Steam Key Quick Activator

quicksteamactivator.user.js

// ==UserScript==
// @name         Steam Key Quick Activator
// @namespace    http://pedrofracassi.me/
// @version      1.2
// @description  Activates Steam Keys Quickly!
// @author       Pedro Fracassi (http://pedrofracassi.me)
// @match        https://store.steampowered.com/account/registerkey?key=*
// @grant        none
// @run-at       document-end
// ==/UserScript==

'use strict';

var key = document.getElementById('product_key').value;
if (key != '') {
// Check for Key
    var flag = true;
    var array = key.split("-");
    if (array.length === 3 || array.length === 5){
      array.forEach(y => {
        if (y.length !== 5) flag = false;
      });
    }
    else flag = false;
    if (flag === true) {
      document.getElementById('accept_ssa').click();
      document.getElementById('register_btn').click();
    }
}

quicksteambookmarklet.js

javascript:
var selection = window.getSelection().toString();

if (selection) {
  location.href = 'https://store.steampowered.com/account/registerkey?key=' + selection;
} else {
  result = prompt('Insert Steam Key');
	if(result != null){
		location.href = 'https://store.steampowered.com/account/registerkey?key=' + result;
	} else {
		location.href = 'https://store.steampowered.com/account/registerkey';
	}
}

How do I install this?
Install tampermonkey, and then click this link

If you want to be even faster, add this code to your bookmarks and click it whenever you need to activate a key.
It will open steam's activation page with your selected text as a key. If you don't have anything selected, it will ask you for a key.

AAAAA-BBBBB-CCCCC

It took way too long, but I managed to work out the regex for the steam key formats:

key.match(/((((?![osuOSU])[a-zA-z1-9]){5}-){2}){1,2}((?![osuOSU])[a-zA-z1-9]){5}$|((?![osuOSU])[a-zA-z1-9]){15}$/g);

It matches to
AAAAA-BBBBB-CCCCC
AAAAA-BBBBB-CCCCC-DDDDD-EEEEE
237ABCDGHJLPRST
With case-insensitive alphabet and digits but without O, S, U, and 0, probably because they look too similar to other characters.