iljaGolubev

var l={
  list: {
    "action_buttons[0]": "View",
    "action_buttons[1]": "Share",
    "action_buttons[2]": "Download",
    "columns[0]": "Date of Study",
    "columns[1]": "Patient",
    "columns[2]": "File name",
    "columns[3]": "Reporting <br>Physician",
    "columns[4]": "Institution"
  }
};

var n = {};
for (var item in l.list) {
    if (l.list.hasOwnProperty(item)) {
    r = item.replace(/\[\d\]/,'');
    if(n[r]==undefined){ n[r]=[] }
    n[r].push(l.list[item]);
    }
}
l.list = n;
console.log(l);