Example: INI

INI (short for initialization) files are simple configuration files. Since there is no standard for the format, we'll write a program that is able to parse this example file:

username = noha
password = plain_text
salt = NaCl




Each line contains a key and value separated by an equals sign; or contains a section name surrounded by square brackets; or else is blank and has no meaning.

Whenever a section name appears, the following keys and values belong to that section, until the next section name. The key–value pairs at the beginning of the file belong to an implicit "empty" section.

Writing the grammar

Start by initializing a new project using Cargo, adding the dependencies pest = "2.6" and pest_derive = "2.6". Make a new file, src/ini.pest, to hold the grammar.

The text of interest in our file — username, /var/www/example.org, etc. — consists of only a few characters. Let's make a rule to recognize a single character in that set. The built-in rule ASCII_ALPHANUMERIC is a shortcut to represent any uppercase or lowercase ASCII letter, or any digit.

char = { ASCII_ALPHANUMERIC | "." | "_" | "/" }

Section names and property keys must not be empty, but property values may be empty (as in the line ip= above). That is, the former consist of one or more characters, char+; and the latter consist of zero or more characters, char*. We separate the meaning into two rules:

name = { char+ }
value = { char* }

Now it's easy to express the two kinds of input lines.

section = { "[" ~ name ~ "]" }
property = { name ~ "=" ~ value }

Finally, we need a rule to represent an entire input file. The expression (section | property)? matches section, property, or else nothing. Using the built-in rule NEWLINE to match line endings:

file = {
    SOI ~
    ((section | property)? ~ NEWLINE)* ~

To compile the parser into Rust, we need to add the following to src/main.rs:

use pest::Parser;
use pest_derive::Parser;

#[grammar = "ini.pest"]
pub struct INIParser;

Program initialization

Now we can read the file and parse it with pest:

use std::collections::HashMap;
use std::fs;

fn main() {
    let unparsed_file = fs::read_to_string("config.ini").expect("cannot read file");

    let file = INIParser::parse(Rule::file, &unparsed_file)
        .expect("unsuccessful parse") // unwrap the parse result
        .next().unwrap(); // get and unwrap the `file` rule; never fails

    // ...

We'll express the properties list using nested HashMaps. The outer hash map will have section names as keys and section contents (inner hash maps) as values. Each inner hash map will have property keys and property values. For example, to access the document_root of server_1, we could write properties["server_1"]["document_root"]. The implicit "empty" section will be represented by a regular section with an empty string "" for the name, so that properties[""]["salt"] is valid.

fn main() {
    // ...

    let mut properties: HashMap<&str, HashMap<&str, &str>> = HashMap::new();

    // ...

Note that the hash map keys and values are all &str, borrowed strings. pest parsers do not copy the input they parse; they borrow it. All methods for inspecting a parse result return strings which are borrowed from the original parsed string.

The main loop

Now we interpret the parse result. We loop through each line of the file, which is either a section name or a key–value property pair. If we encounter a section name, we update a variable. If we encounter a property pair, we obtain a reference to the hash map for the current section, then insert the pair into that hash map.

    // ...

    let mut current_section_name = "";

    for line in file.into_inner() {
        match line.as_rule() {
            Rule::section => {
                let mut inner_rules = line.into_inner(); // { name }
                current_section_name = inner_rules.next().unwrap().as_str();
            Rule::property => {
                let mut inner_rules = line.into_inner(); // { name ~ "=" ~ value }

                let name: &str = inner_rules.next().unwrap().as_str();
                let value: &str = inner_rules.next().unwrap().as_str();

                // Insert an empty inner hash map if the outer hash map hasn't
                // seen this section name before.
                let section = properties.entry(current_section_name).or_default();
                section.insert(name, value);
            Rule::EOI => (),
            _ => unreachable!(),

    // ...

For output, let's simply dump the hash map using the pretty-printed Debug format.

fn main() {
    // ...

    println!("{:#?}", properties);


If you copy the example INI file at the top of this chapter into a file config.ini and run the program, it will not parse. We have forgotten about the optional spaces around equals signs!

Handling whitespace can be inconvenient for large grammars. Explicitly writing a whitespace rule and manually inserting it makes a grammar difficult to read and modify. pest provides a solution using the special rule WHITESPACE. If defined, it will be implicitly run, as many times as possible, at every tilde ~ and between every repetition (for example, * and +). For our INI parser, only spaces are legal whitespace.

WHITESPACE = _{ " " }

We mark the WHITESPACE rule silent with a leading low line (underscore) _{ ... }. This way, even if it matches, it won't show up inside other rules. If it weren't silent, parsing would be much more complicated, since every call to Pairs::next(...) could potentially return Rule::WHITESPACE instead of the desired next regular rule.

But wait! Spaces shouldn't be allowed in section names, keys, or values! Currently, whitespace is automatically inserted between characters in name = { char+ }. Rules that are whitespace-sensitive need to be marked atomic with a leading at sign @{ ... }. In atomic rules, automatic whitespace handling is disabled, and interior rules are silent.

name = @{ char+ }
value = @{ char* }


Try it out! Make sure that the file config.ini exists, then run the program! You should see something like this:

$ cargo run
  [ ... ]
    "": {
        "password": "plain_text",
        "username": "noha",
        "salt": "NaCl"
    "second_server": {
        "ip": "",
        "document_root": "/var/www/example.com",
        "interface": "eth1"
    "server_1": {
        "interface": "eth0",
        "document_root": "/var/www/example.org",
        "ip": ""