JSFiddle - React, Tailwind, and code Playground

JavaScript

var str="itss[BACK][BACK][BACK][BACK][BACK][BACK] it's a test stringgg[BACK][BACK]";

var backspaces = str.match(/\[BACK\]/g).length;

for(i=0; i<backspaces; i++)
{
    str = str.replace(/.?\[BACK\]/, '');
}
document.write( str );