JSFiddle - React, Tailwind, and code Playground

JavaScript

var re = /(.+?)\[(\d+)\]\[(.+?)\]/;
    var result = re.exec("user[2][city]");
    if (result != null)
    {
        var firstString = result[1]; // will contain "user"
        var secondString = result[2]; // will contain "2"
        var thirdString = result[3]; // will contain "city"
        alert(firstString + "\n" + secondString + "\n" + thirdString);
    }