Convert CamelCase to underscore in golang

underscore.go

package main

import (
	"fmt"
	"regexp"
	"strings"
)

var camel = regexp.MustCompile("(^[^A-Z0-9]*|[A-Z0-9]*)([A-Z0-9][^A-Z]+|$)")

func underscore(s string) string {
	var a []string
	for _, sub := range camel.FindAllStringSubmatch(s, -1) {
		if sub[1] != "" {
			a = append(a, sub[1])
		}
		if sub[2] != "" {
			a = append(a, sub[2])
		}
	}
	return strings.ToLower(strings.Join(a, "_"))
}

func main() {
	fmt.Println(
		underscore("ILoveGoAndJSONSoMuch"),
		underscore("CamelCase"),
		underscore("Camel"),
		underscore("CAMEL"),
		underscore("camel"),
		underscore("BIGCase"),
		underscore("SmallCASE"),
		underscore("Camel1"),
		underscore("BIGCase1"),
		underscore("BC1"),
	)
}

underscore("my_case1") will return 1

underscore("mrT") returns "T"