main.go

package main

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

const Shift = 254

type set struct {
	from byte
	to   byte
}

var sets = []set{
	{from: 'a', to: 'z'},
	{from: 'A', to: 'Z'},
	{from: '0', to: '9'},
}

func main() {
	buf, err := ioutil.ReadAll(os.Stdin)
	if err != nil {
		panic(err)
	}

	outbuf := make([]byte, len(buf), len(buf))
	copy(outbuf, buf)

	for _, bound := range sets {
		from, to := bound.from, bound.to
		for i, b := range buf {
			sb := byte(b + Shift)
			if from <= sb && sb <= to {
				outbuf[i] = sb
			}
		}
	}

	fmt.Println(string(outbuf))
}