What is the true behavior of the regex `"\b"`

[TOETOE55]

"\b" represents word boundaries in regex. But it seems to behave differently in different implementations:

In regex crate, which "can't" was separated

fn main() {
    let s = "The quick (\"brown\") fox can't jump 32.3 feet, right?";
    
    let re = Regex::new(r"\b").unwrap();
    let res = re.split(s).collect::<Vec<&str>>();
    // ["", "The", " ", "quick", " (\"", "brown", "\") ", "fox", " ", "can", "'", "t", " ", "jump", " ", "32", ".", "3", " ", "feet", ", ", "right", "?"]
    println!("{res:?}");
}

in swift 5.7

let s = "The quick (\"brown\") fox can't jump 32.3 feet, right?"

let words = s.split(separator: /\b/)
// ["The", " ", "quick", " ", "(", "\"", "brown", "\"", ")", " ", "fox", " ", "can\'t", " ", "jump", " ", "32.3", " ", "feet", ",", " ", "right", "?"]
print(words)
// 

In unicode-segmentation crate, it's same as regex in swift, according to the [Unicode Standard Annex #29(http://www.unicode.org/reports/tr29/) rules.

fn main() {
    let s = "The quick (\"brown\") fox can't jump 32.3 feet, right?";
    
    let res = s.split_word_bounds().collect::<Vec<&str>>();
    // ["The", " ", "quick", " ", "(", "\"", "brown", "\"", ")", " ", "fox", " ", "can't", " ", "jump", " ", "32.3", " ", "feet", ",", " ", "right", "?"]
    println!("{res:?}");
}