package main

import (
    "fmt"
    "strings"
)

func urlEncode(s string) string {
	b := strings.Builder{}
	b.Grow(len(s))

	for _, c := range s {
		if (c >= '0' && c <= '9') || (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '/' {
			b.WriteRune(c)
		} else {
			b.WriteString(fmt.Sprintf("%%%02x", c))
		}
	}

	return b.String()
}

func main() {
    s := "foo/bar+baz $qux!"
    fmt.Println(urlEncode(s))
}