is-identifier.rs

#[derive(Clone,Copy,Debug)]
enum State {
    A,
    B,
    Trap,
}

impl State {
    fn is_accepting(self) -> bool {
        use State::*;
        match self {
            A | B => true,
            Trap => false,
        }
    }
    
    fn transition(self, symbol: char) -> State {
        use State::*;
        match self {
            A => match symbol {
                '_' | 'a' ... 'z' => B,
                _ => Trap,
            },
            B => match symbol {
                '_' | 'a' ... 'z' | '0' ... '9' => B,
                _ => Trap,
            },
            Trap => Trap
        }
    }
}

fn main() {
    let input = std::env::args().nth(1).unwrap_or_default();

    std::process::exit(
        if input.chars().fold(
            State::A,
            State::transition
        ).is_accepting() {
            0
        } else {
            1
        }
    );
}