CopyDigits function ref

copy-digits.go

package main

// ref: https://blog.golang.org/go-slices-usage-and-internals
// 1234 <-- digits

import (
	"fmt"
	"io/ioutil"
	"regexp"
)

var digitRegexp = regexp.MustCompile("[0-9]+")

func CopyDigits(filename string) []byte {
	b, _ := ioutil.ReadFile(filename)
	b = digitRegexp.Find(b)
	return append([]byte{}, b...)
}

func main() {
	digits := CopyDigits("./copy-digits.go")
	fmt.Println(digits)
}

@1Mark I may be wrong, but I think you can see by checking the capacity of the slice. Here's an example that focuses on that.

package main

import (
	"fmt"
	"log"
	"os"
)

const sAndS = "s-and-s.txt"

func main() {
	data, err := os.ReadFile(sAndS)
	if err != nil {
		log.Fatalf("cannot open 's-and-s.txt': %v\n", err)
	}

	fmt.Println("Before slicing:")
	fmt.Printf("\tlen(data) = %d; cap(data) = %d\n", len(data), cap(data))

	data = data[0:4]
	fmt.Println("After data = data[0:4]:")
	fmt.Printf("\tlen(data) = %d; cap(data) = %d\n", len(data), cap(data))

	data = append([]byte{}, data[0:4]...)
	fmt.Println("After data = append([]byte{}, data[0:4]...):")
	fmt.Printf("\tlen(data) = %d; cap(data) = %d\n", len(data), cap(data))
}

I used it on a plaintext version of a book, and the output shows how many bytes the original file has and how many bytes different slices have access to.

$ wc -c s-and-s.txt
  712928 s-and-s.txt
$ ./append
Before slicing:
	len(data) = 712928; cap(data) = 712929
After data = data[0:4]:
	len(data) = 4; cap(data) = 712929
After data = append([]byte{}, data[0:4]...):
	len(data) = 4; cap(data) = 8

You can see that reassigning with a slice expression reduces the length, but the capacity is still as large as the whole file (712929 bytes). But if you use append and force it to create a new backing array, the result is 8 bytes rather than 712929.