How do I replace a match with a variable in rust?

This piece of code replaces "World" with "London":

fn replace_with(foo: &str) -> String {
    use regex::Regex;
    let before = "Hello, World";
    let re = Regex::new(r"(.*?)(World)").unwrap();
    String::from(re.replace_all(before, "$1 London"))
}

I’d instead like to replace it with foo. How do I do that? I’ve tried using "$foo" or "${foo}" and that doesn’t work.

>Solution :

Two options:

format!("{} London", foo)`

In modern Rust (1.58+):

format!("{foo} London")

That’s the closest you can get to JavaScript template strings and the ${x} format.¹

Where that’s the argument to your substitution, as in replace_all(before, format!(...)).

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¹ In Rust it’s more limited, you can’t get as adventurous with arrays and such, just simple variable names. {x[y]} is not a valid substitution, for example.