The feature has been asked for multiple times, but it requires a different database layout than the current.
The new database layout is implemented and I’m currently processing historic data to fill this database.
It is likely that the feature to search for a key will get available at latest at March.
node[~"^addr:.*$"~"^Foo$"]; /* finds addr:* tags with value exactly "Foo" */
node[~"^addr:.*$"~"^Foo"]; /* finds addr:* tags with value starting with "Foo" */
node[~"^addr:.*$"~"Foo$"]; /* finds addr:* tags with value ending with "Foo" */
node[~"^addr:.*$"~"Foo"]; /* finds addr:* tags with value containing the substring "Foo" */
node[~"^addr:.*$"~"."]; /* finds addr:* tags with any value */