Speed or simplicity? Why not both?

pest is a library for writing plain-text parsers in Rust.

Parsers that use pest are easy to design and maintain due to the use of Parsing Expression Grammars, or PEGs. And, because of Rust's zero-cost abstractions, pest parsers can be very fast.

Here is the complete grammar for a simple calculator developed in a (currently unwritten) later chapter:

num = @{ int ~ ("." ~ ASCII_DIGIT*)? ~ (^"e" ~ int)? }
    int = { ("+" | "-")? ~ ASCII_DIGIT+ }

operation = _{ add | subtract | multiply | divide | power }
    add      = { "+" }
    subtract = { "-" }
    multiply = { "*" }
    divide   = { "/" }
    power    = { "^" }

expr = { term ~ (operation ~ term)* }
term = _{ num | "(" ~ expr ~ ")" }

calculation = _{ SOI ~ expr ~ EOI }

WHITESPACE = _{ " " | "\t" }

And here is the function that uses that parser to calculate answers:

# #![allow(unused_variables)]
#fn main() {
lazy_static! {
    static ref PREC_CLIMBER: PrecClimber<Rule> = {
        use Rule::*;
        use Assoc::*;

        PrecClimber::new(vec![
            Operator::new(add, Left) | Operator::new(subtract, Left),
            Operator::new(multiply, Left) | Operator::new(divide, Left),
            Operator::new(power, Right)
        ])
    };
}

fn eval(expression: Pairs<Rule>) -> f64 {
    PREC_CLIMBER.climb(
        expression,
        |pair: Pair<Rule>| match pair.as_rule() {
            Rule::num => pair.as_str().parse::<f64>().unwrap(),
            Rule::expr => eval(pair.into_inner()),
            _ => unreachable!(),
        },
        |lhs: f64, op: Pair<Rule>, rhs: f64| match op.as_rule() {
            Rule::add      => lhs + rhs,
            Rule::subtract => lhs - rhs,
            Rule::multiply => lhs * rhs,
            Rule::divide   => lhs / rhs,
            Rule::power    => lhs.powf(rhs),
            _ => unreachable!(),
        },
    )
}
#}

This book provides an overview of pest as well as several example parsers. For more details of pest's API, check the documentation.

Note that pest uses some advanced features of the Rust language. For an introduction to Rust, consult the official Rust book.