my defaultController is a PageController with a viewAction
www.mysite.com/hello-world displays a page from my database
'urlManager' => array(
'urlFormat' => 'path',
'showScriptName' => false,
'rules' => array(
'<slug:[a-zA-Z0-9_-]+>' => 'page/view'
),
),
other Actions like page/create or page/update are working
I created a module "admin", but www.mysite.com/admin ends up in a 404-Error
what is the missing pattern in rules?
The rule
'<slug:[a-zA-Z0-9_-]+>' => 'page/view'
will catch ‘www.mysite.com/admin’ and dispatch it to ‘page/view’, because ‘admin’ mathces ‘[a-zA-Z0-9_-]+’.
‘admin/default’, ‘page/create’, ‘page/create’ and many others with ‘/’ in them will not fall into the pattern and eventually applied the default rule, and will be routed correctly.
So, the quick and dirty fix may be:
'urlManager' => array(
'urlFormat' => 'path',
'showScriptName' => false,
'rules' => array(
+ 'admin' => 'admin/default',
'<slug:[a-zA-Z0-9_-]+>' => 'page/view'
),
),
You might want some cleaner way. 
indeed, if there is a cleaner way? 
now my code is
'<module:\w+>' => '<module>/default',
'<slug:[a-zA-Z0-9_-]+>' => 'page/view',
Well, I don’t know. I’m using a much more messy set of rules than yours. 
BTW, there’s one thing I’m concerned about. An article with a title without ‘-’ or ‘_’ will fail with your current rules. For example, ‘www.mydomain.com/hello’ will fall into the first rule and end up in 404. Isn’t there any chance that your article will have that kind of title?