Function is_slack_id 

pub fn is_slack_id(text: &str) -> bool

True for a raw Slack id (C0C0T5E6RM1, U9ABCD) rather than a name: an uppercase first character followed by uppercase letters and digits only. Both message.user and a DM’s conversation label reach the prose as one until the producer resolves a handle. The shape is a heuristic, not a guarantee: an all-caps channel name matches it and loses its #.