How do I perform a replacement using a formatted string from a regex capture group? - regex

Question

I am doing multiple replacements at once using the regex crate:
extern crate regex;
use regex::{Captures, Regex};
fn transform(string: &str) {
let rgx = Regex::new(r"(\n)|(/\w+)").unwrap();
let res = rgx.replace_all(string, |caps: &Captures| {
if caps.get(1).is_some() {
return " ";
}
match caps.get(2).map(|m: regex::Match| m.as_str()) {
Some(z) => return "nope", // how to return formatted z instead?
None => (),
}
unreachable!();
});
println!("{}", res);
}
fn main() {
transform("no errors");
transform("big\nbad\n/string");
}
Output as expected:
no errors
big bad nope
Instead of "nope", I would like to return z formatted in some way instead. format! doesn't seem like it can be used here due to String / lifetime issues:
match caps.get(2).map(|m: regex::Match| m.as_str()) {
Some(z) => return format!("cmd: {}", z),
None => (),
}
error[E0308]: mismatched types
--> src/main.rs:12:31
|
12 | Some(z) => return format!("cmd: {}", z),
| ^^^^^^^^^^^^^^^^^^^^^ expected &str, found struct `std::string::String`
|
= note: expected type `&str`
found type `std::string::String`
= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
What should be done instead?

Answer 1

Note in the error message:
expected &str
It expects a &str because that's the first type returned by your closure:
return " ";
A closure / function can have only one return type, not two.
The simplest fix is to return a String in both cases:
let res = rgx.replace_all(string, |caps: &Captures| {
if caps.get(1).is_some() {
return String::from(" ");
}
let m = caps.get(2).unwrap();
format!("cmd: {}", m.as_str())
});
To be slightly more efficient, you can avoid the String allocation for the space character:
use std::borrow::Cow;
let res = rgx.replace_all(string, |caps: &Captures| {
if caps.get(1).is_some() {
return Cow::from(" ");
}
let m = caps.get(2).unwrap();
Cow::from(format!("cmd: {}", m.as_str()))
});
playground
I've also replaced the match with the => () arm paired with the unreachable! with the shorter unwrap.
See also:
Cannot use `replace_all` from the regex crate: expected (), found String
Using str and String interchangably
Return local String as a slice (&str)