JSFiddle - React, Tailwind, and code Playground

HTML

<div id="sample">[abc][123]</div>
<div id="log"></div>

CSS

#log label, #log span {
    float: left;
}
#log label {clear: left; font-weight: bold;}
#log label:after {
    content: " ";
}

JavaScript

(function () {
    "use strict";
    var duplicate = document.getElementById('sample').innerText,
        log = document.getElementById('log'),
        section = 'abc',
        pat = '\\[' + section + '\\]\\[(\\d+)\\]',
        reg = new RegExp(pat, 'g'),
        num = duplicate.replace(reg, "$1++");
    log.innerHTML = '<label>Pattern:</label><span>' + pat + '</span><label>Replace Result:</label><span>' + num + '</span>';
}());