You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: expand.rs
    path of file: ./repos/phaser/phaser/vendor/regex/src
    
    the code of the file until where you have to start completion: use std::str;

use crate::find_byte::find_byte;

use crate::re_bytes;
use crate::re_unicode;

pub fn expand_str(
    caps: &re_unicode::Captures<'_>,
    mut replacement: &str,
    dst: &mut String,
) {
    while !replacement.is_empty() {
        match find_byte(b'$', replacement.as_bytes()) {
            None => break,
            Some(i) => {
                dst.push_str(&replacement[..i]);
                replacement = &replacement[i..];
            }
        }
        if replacement.as_bytes().get(1).map_or(false, |&b| b == b'$') {
            dst.push_str("$");
            replacement = &replacement[2..];
            continue;
        }
        debug_assert!(!replacement.is_empty());
        let cap_ref = match find_cap_ref(replacement.as_bytes()) {
            Some(cap_ref) => cap_ref,
            None => {
                dst.push_str("$");
                replacement = &replacement[1..];
                continue;
            }
        };
        replacement
    
fn find_cap_ref(replacement: &[u8]) -> Option<CaptureRef<'_>> {
    let mut i = 0;
    let rep: &[u8] = replacement.as_ref();
    if rep.len() <= 1 || rep[0] != b'$' {
        return None;
    }
    i += 1;
    if rep[i] == b'{' {
        return find_cap_ref_braced(rep, i + 1);
    }
    let mut cap_end = i;
    while rep.get(cap_end).map_or(false, is_valid_cap_letter) {
        cap_end += 1;
    }
    if cap_end == i {
        return None;
    }
    // We just verified that the range 0..cap_end is valid ASCII, so it must
    // therefore be valid UTF-8. If we really cared, we could avoid this UTF-8
    // check via an unchecked conversion or by parsing the number straight from
    // &[u8].
    let cap =
        str::from_utf8(&rep[i..cap_end]).expect("valid UTF-8 capture name");
    Some(CaptureRef {
        cap: match cap.parse::<u32>() {
            Ok(i) => Ref::Number(i as usize),
            Err(_) => Ref::Named(cap),
        },
        end: cap_end,
    })
}

fn find_cap_ref_braced(rep: &[u8], mut i: usize) -> Option<CaptureRef<'_>> {
    let start = i;
    while rep.get(i).map_or(false, |&b| b != b'}') {
        i += 1;
    }
    if !rep.get(i).map_or(false, |&b| b == b'}') {
        return None;
    }