Can I use grok filter on a nested filed

The log like this:
{
meta:{ path: '/app/app1'}
}
and my filter like this:
grok {
'meta.path' => '/app/%{WORD:appid}'
}

it didn't work.
How can I get the result I want?

https://www.elastic.co/guide/en/logstash/current/event-dependent-configuration.html#logstash-config-field-references