Function is_literal_dotted 

fn is_literal_dotted(pattern: &str) -> bool

Whether a pattern is a plain dotted name rather than an expression.

True only for a pattern holding a dot, holding something more than dots, and built from characters a server name or a localpart may contain, none of which carry regex meaning. Anything using regex syntax of its own is left alone, + included, as is a pattern of bare dots, which is the any-character idiom rather than a name.