JSFiddle - React, Tailwind, and code Playground

HTML

<a href="group1/A"></a>
<a href="group1/B"></a>
<a href="group1/C"></a>
<a href="group2/C"></a>
<a href="group2/D"></a>
<a href="group2/E"></a>
<a href="group3/F"></a>
<a href="group3/G"></a>
<a href="group3/H"></a>

JavaScript

var subjects = {};

$('a').each(function(index) {
    var prefix = $(this).attr("href").replace(/([^\/]*)\/.*/, "$1");
    var name = $(this).attr("href").replace(/[^\/]*\/(.*)/, "$1");

    if (!(prefix in subjects)) {
        subjects[prefix] = new Array();
    }

    subjects[prefix].push(name);
});

console.log(subjects);