Those of StringUtils.substringBefore(), substringAfter(), and subStringBetween() are following in Rust. JavaのStringUtils.substringBefore(), substringAfter(), substringBetween()はRustでは以下かと（文字列操作）。 https://play.rust-lang.org/?gist=31abc000d4c0973c58c395d1d28a4332&version=undefined `cargo test -- --nocapture` for println!()

StringUtils.rs

fn substring_before(body: &str, separator: &str) -> String {
    match body.find(separator) {
        Some(i) => body.get(..i).unwrap().to_string(),
        None    => body.to_string(),
    }
}

fn substring_after(body: &str, separator: &str) -> String {
    match body.find(separator) {
        Some(i) => body.get(i+separator.len()..).unwrap().to_string(),
        None    => body.to_string(),
    }
}

fn substring_between(body: &str, open: &str, close: &str) -> String {
    match body.find(open) {
        Some(i) => {
            let after = body.get(i+open.len()..).unwrap();
            match after.find(close) {
                Some(i) => after.get(..i).unwrap().to_string(),
                None    => body.to_string(),
            }
        },
        None => body.to_string(),
    }
}

#[test]
fn test_substring_before() {
    let test_url          = "https://doc.rust-lang.org/book/first-edition/documentation.html";
    let test_expected_url = "https://doc.rust-lang.org";
    assert_eq!(test_expected_url, substring_before(test_url, "/book"));
    
    let test_emojis = "🗻∈🌏";
    assert_eq!(Some("🗻"), test_emojis.get(0..4));
    println!("{:?}", test_emojis.get(..test_emojis.len()));
    assert_eq!("🗻∈", substring_before(test_emojis, "🌏"));
}

#[test]
fn test_substring_after() {
    let test_url          = "https://doc.rust-lang.org/book/first-edition/documentation.html";
    let test_expected_url = "doc.rust-lang.org/book/first-edition/documentation.html";
    assert_eq!(test_expected_url, substring_after(test_url, "https://"));
    
    let test_emojis = "🗻∈🌏";
    assert_eq!(Some("🗻"), test_emojis.get(0..4));
    println!("{:?}", test_emojis.get(..test_emojis.len()));
    assert_eq!("∈🌏", substring_after(test_emojis, "🗻"));
}

#[test]
fn test_substring_between() {
    let test_url          = "https://doc.rust-lang.org/book/first-edition/documentation.html";
    let test_expected_url = "doc.rust-lang.org";
    assert_eq!(test_expected_url, substring_between(test_url, "https://", "/"));
    
    let test_emojis = "🗻∈🌏";
    assert_eq!(Some("🗻"), test_emojis.get(0..4));
    println!("{:?}", test_emojis.get(..test_emojis.len()));
    assert_eq!("∈", substring_between(test_emojis, "🗻", "🌏"));
}