RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s/+user\.php\?show=([^\s]+) [NC]
RewriteRule ^ /%1? [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-l
RewriteRule ^(.+)$ /user.php?show=$1 [L,QSA,NC]