nginx.required-param-conditional-redirect.conf

location /validate {
    # IF:
    #   /validate?param1=value
    #   /validate?param0=x&param1=value
    rewrite ^/validate?.*param1\=(.+).*$ /success/$1 last;
    
    # ELSE:
    #   /validate?param0=x
    #   /validate?param0=x&param1
    #   /validate?param0=x&param1=
    rewrite ^/validate?.*$ /fail last;
    
    # FALLBACK: Not a case, but just for the cause...
    return 403;
}

location ~ /fail {
    return 400;
}

location ~ /success/(.+) {
    return 200 $1;
}