.htaccess: append subdomain as query to existing queries - regex

Question

I'm trying to append the subdomain as a query to eventual existing queries using htaccess.
http://test.domain.com should be http://test.domain.com?x=test
http://test.domain.com?id=1 should be http://test.domain.com?id=1&x=test
This is what I have done, but it doesn't work and I can figure out why:
RewriteCond %{HTTP_HOST} ^([a-z0-9_-]+)\.domain\.com$ [NC]
// exclude www.domain.com
RewriteCond %1 !^(www)$ [NC]
RewriteRule ^[^\?]*(?:\?(.*))?$ index.php?$1&x=%1 [L]
My understanding was
[^\?]* all characters except ?, match 0 or more times
(?: start of a non capturing group
\? match ? literally
(.*) all characters after ? as a group
)? end of the non capturing group, match 0 or 1 times
But it does not work. Where is my mistake?
UPDATE 1:
I could make it work by using the following rule
RewriteRule (.*) index.php?$1&x=%1 [QSA,L]
http://test1.domain.com?y=test1 brings me [x=>test1,y=>test2]
but
http://test1.domain.com?y=test1&x=test3 brings me [x=>test3,y=>test2]
So it overrides my x value. Is there a way to block that?
UPDATE 2
This is the code I'm using now:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^(?!www)([\w-]+)\. [NC]
RewriteCond %1::%{QUERY_STRING} !^(.+?)::x=\1(?:&|$) [NC]
RewriteRule ^ index.php?%{QUERY_STRING}&x=%1 [L]

Answer 1

Try this rule:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^(?!www)([\w-]+)\. [NC]
RewriteCond %1::%{QUERY_STRING} !^(.+?)::x=\1(?:&|$) [NC]
RewriteRule ^ index.php?%{QUERY_STRING}&x=%1 [L]
Make sure this is the only rule you have in .htaccess while testing.
Explanation of:
RewriteCond %{HTTP_HOST} ^(?!www)([\w-]+)\. [NC]
RewriteCond %1::%{QUERY_STRING} !^(.+?)::x=\1(?:&|$) [NC]
We are capturing starting part of hostname from this group: ([\w-]+) which is denoted by %1. Note that we cannot use %1 in RHS of a condition.
We are then appending %1 and %{QUERY_STRING} together in %1::%{QUERY_STRING}. Here we could use any other arbitrary delimiter like ## as well.
In RHS we have ^(.+?)::x=\1(?:&|$) which means %1 followed by delimiter :: followd by literal x= and then \1 which is back-reference for %1 (goup before ::). ! before ^ is there to negate the condition. In simple words this condition means execute this rule only if we already don't have x=subdomain in QUERY_STRING.

Answer 2

Looks like you are trying to match the query string content with your RewriteRule’s pattern – that is not possible, it searches only the path component of the requested URL.
But, no worries – there’s an easy solution that helps combine the original query string, and what the pattern matched: The QSA flag.
So this should do the trick (combined with your existing RewriteConds):
RewriteRule (.*) index.php?$1 [QSA,L]