Shared via Rust Playground

playground.rs

struct Scanner {
    input: String,
    output: Vec<Token>
}

#[derive(Debug)]
enum Token {
    Integer(i64),
    Addition,
    Subtraction,
    Multiplication,
    Division,
    Modulus,
    Exponent,
    Assignment,
    Terminator,
    Variable(char),
    Unknown(char),
}

impl Scanner {
    fn new(input: &str) -> Scanner {
        Scanner{
            input: input.to_string(),
            output: Vec::new(),
        }
    }

    fn scan(&mut self) {
        let mut chars = self.input.chars().peekable();
        while let Some(c) = chars.next() {
            let tok = match c {
                '0'...'9' => {
                    let mut number = 0;
                    let mut current = c;
                    loop {
                        number *= 10;
                        number += ((current as u8) - ('0' as u8)) as i64;
                        match chars.peek() {
                            Some(&next) if '0' <= next && next <= '9' => {
                                current = next;
                                chars.next();
                            }
                            _ => break,
                        };
                    }
                    Token::Integer(number)
                },
                '+' => Token::Addition,
                '-' => Token::Subtraction,
                '*' => Token::Multiplication,
                '/' => Token::Division,
                '%' => Token::Modulus,
                '^' => Token::Exponent,
                '=' => Token::Assignment,
                ';' => Token::Terminator,
                'a'...'z' => Token::Variable(c),
                'A'...'Z' => Token::Variable(std::ascii::AsciiExt::to_ascii_lowercase(&c)),
                _ => Token::Unknown(c)
            };
            self.output.push(tok);
        }
    }

    fn output(&self) -> &[Token] {
        &self.output
    }
}

fn main() {
    let program = "+ - 34 ; quit a 3 3 - - 1 * ^ 7";
    let mut s = Scanner::new(program);
    s.scan();
    for line in s.output() {
        println!("{:?}",line);
    }
}