CODEWITHSUNDEEP
rust
vitiral
vitiral

Reputation: 9220

How to make a compiled Regexp a global variable?

I have several regular expressions that are defined at runtime and I would like to make them global variables.

To give you an idea, the following code works:

use regex::Regex; // 1.1.5

fn main() {
    let RE = Regex::new(r"hello (\w+)!").unwrap();
    let text = "hello bob!\nhello sue!\nhello world!\n";
    for cap in RE.captures_iter(text) {
        println!("your name is: {}", &cap[1]);
    }
}

But I would like it to be something like this:

use regex::Regex; // 1.1.5

static RE: Regex = Regex::new(r"hello (\w+)!").unwrap();

fn main() {
    let text = "hello bob!\nhello sue!\nhello world!\n";
    for cap in RE.captures_iter(text) {
        println!("your name is: {}", &cap[1]);
    }
}

However, I get the following error:

error[E0015]: calls in statics are limited to constant functions, tuple structs and tuple variants
 --> src/main.rs:3:20
  |
3 | static RE: Regex = Regex::new(r"hello (\w+)!").unwrap();
  |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Does this mean that I need nightly Rust in order to make these variables global, or is there another way to do it?

Upvotes: 44

Views: 15658

Answers (3)

Yuri Astrakhan
Yuri Astrakhan

Reputation: 9965

Since 1.80.0, you do not need any 3rd party crates - use the built-in std::sync::LazyLock. Note that only the static line has changed, the rest is identical to the question.

use regex::Regex;
use std::sync::LazyLock;

static RE: LazyLock<Regex> =
    LazyLock::new(|| Regex::new(r"hello (\w+)!").unwrap());

// This code is identical to the question
fn main() {
    let text = "hello bob!\nhello sue!\nhello world!\n";
    for cap in RE.captures_iter(text) {
        println!("your name is: {}", &cap[1]);
    }
}

Upvotes: 9

womble
womble

Reputation: 12407

This is such a common pattern that there's a whole crate specifically for this use-case (with the added bonus of compile-time regex correctness checking): regex_static.

static RE: Lazy<Regex> = regex_static::lazy_regex!("^Stack Overflo{1,50}w!$");

Upvotes: 1

squiguy
squiguy

Reputation: 33380

You can use the lazy_static macro like this:

use lazy_static::lazy_static; // 1.3.0
use regex::Regex; // 1.1.5

lazy_static! {
    static ref RE: Regex = Regex::new(r"hello (\w+)!").unwrap();
}

fn main() {
    let text = "hello bob!\nhello sue!\nhello world!\n";
    for cap in RE.captures_iter(text) {
        println!("your name is: {}", &cap[1]);
    }
}

If you are using the 2015 edition of Rust, you can still use lazy_static via:

#[macro_use]
extern crate lazy_static;

Upvotes: 48

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