JSFiddle - React, Tailwind, and code Playground

JavaScript

var str = "itss[BACK][BACK][BACK][BACK][BACK][BACK] it's a test stringgg[BACK][BACK]";
var word = '[BACK]';
var substrings = str.split(word);
var cnt = substrings.length - 1;
for (i = 0; i < cnt; i++) {
    pos = str.search("[BACK]");
    if (pos - 1 > 0) {
        str = str.replace(str.substring(pos - 2, pos + 5), '');
    } else {
        str = str.replace(str.substring(pos - 1, pos + 5), '');
    }
}
alert(str);