Remove index.php from URL only on root with Nginx rewrite

I'm using Wordpress as root of my website and Invision Power Boards as forum.

http://localhost -> Wordpress
http://localhost/forum -> IPB

I have removed "index.php" from Wordpress URLs successfully with Nginx-rewrite however when I try to use SEO Friendly URLs on IPB, nginx simply returns to Wordpress' 404 page.

My configuration is like this:

#This removes "index.php" from Wordpress URLs
location / {
   index index.php index.html index.htm;
   try_files    $uri $uri/ /index.php?q=$uri&$args;
} 

Then I follow this link to modify my nginx conf file in order to be able to use SEO friendly URLs of IPB: http://www.devcu.com/forums/topic/262-furl-friendly-urls-with-ipb-and-nginx/

#This part is to be able to use IPB SEO
location /forum/ {
    index index.php;
    try_files $uri $uri/ /forum/index.php?$uri&$args;
    rewrite ^ /index.php? last;
}

When I click a link on my forum (for example: http://localhost/forum/index.php/forum/51-sport/) nginx simply redirects me to (http://localhost/forum/forum/51-sport/) which displays Wordpress 404 error page.

I have very little knowledge about regex so any help would be appreciated.

This is my whole conf file after modifications, little messy I accept that.

server {
    listen      80; ## listen for ipv4; this line is default and implied
    #listen     [::]:80 default ipv6only=on; ## listen for ipv6

    root    /home/user_name/public_html;

    access_log  /var/log/nginx/a/access.log;
    error_log  /var/log/nginx/a/error.log

    server_name localhost;
    server_tokens off;

    location / {
        try_files   $uri $uri/ @wordpress;
    }

    location @wordpress {
        fastcgi_pass php-fpm;
            fastcgi_param SCRIPT_FILENAME /home/user_name/public_html$fastcgi_script_name;
            include /etc/nginx/fastcgi_params;
            fastcgi_index index.php;
        fastcgi_param SCRIPT_NAME /index.php;
    }

    location /forum {
        try_files $uri $uri/ try_files $uri $uri/ /forum/index.php?q=$uri;
    }

    location /forum/ {
        try_files $uri $uri/ try_files $uri $uri/ /forum/index.php?q=$uri;
    }

    #location / {
        #index      index.php index.html index.htm;
        #try_files  $uri $uri/ /index.php?q=$uri&$args;
    #}

    location = /favicon.ico {
        log_not_found off;
        access_log off;
    }

    location = /robots.txt {
        allow all;
        log_not_found off;
        access_log off;
    }

    location ~ \.php$ {
        fastcgi_split_path_info ^(/)(/.*)$;
    }

    # Add trailing slash to */wp-admin and */forum requests.
    rewrite /wp-admin$ $scheme://$host$uri/ permanent;

    # pass the PHP scripts to FastCGI server listening on 127.0.0.1:9001
        #location ~ \.php$ {
    #   fastcgi_split_path_info ^(/)(/.*)$;
    #   fastcgi_index   index.php;
        #       fastcgi_param SCRIPT_FILENAME /home/user_name/public_html$fastcgi_script_name;
        #       fastcgi_param PATH_INFO $fastcgi_script_name;
        #       include /etc/nginx/fastcgi_params;

        #REMOVE THIS        
        #fastcgi_read_timeout 60000;
        #fastcgi_send_timeout 6000;
        #}
}

Since the last post, I have played with IPB's SEO configurations and I managed to remove "index.php" from URLs. It doesn't effect the result of course. But it seems that location / decides what to do and therefore link is being considered as a Wordpress permalink.

EDIT - Solution

    # Upstream to abstract backend connection(s) for php
upstream php {
#        server unix:/tmp/php-cgi.socket;
        server 127.0.0.1:9001;
}

server {
        ## Your website name goes here.
        server_name localhost;
        ## Your only path reference.
        root /home/username/public_html;
        ## This should be in your http block and if it is, it's not needed here.
        index index.php;

        location = /favicon.ico {
                log_not_found off;
                access_log off;
        }

        location = /robots.txt {
                allow all;
                log_not_found off;
                access_log off;
        }

        location / {
                # This is cool because no php is touched for static content
                try_files $uri $uri/ /index.php;
        }

    location /forum {       
        try_files $uri $uri/ /forum/index.php;
        rewrite ^ /forum/index.php? break;
    }

    location ~ ^/forum/index.php {
        if ($args != "") {
            rewrite ^ http://www.google.com/ permanent;
        }
        try_files $uri $uri/ /forum/index.php;
        rewrite ^ /forum/index.php? last;
    }

    location /forum/admin/ {
        try_files $uri $uri/ /forum/admin/index.php;
        rewrite ^ /forum/admin/index.php? last;
    }



        location ~ \.php$ {
                #NOTE: You should have "cgi.fix_pathinfo = 0;" in php.ini
            include /etc/nginx/fastcgi_params;
                fastcgi_intercept_errors on;
                fastcgi_pass php;
        }

        location ~* \.(js|css|png|jpg|jpeg|gif|ico)$ {
                expires max;
                log_not_found off;
        }
}