Rsa97

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var t = {
  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 list = {};
var re = /^(.*?)\[(\d+)\]$/;
var match;
for (var prop in t.list) {
  match = prop.match(re);
  if (typeof list[match[1]] == "undefined") {
    list[match[1]] = [];
  }
  list[match[1]][match[2]] = t.list[prop];
}
t.list = list;
console.log(t);

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