Regular expressions contain a special language, used to write a pattern to extract data from a string. We need three patterns for our example:

• A path for the index page
• A path for user management
• A path for the list of users (a new feature for our example server)

There's a Regex::new function that creates regular expressions. Remove the previous USER_PATH constant and add three new regular expression constants in a lazy static block:

lazy_static! {
    static ref INDEX_PATH: Regex = Regex::new("^/(index\.html?)?$").unwrap();
    static ref USER_PATH: Regex = Regex::new("^/user/((?P<user_id>\d+?)/?)?$").unwrap();
    static ref USERS_PATH: Regex = Regex::new("^/users/?$").unwrap();
}

As you can see, regular expressions look complex. To understand them better, let's analyze them.