.htaccess redirect only if GET parameter exists - php

Question

I have a client with an old website without 'pretty' URLs. So currently it looks like this:
http://www.domain.com/?w=42&a=5&b=3
The parameter values are numbers only.
Now they want to move the old site to a subdomain and main (www) domain would be home to a new website (WP with SEO friendly URLs).
Now what I would like to do is redirect all requests that come to the /?w=<num> (and ONLY those) to sub.domain.com/?w=<num>, so that existing links (mostly from Google) get redirected to the subdomain page, while the new page works serving new content thorough pretty URLs.
I tried this:
# This works, but redirects the entire www.domain.com
# to sub.domain.com no mather what
RewriteCond %{HTTP_HOST} ^www\.domain\.com$ [NC]
RewriteRule ^(.*)$ http://sub.domain.com/$1 [R=301,L]
# But this DOESN'T work
RewriteRule ^/?w(.*) http://sub.domain.com/?w$1 [R=301,L]
# Also tried to redirect 'by hand', but DIDN'T work either
Redirect 301 /?w=42 http://sub.domain.com/?w=42
What am I doing wrong? I searched high and low but always end up with this kind of suggestions. Or maybe I'm just searching for wrong keywords ...
Thank you!

Answer 1

You can't match against the query string inside a rewrite rule or a redirect directive. You need to match against the %{QUERY_STRING} variable. Try:
RewriteCond %{QUERY_STRING} (^|&)w=[0-9]+(&|$)
RewriteRule ^(.*)$ http://sub.domain.com/$1 [L,R=301]
Note that the query string gets automatically appended to the end of the rule's destination.

Answer 2

Just for documentation: If you want to redirect one directory (path) only if there is a URL parameter present, from one path to another, while maintaining the URL parameter, you can use this in your htaccess file:
# /programs/?id=1 to new path /loadprog/?id=1
RewriteCond %{REQUEST_URI} ^/programs/
RewriteCond %{QUERY_STRING} id=
RewriteRule ^programs\/$ /loadprog/$1 [R=301,L]
I am sure this will help others since I stumbled over the question above trying to find this answer.