$_GET is empty after defining a rewrite rule - wordpress

Question

Hello Wordpress ninjas,
I am new to wordpress develpoment. I define a rewrite rule (with a tag), similar to what you can find in examples here and here.
$wp_rewrite->add_rewrite_tag('%filterx_location%', '([^&/]+)', 'filterx_location=');
$wp_rewrite->add_rule('property-location/(.+)$','index.php?post_type=property&filterx_location=$matches[1]', 'top');
Now, the problem is, that when the query is completed (the page is shown), the filterx_location paramater is not set. In fact, var_dump($_GET) gives me an array(0) { }.
I have a black out or something, seems like something simple I miss here, just cannot figure it out :-/
Any help is much appreciated!
UPDATE
what is even stranger is that when I generate rewrite rules with:
$wp_rewrite->add_rewrite_tag('%filterx_location%', '([^/]+)', 'filterx_location=');
$permalink_prefix = 'property-location';
$permalink_structure = '%filterx_location%/';
$rewrite_rules = $wp_rewrite->generate_rewrite_rules($permalink_prefix.'/'.$permalink_structure, EP_ALL, true, true, true, true, true);
I see a bunch of generated urls matches and redirects, if I print them. One of them is:
property-location/([^/]+)/page/?([0-9]{1,})/?$ => index.php?filterx_location=$matches[1]&paged=$matches[2]&post_type=property
I add all the generated urls with:
foreach($rewrite_rules as $regex => $redirect) {
if(strpos($redirect, 'attachment=') === false) {
//add the post_type to the rewrite rule
$redirect .= '&post_type=property';
}
//turn all of the $1, $2,... variables in the matching regex into $matches[] form
if(0 < preg_match_all('#\$([0-9])#', $redirect, $matches)) {
for($i = 0; $i < count($matches[0]); $i++) {
$redirect = str_replace($matches[0][$i], '$matches['.$matches[1][$i].']', $redirect);
}
}
//add the rewrite rule to wp_rewrite
$wp_rewrite->add_rule($regex, $redirect, 'top');
}
And If I go to the URL /property-location/madrid/page/2/ then the query_vars correctly do have ["paged"]=> int(2). But the filterx_location is totally ignored!

Answer 1

Ok, I got it working, in a strange turn of events. First I replaced
$wp_rewrite->add_rewrite_tag('%filterx_location%', '([^/]+)', 'filterx_location=');
with:
add_rewrite_tag('%filterx_location%', '([^/]+)', 'filterx_location=');
I noticed those are two different things. I took a look into the wordpress code of add_rewrite_tag:
function add_rewrite_tag($tagname, $regex) {
//validation
if ( strlen($tagname) < 3 || $tagname[0] != '%' || $tagname[strlen($tagname)-1] != '%' )
return;
$qv = trim($tagname, '%');
global $wp_rewrite, $wp;
$wp->add_query_var($qv);
$wp_rewrite->add_rewrite_tag($tagname, $regex, $qv . '=');
}
I thought the "add_query_var" part might be essential.
But it still did not work!
So what I did finally was add another call to add_query_var() after the add_rewrite_tag() call:
add_rewrite_tag('%filterx_location%', '([^/]+)', 'filterx_location=');
$wp->add_query_var('filterx_location');
and only then everything started to work. Initially I assumed that $_GET should be holding my variable, but then even calls to get_query_var('filterx_location') were empty. Now it get the right stuff.