Regex positional capture in map

[B.R.]

map's documentation
<http://nginx.org/en/docs/http/ngx_http_map_module.html#map> states:
A regular expression can contain named and positional captures that can
later be used in other directives along with the resulting variable.

Trying to do the following failed validation:
map $host $foo {
    "~*^www\.(.*)$"    $1; # Positional capture fails
    default                    $foo;
}

What am I doing wrong?
---
*B. R.*

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