NGINX: how to block access to all files inside some directories

I have different files inside three different directories.

I'd like to block access to all files inside those (also block directory listing).

This way, I'd block, for example:

Whatever the file extension...

This is what I am trying:

location /a/(b|c|d) {
    allow 1.2.3.4; #IP
    deny all;
    return 403;
}

But it's not working.

You need ~ in your location config - without it means literal prefix matching, while you're attempting to use regex syntax.

location ~ /a/(b|c|d) {

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