can some1 please convert this htaccess to nginx rewrite?

[Lemon Head]

i got a signup verification script for my forum that shows images
and asks the user to match it with the provided word
it worked fine when i was using apache but today after
i made the switch to nginx the images dont load anymore
this script have an .htaccess file that contains:
RewriteEngine on
RewriteRule (.*) /show.php

can someone please convert it to nginx rewrite?
also i'm not sure if it matters but the full path
to the .php file is /usr/share/nginx/html/forum
another small problem i have is that if i get into
http://mysite/forum it doesnt load the page,but if i add
a secondary slash however as in /forum/ it works fine
how can i fix it? thanks in advance
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