How can I optimise and improve getting 6 possible URL parameters - php

Question

I am in the midst of writing a simple job board and I have the URL's filtering and working how I want, everything works - I just want to know if I can improve it, if there are any security concerns, and if it is horribly inefficient.
Take the following URL section - foo.com/in/london/at/google/do/design
I assign each section a variable and work out the location, company, and category.
Other use cases that work:
Switching the order - foo.com/at/google/in/london/do/design
Having less parameters - foo.com/in/london/at/google
My code to figure out all these variables is:
// Get the different possible page parameters
// Only allow alphanumeric, dashes, and commas
$regex = "[^a-zA-Z0-9,-]";
$a = isset($_GET["a"]) ? preg_replace("/".$regex."/", "", $_GET["a"]) : "";
$aa = isset($_GET["aa"]) ? preg_replace("/".$regex."/", "", $_GET["aa"]) : "";
$b = isset($_GET["b"]) ? preg_replace("/".$regex."/", "", $_GET["b"]) : "";
$bb = isset($_GET["bb"]) ? preg_replace("/".$regex."/", "", $_GET["bb"]) : "";
$c = isset($_GET["c"]) ? preg_replace("/".$regex."/", "", $_GET["c"]) : "";
$cc = isset($_GET["cc"]) ? preg_replace("/".$regex."/", "", $_GET["cc"]) : "";
if ($a == "in" && $aa != null) { $searchLocation = $aa; }
if ($b == "in" && $bb != null) { $searchLocation = $bb; }
if ($c == "in" && $cc != null) { $searchLocation = $cc; }
if ($a == "at" && $aa != null) { $searchCompany = $aa; }
if ($b == "at" && $bb != null) { $searchCompany = $bb; }
if ($c == "at" && $cc != null) { $searchCompany = $cc; }
if ($a == "do" && $aa != null) { $searchCategory = $aa; }
if ($b == "do" && $bb != null) { $searchCategory = $bb; }
if ($c == "do" && $cc != null) { $searchCategory = $cc; }
if ($a == "missinghtml") { $errorUrl = true; }
I'm looking at this thinking there must be a better to do this, and is this section secure?
Any thoughts on this are much appreciated. Like I say it works, but can it be better? Thanks :)