func Grep(r string) Filter {
	re, err := regexp.Compile(r)
	if err != nil {
		return FilterFunc(func(Arg) error { return err })
	}
	return If(re.MatchString)
}
func Grep(r string) Filter {
	re, err := regexp.Compile(r)
	if err != nil {
		return FilterFunc(func(Arg) error { return err })
	}
	return If(re.MatchString)
}
func Grep ( r string ) Filter { re , err := regexp . Compile ( r ) if err != nil { return FilterFunc ( func ( Arg ) error { return err } ) } return If ( re . MatchString ) }
// Grep emits every input x that matches the regular expression r.
Grep emits every input x that matches the regular expression r .