Grammars

Like many parsing tools, pest operates using a formal grammar that is distinct from your Rust code. The format that pest uses is called a parsing expression grammar, or PEG. When building a project, pest automatically compiles the PEG, located in a separate file, into a plain Rust function that you can call.

How to activate pest

Most projects will have at least two files that use pest: the parser (say, src/parser/mod.rs) and the grammar (src/parser/grammar.pest). Assuming that they are in the same directory:

#![allow(unused)]
fn main() {
use pest::Parser;

#[derive(Parser)]
#[grammar = "parser/grammar.pest"] // relative to project `src`
struct MyParser;
}

Whenever you compile this file, pest will automatically use the grammar file to generate items like this:

#![allow(unused)]
fn main() {
pub enum Rules { /* ... */ }

impl Parser for MyParser {
    pub fn parse(Rules, &str) -> pest::Pairs { /* ... */ }
}
}

You will never see enum Rules or impl Parser as plain text! The code only exists during compilation. However, you can use Rules just like any other enum, and you can use parse(...) through the Pairs interface described in the Parser API chapter.

Load multiple grammars

If you have multiple grammars, you can load them all at once:

#![allow(unused)]
fn main() {
use pest::Parser;

#[derive(Parser)]
#[grammar = "parser/base.pest"]
#[grammar = "parser/grammar.pest"]
struct MyParser;
}

Then pest will generate a Rules enum that contains all the rules from both. This is useful if you have a base grammar that you want to extend in multiple grammars.

Warning about PEGs!

Parsing expression grammars look quite similar to other parsing tools you might be used to, like regular expressions, BNF grammars, and others (Yacc/Bison, LALR, CFG). However, PEGs behave subtly differently: PEGs are eager, non-backtracking, ordered, and unambiguous.

Don't be scared if you don't recognize any of the above names! You're already a step ahead of people who do — when you use pest's PEGs, you won't be tripped up by comparisons to other tools.

If you have used other parsing tools before, be sure to read the next section carefully. We'll mention some common mistakes regarding PEGs.