main.go

func validateSFID(input string) bool {
	// https://stackoverflow.com/a/29299786/1333724
	if len(input) != 18 {
		return false
	}
	parts := []string{input[0:5], input[5:10], input[10:15]}
	chars := make([]byte, 3)
	for j, word := range parts {
		for i, char := range []byte(word) {
			// fmt.Println([]byte("AZ")) => [65 90]
			if char >= 65 && char <= 90 {
				chars[j] += 1 << uint64(i)
			}
		}
	}
	for i, c := range chars {
		if c <= 25 {
			chars[i] = c + 65
		} else {
			chars[i] = c - 25 + 48
		}
	}
	if string(chars) == input[15:] {
		return true
	}
	return false
}

Warning, as of 10/2022 this does not appear to work for all valid Salesforce IDs. It appears to fail on certain ranges of their semi-sequential ids. We got bit by this. Unfortunately I don't have a working version but if I right one up I will link to it here.