Rewrite URL when parameter is missing - regex

Question

I'm looking for an Apache Rewrite Condition that fires if a certain URL parameter is not set and then rewrites the URL by adding this parameter.
So basically if my URL looks like http://www.heco.de/index.php?id=123&catId=456, I want it to redirect to http://www.heco.de/index.php?id=123&catId=456&cHasH=456
The first parameter is a static page ID, but the catId varies from a single digit up to 6 digits.
I tried to achieve this by adding this code snippet to my .htaccess:
RewriteEngine On
RewriteBase /
RewriteCond %{QUERY_STRING} catId=([0-9]{1,6})
RewriteCond %{QUERY_STRING} !cHash=
RewriteRule ^(.*)/?id=123&catId=([0-9]{1,6})$ index.php?id=123&catId=$2&cHash=$2
#RewriteRule ^(.*)/?id=123&catId=([0-9]+)$ http://www.stackoverflow.com
Alas, it's not working. I tried testing this by uncommenting the last line, but no redirect takes place at all. Hence I would be really grateful for any insights, ideas or further advice...!
Thx in advance,
Martin

Answer 1

You can't match QUERY_STRING in RewriteRule.
You can use this rule:
RewriteCond %{QUERY_STRING} !(?:^|&)cHash=[^&]+ [NC]
RewriteCond %{QUERY_STRING} (?:^|&)catId=([0-9]{1,6}) [NC]
RewriteRule ^index\.php$ $0?cHash=%1 [L,QSA,NC]