extern crate regex;
use regex::Regex;

fn main() {
    let args: Vec<String> = std::env::args().collect();

    if args.len() < 4 {
	println!("Usage: {} <str> <regex> <group>", args[0]);
    } else {
	let re = Regex::new(&args[2]).unwrap();
	let text = &args[1];
	let group: usize = args[3].parse().expect("Invalid group number.");

	let groups = re.captures(&text).unwrap();

	if group > groups.len() {
	    eprintln!("Invalid group number.");
	} else {
	    println!("{}", groups.get(group).unwrap().as_str());
	}
    }
}