Greetings.

I'm trying to do some fairly intricate URI rewriting, and the behaviour
of the 'rewrite' statement does not correspond to anything I can explain
from the docs.

The goal is that /A/foo/modified is rewritten to /_newroot/foo and
/B/foo/modified to /_defaultroot/foo. I hope to achieve this with

map $uri $modroot {
default _defaultroot;
~^/A _newroot;
}

and

location ~ /modified$ {
rewrite ^(.+)/modified$ /$modroot/-$1-;
}

(in the real config, /_newroot is reverse-proxied to a web service, so
that the URIs it handles end up grafted on to selected trees; the 'map'
is intended to select/limit which URIs are passed on to this service).

The complete nginx.conf is at the bottom.

Looking in the error log, when I retrieve /hello/modified I find

2020/04/26 12:23:28 [notice] 63328#0: *5 "^(.+)/modified$" matches
"/hello/modified", client: 127.0.0.1, server: localhost, request: "GET
/hello/modified HTTP/1.1", host: "localhost"
2020/04/26 12:23:28 [notice] 63328#0: *5 rewritten data:
"/_defaultroot/-/hello-", args: "", client: 127.0.0.1, server:
localhost, request: "GET /hello/modified HTTP/1.1", host: "localhost"

....which is fine: the map defines $modroot as /_defaultroot, and the
rewrite captures the /hello.

But retrieving /A/hello/modified,

2020/04/26 13:00:51 [notice] 63828#0: *6 "^(.+)/modified$" matches
"/A/hello/modified", client: 127.0.0.1, server: localhost, request: "GET
/A/hello/modified HTTP/1.1", host: "localhost"
2020/04/26 13:00:51 [notice] 63828#0: *6 rewritten data: "/_newroot/--",
args: "", client: 127.0.0.1, server: localhost, request: "GET
/A/hello/modified HTTP/1.1", host: "localhost"

I would expect this to be rewritten to /_newroot/-/A/hello-

Here, the map has defined $modroot as /_newroot (which is correct). The
'rewrite' _has_ matched, but the $1 in that line appears to be empty.
Note the '+' in the regexp: there is supposed be be a string of non-zero
length in there (ie, this is ruling out that I'm inadvertently matching,
and replacing, an empty string, as a result of being somehow confused
about where in the string '^' is matching).

It's as if the regexp match in the 'map' is somehow interfering with the
group-capturing in the 'rewrite'.

As a workaround, I can get this to work with ^(?<newprefix>/A) in the
'map', and using $newprefix in the 'rewrite', but that's fiddly/ugly and
more confusing than localising the rewriting to the 'rewrite' statement.

Am I misunderstanding how 'rewrite' matches things, or is there an issue
here?

Best wishes,

Norman






% ../sbin/nginx -V
nginx version: nginx/1.18.0
built by clang 11.0.3 (clang-1103.0.32.29)
configure arguments: --prefix=/Data/tools/nginx-1.18
--with-pcre=../pcre-8.44

Both nginx and pcre built, as shown, from source.

This is on macOS 10.15.3, but I get the same results with (packaged)
nginx/1.16.1 on FreeBSD 12.1


Complete nginx.conf:

worker_processes 1;

events {
worker_connections 1024;
}


http {
include mime.types;
default_type application/octet-stream;

error_log logs/error.log debug;
rewrite_log on;

sendfile on;
keepalive_timeout 65;

# selected URIs are dynamically 'rehomed'
map $uri $modroot {
default _defaultroot;
~^/A _newroot;
}

server {
listen 80;
server_name localhost;

location / {
root html;
index index.html index.htm;
}
location ~ /modified$ {
rewrite ^(.+)/modified$ /$modroot/-$1-;
}
location /_defaultroot { # not a 'rehomed' one
internal;
error_page 404 /404-private.html;
}
location /_newroot {
internal;
root html/x;
}

error_page 500 502 503 504 /50x.html;
location = /50x.html {
root html;
}
}
}


--
Norman Gray : https://nxg.me.uk
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