABCD ABCB ABCD BCD ABC";
console.log(3, doit(myStr));

myStr = "ABCA ABCB ABC";
console.log(0, doit(myStr));