Clippy lint · Performance · checked with Clippy 0.1.99

clippy::regex_creation_in_loops: Checks for regex compilation inside a loop with a literal

What it meansChecks for regex compilation inside a loop with a literal. It is in the performance group and warns by default.

Does it matter?

Compiling a regex is a much more expensive operation than using one, and a compiled regex can be used multiple times. This is documented as an antipattern on the regex documentation

Code that could be written to run faster. Warns by default.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

for haystack in haystacks {
    let regex = regex::Regex::new(MY_REGEX).unwrap();
    if regex.is_match(haystack) {
        // Perform operation
    }
}

Write this instead

let regex = regex::Regex::new(MY_REGEX).unwrap();
for haystack in haystacks {
    if regex.is_match(haystack) {
        // Perform operation
    }
}

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::regex_creation_in_loops)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
regex_creation_in_loops = "allow"

One run

cargo clippy -- -A clippy::regex-creation-in-loops

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Performancegroup

warndefault level

nofixed by cargo clippy --fix

1.84.0added in Rust

Clippy's own documentation

What it does

Checks for regex compilation inside a loop with a literal.

Why is this bad?

Compiling a regex is a much more expensive operation than using one, and a compiled regex can be used multiple times. This is documented as an antipattern on the regex documentation

Example

for haystack in haystacks {
    let regex = regex::Regex::new(MY_REGEX).unwrap();
    if regex.is_match(haystack) {
        // Perform operation
    }
}

can be replaced with

let regex = regex::Regex::new(MY_REGEX).unwrap();
for haystack in haystacks {
    if regex.is_match(haystack) {
        // Perform operation
    }
}

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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