// include the latest version of the regex crate in your Cargo.toml
extern crate regex;
use regex::Regex;
fn main() {
let regex = Regex::new(r"(?s)(?:^|\n)(\nsecond thing:\s*[^\n]+\n)|(first thing:\s*[^\n]+\n(?!second thing:))|(second thing:\s*[^\n]+\n(?!third thing:))|(third thing:\s*[^\n]+\n\n)").unwrap();
let string = "first thing: content 1
second thing: content 2
fourth thing: content 4
";
let substitution = "\\3third thing: NA\\n";
// result will be a String with the substituted value
let result = regex.replace_all(string, substitution);
println!("{}", result);
}
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Rust, please visit: https://docs.rs/regex/latest/regex/