1. Create a Rust project to work on during this chapter with cargo new chapter_five.
2. Navigate into the newly created chapter_five folder. For the rest of this chapter, we will assume that your command line is currently in this directory.
3. Inside the src folder, create a new folder called bin.
4. Delete the generated lib.rs file, as we are not creating a library.

5. Open the Cargo.toml file that was generated earlier for you.

6. Under [dependencies], add the following lines:

lazy_static = "1.0"
regex = "0.2"

If you want, you can go to the crates.io web pages for lazy_static (https://crates.io/crates/lazy_static) and regex (https://crates.io/crates/regex) to check for the newest version and use that one instead.

7. In the src/bin folder, create a file called lazy_static.rs
8. Add the following code and run it with cargo run --bin lazy_static:

1    #[macro_use] 
2    extern crate lazy_static;
3    extern crate regex;
4 
5    use regex::Regex;
6    use std::collections::HashMap;
7    use std::sync::RwLock;
8 
9    // Global immutable static
10   lazy_static! {
11     static ref CURRENCIES: HashMap<&'static str, &'static str> = 
       {
12       let mut m = HashMap::new();
13       m.insert("EUR", "Euro");
14       m.insert("USD", "U.S. Dollar");
15       m.insert("CHF", "Swiss Francs");
16       m
17     };
18   }
19 
20   // Global mutable static
21   lazy_static! {
22     static ref CLIENTS: RwLock<Vec<String>> = 
       RwLock::new(Vec::new());
23   }
24 
25   // Local static
26   fn extract_day(date: &str) -> Option<&str> {
27     // lazy static objects are perfect for
28     // compiling regexes only once
29     lazy_static! {
30       static ref RE: Regex =
31       Regex::new(r"(d{2}).(d{2}).(d{4})")
32         .expect("Failed to create regex");
33     }
34     RE.captures(date)
35       .and_then(|cap| cap.get(1).map(|day| day.as_str()))
36   }
37 
38   fn main() {
39     // The first access to CURRENCIES initializes it
40     let usd = CURRENCIES.get("USD");
41     if let Some(usd) = usd {
42       println!("USD stands for {}", usd);
43     }
44 
45     // All accesses will now refer to the same,
46     // already constructed object
47     if let Some(chf) = CURRENCIES.get("CHF") {
48       println!("CHF stands for {}", chf);
49     }
50 
51     // Mutable the global static
52     CLIENTS
53       .write()
54       .expect("Failed to unlock clients for writing")
55       .push("192.168.0.1".to_string());
56 
57     // Get an immutable reference to the global static
58     let clients = CLIENTS
59       .read()
60       .expect("Failed to unlock clients for reading");
61     let first_client = clients.get(0).expect("CLIENTS is
       empty");
62     println!("The first client is: {}", first_client);
63 
64     let date = "12.01.2018";
65     // The static object is nicely hidden inside
66     // the definition of extract_day()
67     if let Some(day) = extract_day(date) {
68       println!("The date "{}" contains the day "{}"", date, 
         day);
69     }
70   }