RewriteEngine On
RewriteCond     %{REQUEST_FILENAME}             !-f
RewriteCond     %{REQUEST_FILENAME}             !-d
RewriteRule    ^([A-Za-z0-9_.-)+/(.*)$         script.php?username=$1&do=$2       [L]
RewriteRule    ^([A-Za-z0-9_.-)+$              script.php?username=$1             [L]