How to only redirect urls located in a nginx Map

Question

I am in the process of moving my blog from a.com to b.com. Now I want to tell Google /bookmarks that all blogposts (around 100) have been moved to b.com. I only want to redirect the blogposts and nothing else.

After reading about the map module in nginx I tried the following:

map_hash_bucket_size 128;
map $uri $new {
  /post http://b.com/post
  # (repeated for all 100 posts)
}

And when I put the following line inside the server block:

rewrite ^ $new redirect;

It will redirect all 100 posts but all the other pages on my domain will error with: 302 Found.

Here is my whole server block inside the config:

server {

    listen 80;
    server_name b.com;
    root /my/old/path/;
    index index.html;

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

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

    # Deny all attempts to access hidden files such as .htaccess, .htpasswd, .DS_Store (Mac).
    location ~ /\. {
        deny all;
        access_log off;
        log_not_found off;
    }

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

    # example.com/index gets redirected to example.com/

    location ~* ^(.*)/index$ {
        return 301 $scheme://$host$1/;
    }

    # example.com/foo/ loads example.com/foo/index.html

    location ~* ^(.*)/$ {
        try_files $1/index.html @backend;
    }

    # example.com/a.html gets redirected to example.com/a

    location ~* \.html$ {
        rewrite ^(.+)\.html$ $scheme://$host$1 permanent;
    }

    # anything else not processed by the above rules:
    # * example.com/a will load example.com/a.html
    # * or if that fails, example.com/a/index.html

    location / {
        try_files $uri.html $uri/index.html $uri @backend;
    }

    # default handler
    # * return error or redirect to base index.html page, etc.
    location @backend {
        try_files /404.html 404;
    }

    location ~ \.php$ {
            expires off;
            try_files $uri =404;
            fastcgi_split_path_info ^(.+\.php)(/.+)$;
            #NOTE: You should have "cgi.fix_pathinfo = 0;" in php.ini

            include fastcgi_params;
            fastcgi_index index.php;
            fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
            fastcgi_intercept_errors on;
            fastcgi_pass php;
    }

    # apply the rewrite map
    rewrite ^ $new redirect;
}

I am thinking the map interferes with the try_files call for location \ (which I need).

Solution

I found a solution for your problem on serverfault:

This is probably failing because you're trying to redirect all requests, whether they matched something in the map or not.

To prevent this, check to see if there was a match first.

if ($new) {
    return 301 $new;
}

So replace

rewrite ^ $new redirect;

with

if ($new) {
    return 301 $new;
}

And you should be good to go