Strings and Text Processing

contains() and starts_with()

The .contains(), .starts_with(), and .ends_with() methods on str test whether a string slice matches a given pattern (&str, char, &[char], or a char predicate closure) and return a bool.

Signatures and Supported Patterns

pub fn contains<P: Pattern>(&self, pat: P) -> bool
pub fn starts_with<P: Pattern>(&self, pat: P) -> bool
pub fn ends_with<P: Pattern>(&self, pat: P) -> bool
Pattern Type P Example Call What It Tests
&str s.starts_with("https://") Exact substring match
char s.ends_with('/') Single character match (faster than 1-char "/" literal)
&[char] s.starts_with(&['#', ';'][..]) Matches any character in the slice
FnMut(char) -> bool s.contains(char::is_numeric) Matches any character satisfying the predicate

Testing Prefixes, Suffixes, and Substrings

fn main() {
    let url = "https://crates.io/crates/serde";

    println!("Is HTTPS:         {}", url.starts_with("https://"));
    println!("Ends with serde:  {}", url.ends_with("/serde"));
    println!("Contains crates:  {}", url.contains("crates"));

    // Using a char predicate closure:
    let password = " correct-horse-9 ";
    println!("Has digit:        {}", password.contains(|c: char| c.is_ascii_digit()));
    println!("Has whitespace:   {}", password.contains(char::is_whitespace));
}

Is HTTPS:         true
Ends with serde:  true
Contains crates:  true
Has digit:        true
Has whitespace:   true

Case-Insensitive ASCII Matching (eq_ignore_ascii_case)

When comparing strings or prefixes case-insensitively for ASCII protocols (such as HTTP headers or SQL keywords), use s.eq_ignore_ascii_case(other) rather than allocating a new lowercase String via .to_lowercase():

fn is_bearer_token(header_value: &str) -> bool {
    header_value
        .get(..7)
        .is_some_and(|prefix| prefix.eq_ignore_ascii_case("bearer "))
}

fn main() {
    println!("'Bearer xyz': {}", is_bearer_token("Bearer xyz"));
    println!("'BEARER xyz': {}", is_bearer_token("BEARER xyz"));
    println!("'Basic xyz':  {}", is_bearer_token("Basic xyz"));
}

'Bearer xyz': true
'BEARER xyz': true
'Basic xyz':  false