Go "Not" Regex Prefix

not.go

package main

import (
	"bytes"
	"fmt"
	"regexp"
)

func main() {
	var valid = regexp.MustCompile(Not("abcd"))

	// Matches
	fmt.Println(valid.MatchString("xabcd"))
	fmt.Println(valid.MatchString("xyz"))
	fmt.Println(valid.MatchString("abxyz"))
	fmt.Println(valid.MatchString("a"))
	fmt.Println(valid.MatchString("ab"))
	fmt.Println(valid.MatchString("abc"))

	// No Match
	fmt.Println(valid.MatchString("abcdxyz"))
	fmt.Println(valid.MatchString("abcd"))
	fmt.Println(valid.MatchString("abcde"))
}

// Generate a regexp which matches strings without the prefix m
func Not(m string) string {
	buf := bytes.NewBufferString("")
	sep := ""
	opt := ""

	for i, a := range m {
		fmt.Fprintf(buf, "%v%v([^%v].*)%s", sep, m[:i], string(a), opt)
		sep = "|"
		opt = "?"
	}

	return fmt.Sprintf("^(%s)$", buf.String())
}

On the playground:
https://play.golang.org/p/piSoC9RB_l