Can someone please help with trying to use mod rewrite so foo.domain.com is rewritten to domain.com/p.php?s=foo and foo.domain.com/bar to domain.com/p.php?s=foo&p=bar?

Currently my .htaccess is:

RewriteEngine On

# Remove www
RewriteCond %{HTTP_HOST} ^www.domain.com$ [NC]
RewriteRule ^(.*)$ http://domain.com/$1 [R=301,L]

# Rewrite subdomain etc.
RewriteCond %{HTTP_HOST} ^(.*)\.domain\.com$ [NC]
RewriteRule ^([^/]+)/?$ p.php?s=%1&p=$2 [QSA,L,NC]

But I can't work out how to grab the second variable (being optional and after a slash at the end of the URI. I've tried changing the end of the condition to ?/(.*)$, but to no avail and my mod rewrite skills are certainly naff!