Case-insensitive string matching in Rust

You can always convert both strings to the same casing. This will work for some cases:

let needle = "μτς";
let haystack = "ΜΤΣ";

let needle = needle.to_lowercase();
let haystack = haystack.to_lowercase();

for i in haystack.matches(&needle) {
    println!("{:?}", i);
}

See also str::to_ascii_lowercase for ASCII-only variants.

In other cases, the regex crate might do enough case-folding (potentially Unicode) for you:

use regex::RegexBuilder; // 1.4.3

fn main() {
    let needle = "μτς";
    let haystack = "ΜΤΣ";

    let needle = RegexBuilder::new(needle)
        .case_insensitive(true)
        .build()
        .expect("Invalid Regex");

    for i in needle.find_iter(haystack) {
        println!("{:?}", i);
    }
}

However, remember that ultimately Rust's strings are UTF-8. Yes, you need to deal with all of UTF-8. This means that picking upper- or lower-case might change your results. Likewise, the only correct way to change text casing requires that you know the language of the text; it's not an inherent property of the bytes. Yes, you can have strings which contain emoji and other exciting things beyond the Basic Multilingual Plane.

See also:

• How can I case fold a string in Rust?
• Why is capitalizing the first letter of a string so convoluted in Rust?

If you're using the regex crate, you can make the pattern case insensitive:

let re = Regex::new("(?i)μτς").unwrap();
let mat = re.find("ΜΤΣ").unwrap();