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]

ErrorDocument 400 / 400.html
ErrorDocument 401 / 401.html
ErrorDocument 403 / 403.html
ErrorDocument 404 / 404.html
ErrorDocument 500 / 500.html

RewriteEngine on
RewriteRule ^user/([-a-zA-Z0-9_]+)/? user.php?&show=$1 [L]