Code shared from the Rust Playground

playground.rs

fn main() {
    let regex = regex::Regex::new(r"(?P<a_only>a$)|(?:a:)(?P<a_colon>[^/].+)|(?P<a_slash>a/\d{1,2})").unwrap();
    
    [
        "a",
        "a:example.com",
        "a:mailers.example.com",
        "a/24",
        "a:offsite.example.com/24]",
    ]
    .iter()
    .copied()
    .map(|string| regex.captures(string))
    .for_each(|cap| println!("{:?}", cap))
}