RewriteRule includding only last part of REQUESTI_URI - mod-rewrite

Question

<LocationMatch /foo/>
ProxyPreserveHost On
RewriteEngine on
RewriteCond %{HTTP:Upgrade} websocket [NC]
RewriteCond %{HTTP:Connection} upgrade [NC]
RewriteRule .* "ws://192.168.0.101:1234%{REQUEST_URI}" [P]
when client connects to
ws://www.example.com/foo/whatever_arbitrary_random
this works great, but the outcome is
ws://192.168.0.101:1234/foo/whatever_arbitrary_random
how can i get rid of the /foo/ so the outcome would be
ws://192.168.0.101:1234/whatever_arbitrary_random
client must still need to connect to /foo/ to trigger this
EDIT: I found how to do it, replace last line with
RewriteRule ([^/]+)/?$ ws://192.168.0.101:1234/$1 [P,L]
But please read the first answer suggesting not to do this in a Location

Answer 1

You shouldn't use mod_rewrite directives inside <Location> (and <LocationMatch>) containers.
UPDATE: As stated in the Apache docs for the RewriteRule directive:
Although rewrite rules are syntactically permitted in <Location> and <Files> sections (including their regular expression counterparts), this should never be necessary and is unsupported. A likely feature to break in these contexts is relative substitutions.
<Location> sections are merged very late. When used inside a <Location> section, the RewriteRule directive matches the absolute filesystem-path, not the URL-path as would ordinarily be expected.
If .htaccess overrides are disabled then you can do it like this instead inside the appropriate <Directory> container:
<Directory /path/to/foo>
# Disable .htaccess overrides if not already
AllowOverride None
ProxyPreserveHost On
RewriteEngine on
RewriteCond %{HTTP:Upgrade} websocket [NC]
RewriteCond %{HTTP:Connection} upgrade [NC]
RewriteRule .* ws://192.168.0.101:1234/$0 [P]
</Directory>
The backreference $0 naturally excludes /foo/.
UPDATE:
RewriteRule ([^/]+)/?$ ws://192.168.0.101:1234/$1 [P,L]
This only matches the last path segment, it doesn't strictly match everything after /foo/. This may or may not be OK, depending on your requests. eg. It will redirect a request for /foo/bar/baz to /baz only, not /bar/baz.
The regex should really be anchored. However, you've probably written it this way because the directive is inside a <Location> section and matches the absolute file-path, rather than the requested URL-path.
Incidentally, you don't need the L flag when used with P - it is implied.
An alternative to the above... you don't need to use these directvies in a directory context (ie. inside a <Directory> or <Location> section). You can instead place these rules directly in the <VirtualHost> container (a virtualhost context), in which case they should be written like this instead:
ProxyPreserveHost On
RewriteEngine on
RewriteCond %{HTTP:Upgrade} websocket [NC]
RewriteCond %{HTTP:Connection} upgrade [NC]
RewriteRule ^/foo/(.*) ws://192.168.0.101:1234/$1 [P]