Step 5 of 6

Changing case, searching, replacing and trimming

Here are the everyday tools for cleaning up and inspecting text, applied to a messy log line:

fn main() {
    let line = "  ERROR: Disk full on /dev/sda1  ";
    let clean = line.trim();
    println!("[{clean}]");
    println!("{}", clean.to_lowercase());
    println!("{}", clean.contains("Disk"));
    println!("{}", clean.contains("disk"));
    println!("{}", clean.replace("/dev/sda1", "drive 1"));
    println!("[{line}]");
}

[ERROR: Disk full on /dev/sda1]
error: disk full on /dev/sda1
true
false
ERROR: Disk full on drive 1
[  ERROR: Disk full on /dev/sda1  ]

• trim() removes whitespace (spaces, tabs, newlines) from both ends. trim_start() and trim_end() do one side. You'll call it constantly in the next module, because every line a user types ends with a newline.
• to_uppercase() / to_lowercase() return a new String with the case changed. They handle non-English letters too: "straße".to_uppercase() is "STRASSE".
• contains(pattern) returns true if the pattern appears anywhere. It's case-sensitive: "Disk" is found, "disk" isn't.
• replace(from, to) returns a new String with every occurrence replaced. The pattern can also be a char: s.replace(' ', "_").

Look at the last line of output: none of these methods change the original. to_lowercase and replace build a new String; trim returns a &str view of the middle part (nothing is copied); contains just answers a question.

Chaining

Each method returns a value that has methods of its own, so you can chain them. Read the chain left to right:

fn main() {
    let raw = "  Hello World  ";
    let slug = raw.trim().to_lowercase().replace(' ', "-");
    println!("{slug}");
    let shouted = "is anyone there?".to_uppercase();
    println!("{}", shouted.contains("ANYONE"));
}

hello-world
true

To search ignoring case, lowercase the text first and search for a lowercase pattern: line.to_lowercase().contains("error") finds ERROR, Error and error.

Common mistakes

• Expecting the original to change. This line computes an uppercase copy and throws it away:

name.to_uppercase(); // warning: unused return value of `str::<impl str>::to_uppercase` that must be used

The compiler warns you, and the note says: this returns the uppercase string as a new String, without modifying the original. Keep the result: let name = name.to_uppercase(); (shadowing works nicely here).

• Forgetting that contains is case-sensitive. "ERROR".contains("error") is false.
• Mixing up the argument order of replace: it's replace(what_to_find, what_to_put_instead).

Your task

Write normalize_tag(tag), which cleans up a tag typed by a user: remove spaces at the ends, make it lowercase, and replace each remaining space with -. For example normalize_tag(" Rust Lang ") returns "rust-lang".

Challenge 2

Write is_alert(line), which returns true when a log line mentions error or fatal, in any mix of upper and lower case.

Challenge 3

Write censor(text, word), which replaces every occurrence of word with stars, one * per character of the word. For example censor("darn it, darn", "darn") returns "**** it, ****", and censor("café café", "café") returns "**** ****".

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