Rust Pattern Matching

Patterns come in two forms: refutable and irrefutable.
Patterns that will match for any possible value passed are irrefutable. An example would be x in the statement let x = 5; because x matches anything and therefore cannot fail to match. Patterns that can fail to match for some possible value are refutable. An example would be Some(x) in the expression if let Some(x) = a_value because if the value in the a_value variable is None rather than Some, the Some(x) pattern will not match.

Cases:

Multiple Patterns

In match expressions, you can match multiple patterns using the | syntax, which is the pattern or operator. For example, in the following code we match the value of x against the match arms, the first of which has an or option, meaning if the value of x matches either of the values in that arm, that arm’s code will run:

fn main() {
    let x = 1;

    match x {
        1 | 2 => println!("one or two"),
        3 => println!("three"),
        _ => println!("anything"),
    }
}

This code prints one or two.

Matching Ranges of Values with ..=

The ..= syntax allows us to match to an inclusive range of values. In the following code, when a pattern matches any of the values within the given range, that arm will execute:

fn main() {
    let x = 5;

    match x {
        1..=5 => println!("one through five"),
        _ => println!("something else"),
    }
}

If x is 1, 2, 3, 4, or 5, the first arm will match. This syntax is more convenient for multiple match values than using the | operator to express the same idea; if we were to use | we would have to specify 1 | 2 | 3 | 4 | 5. Specifying a range is much shorter, especially if we want to match, say, any number between 1 and 1,000!

The compiler checks that the range isn’t empty at compile time, and because the only types for which Rust can tell if a range is empty or not are char and numeric values, ranges are only allowed with numeric or char values.

Here is an example using ranges of char values:

fn main() {
    let x = 'c';

    match x {
        'a'..='j' => println!("early ASCII letter"),
        'k'..='z' => println!("late ASCII letter"),
        _ => println!("something else"),
    }
}

Rust can tell that 'c' is within the first pattern’s range and prints early ASCII letter.

fn main() {
    let favorite_color: Option<&str> = None;
    let is_tuesday = false;
    let age: Result<u8, _> = "34".parse();

    if let Some(color) = favorite_color {
        println!("Using your favorite color, {color}, as the background");
    } else if is_tuesday {
        println!("Tuesday is green day!");
    } else if let Ok(age) = age {
        if age > 30 {
            println!("Using purple as the background color");
        } else {
            println!("Using orange as the background color");
        }
    } else {
        println!("Using blue as the background color");
    }
}