Function is_syntactic_r_name 

pub(crate) fn is_syntactic_r_name(name: &str) -> bool

Whether name is a syntactic R name: ASCII letters, digits, . and _ only; starts with a letter, or with a . that is not followed by a digit; and is not a reserved word. Anything else ([.foo, $.foo, ==.foo, %in%.foo) has to be backtick-quoted in source.