Shared via Rust Playground

playground.rs

enum MachineState {
    Normal,
    Comment,
    Upper,
    Lower,
}

fn machine_cycle(state: &MachineState, character: char) -> (Option<char>, MachineState) {
    use std::ascii::AsciiExt;

    let mut return_char = None;
    let return_state;

    match *state {
        MachineState::Normal => {
            match character {
                '#' => {
                    return_state = MachineState::Comment;
                }
                '^' => {
                    return_state = MachineState::Upper;
                }
                '_' => {
                    return_state = MachineState::Lower;
                }
                _ => {
                    return_char = Some(character);
                    return_state = MachineState::Normal;
                }
            }
        }
        MachineState::Comment => {
            if character == '#' {
                return_state = MachineState::Normal;
            }
            else {
                return_state = MachineState::Comment;
            }
        }
        MachineState::Upper => {
            if character == '^' {
                return_state = MachineState::Normal;
            }
            else {
                return_char = Some(character.to_ascii_uppercase());
                return_state = MachineState::Upper;
            }
        }
        MachineState::Lower => {
            if character == '_' {
                return_state = MachineState::Normal;
            }
            else {
                return_char = Some(character.to_ascii_lowercase());
                return_state = MachineState::Lower;
            }
        }
    }

    (return_char, return_state)
}

fn main() {
    let mut state = MachineState::Normal;
    let input = "this is some _LoWER CASe_ text,#this is a comment,# ^and this part is upper casE^.";
    let mut processed_string = String::new();
    
    for character in input.chars() {
        let (output, new_state) = machine_cycle(&state, character);
        // No match needed here, as it's either Some(character), or None, and we don't care about 
        // None.
        if let Some(c) = output {
            processed_string.push(c);
        }
        state = new_state;
    }
    println!("Input:\t{}\nOutput:\t{}", input, processed_string);
}