Code shared from the Rust Playground

playground.rs

fn main() {
    let regex = regex::Regex::new(r"^(?P<qualifier>[+?~-])?(?P<is_a>a)(?:[:]|)?(?P<mechamism>.+)?").unwrap();
    
    [
        "a",
        "a:example.com",
        "~a:example.com",
        "a:mailers.example.com",
        "-a:mailers.example.com",
        "a/24",
        "-a:offsite.example.com/24]",
        "mx",
        "+mx:mailer.com",
    ]
    .iter()
    .copied()
    .map(|string| regex.captures(string))
    .for_each(|cap| println!("{:?}", cap))
}