Valid characters for PostgreSQL session parameters

Where can I find the list of characters which are valid for PostgreSQL configuration parameter names? By trial and error I found that '.' and '_' are valid, but ':' isn't. Quoting doesn't seem to help. set claims.custom_id to asdf; -- accepted set "claims.custom:id" to asdf; -- invalid configuration parameter name The PostgreSQL docs don't seem to cover this point - https://www.postgresql.org/docs/current/config-setting.html