Rust Programming By Example

Repetitions

In a macro pattern, it is also possible to match against an unlimited number of patterns, using the repetition operators + and *. They behave exactly like the same operators in regular expressions:

• + matches 1 or more times.
• * matches 0, 1, or more times.

Let's write a very useful macro, a macro to provide syntactic sugar to create HashMaps:

Note: A HashMap is a data structure from Rust's standard library that maps keys to values.

macro_rules! hash {
    ($( $key:expr => $value:expr ),*) => {{
        let mut hashmap = ::std::collections::HashMap::new();
        $(hashmap.insert($key, $value);)*
        hashmap
    }};
}

As we can see, we use the * operator here. The comma before it specify the separator token: this token must be present between each occurrence ...