whole file regexp replacement in go

replace.go

package main

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

func main() {
  regexp, _ := regexp.Compile(os.Args[1])
  replacement := []byte(os.Args[2])
  var b []byte
  var out *os.File
  if len(os.Args) > 3 {
    b, _ = ioutil.ReadFile(os.Args[3])
    out, _ = os.OpenFile(os.Args[3], os.O_WRONLY, 644)
  } else {
    b, _ = ioutil.ReadAll(os.Stdin) 
    out = os.Stdout
  }
  out.Write(regexp.ReplaceAll(b, replacement))
}