func FatalUsage(format string, a ...interface{}) {
	if format != "" {
		fmt.Fprintf(os.Stderr, format, a...)
	}
	flag.Usage()
	os.Exit(2)
}
FatalUsage report usage error in same way as flag.Parse().
package main
import (
	"flag"
	"fmt"
	"net/url"
	"os"
	"regexp"
)