By now, you're comfortable with the difference between String and &str. But in the real world, you spend a huge amount of your time just poking and prodding at string slices to see if they contain what you expect. I've found that instead of reaching for a heavy regex crate for every little thing, most of the time, the built-in &str methods are all you actually need.

Let's build a simple username validator. I want to make sure a username isn't empty, doesn't start with a forbidden character, and doesn't contain any illegal whitespace. This is a perfect playground for string slices.

Cleaning up the raw input

Usually, when you get a string from a user or an API, it's messy. There are trailing spaces or accidental newlines. I always start by trimming the input. The .trim() method is great because it doesn't allocate a new string; it just returns a sub-slice of the original that excludes the whitespace.

let raw_input = "  rust_ace_99  \n";
let username = raw_input.trim();

if username.is_empty() {
    println!("Username cannot be blank!");
}

Notice how I used .is_empty(). I could have checked if username.len() == 0, but is_empty() is more idiomatic and arguably more readable. It tells the next developer exactly what you're checking for.

Setting some boundary rules

In my imaginary system, usernames can't start with an underscore—it's just a stylistic choice I'm making for this example. This is where starts_with and ends_with come in handy. They return a simple boolean, making them perfect for if statements.

if username.starts_with('_') {
    println!("Usernames cannot start with an underscore.");
}

if username.ends_with('!') {
    println!("No exclamation points at the end, please.");
}

Hunting for forbidden characters

Next, I want to make sure there aren't any spaces inside the username. Even though we trimmed the ends, we could still have "rust ace". The .contains() method is my go-to here.

if username.contains(' ') {
    println!("Usernames cannot contain spaces.");
}

I prefer passing a char (single quotes) to contains rather than a string slice when I'm looking for a single character. It's a tiny bit more efficient and makes it clear I'm looking for one specific glyph.

Oops, I forgot how split works

Now, I want to check if the username has too many segments. I've decided that usernames are separated by dots (like dev.rust.user), and I only want a maximum of three segments. Here is where I usually trip up when I'm rushing.

// My first attempt (This won't work!)
let segments = username.split('.');
if segments.len() > 3 {
    println!("Too many segments!");
}

I just tried to call .len() on the result of split(). Rust compiler immediately yelled at me. Why? Because split() doesn't return a Vec or a slice; it returns a Split iterator. Iterators in Rust are lazy—they don't know how many elements they have until they actually traverse them.

To fix this, I have two choices: I can collect the iterator into a Vec, or I can use the count() method. Since I just need the number, count() is cleaner.

// The corrected version
let segment_count = username.split('.').count();
if segment_count > 3 {
    println!("Too many segments! Max 3 allowed.");
}

Just a heads up: count() consumes the iterator. If you needed to actually use those segments later, you'd be better off using .collect::>().

📋 Practical Task

Build a URL Slug Sanitizer

Your task is to write a function that takes a raw page title (a &str) and converts it into a URL-friendly "slug".

Implement a function fn create_slug(title: &str) -> String that does the following:

• Trims leading and trailing whitespace.
• Converts the entire string to lowercase.
• Replaces all spaces with hyphens (-).
• Checks if the resulting slug starts with a hyphen; if it does, remove it (you can use .strip_prefix("-") or a similar slice method).

Test your function with: " My First Rust Blog Post ".
Expected Output: "my-first-rust-blog-post"