Capturing grouping for regex function replace in Go

example.go

func main() {
	str := "abc foo:bar def baz:qux ghi"
	re := regexp.MustCompile("([a-z]+):([a-z]+)")
	result := ReplaceAllStringSubmatchFunc(re, str, func(groups []string) string {
		return groups[1] + "." + groups[2]
	})
	fmt.Printf("'%s'\n", result)
}

main.go

import "regexp"

func ReplaceAllStringSubmatchFunc(re *regexp.Regexp, str string, repl func([]string) string) string {
	result := ""
	lastIndex := 0

	for _, v := range re.FindAllSubmatchIndex([]byte(str), -1) {
		groups := []string{}
		for i := 0; i < len(v); i += 2 {
			groups = append(groups, str[v[i]:v[i+1]])
		}

		result += str[lastIndex:v[0]] + repl(groups)
		lastIndex = v[1]
	}

	return result + str[lastIndex:]
}