RewriteEngine On

# If requested resource exists as a file or directory, skip next two rules
RewriteCond %{DOCUMENT_ROOT}/$1 -f [OR]
RewriteCond %{DOCUMENT_ROOT}/$1 -d
RewriteRule (.*) - [S=2]
#
# Else rewrite requests for non-existent resources to /index.php
RewriteRule (.*) /index.php?q=$1 [L]