Regex

Capture first match

NB: If regex is not executed more than once then you may opt to not use the Cells but they remove the need to recompile the regex.

Dependencies (anyhow is not needed by makes the code cleaner)

cargo add regex
cargo add anyhow

use anyhow::Context as _;
use regex::Regex;
use std::sync::OnceLock;

fn main() -> anyhow::Result<()> {
    static CELL: OnceLock<Regex> = OnceLock::new();
    let re = CELL.get_or_init(|| {
        Regex::new(r"(\d+)").expect("failed to compile static regex that was known at design time")
    });

    let haystack = "There are 20 bad apples in the batch of 100";
    let captures = re
        .captures(haystack)
        .with_context(|| format!("failed to find a match in: {haystack:?}"))?;
    let number = captures
        .get(1)
        .with_context(|| format!("failed to extract number from: {haystack:?}"))?
        .as_str();
    // Prints: The number 20 was found in "There are 20 bad apples in the batch of 100"
    println!("The number {number} was found in {haystack:?}");
    Ok(())
}

Capture all matches

See notes on Capture first match

cargo add regex
cargo add anyhow

use anyhow::Context as _;
use regex::Regex;
use std::sync::OnceLock;

fn main() -> anyhow::Result<()> {
    static CELL: OnceLock<Regex> = OnceLock::new();
    let re = CELL.get_or_init(|| {
        Regex::new(r"(\d+)").expect("failed to compile static regex that was known at design time")
    });

    let haystack = "There are 20 bad apples in the batch of 100";
    let captures_iter = re.captures_iter(haystack);
    for captures in captures_iter {
        let number = captures
            .get(1)
            .with_context(|| format!("failed to extract number from: {haystack:?}"))?
            .as_str();
        println!("A number {number} was found in {haystack:?}");
    }

    // Prints:
    // A number 20 was found in "There are 20 bad apples in the batch of 100"
    // A number 100 was found in "There are 20 bad apples in the batch of 100"

    Ok(())
}