msa.go

package main

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

func main() {
	flag.Parse()
	args := (flag.Args())
	zws := '\u200B'
	charmatch := regexp.MustCompile("([A-Za-z\\d])")
	spacematch := regexp.MustCompile("\\s")
	emojimatch := regexp.MustCompile(fmt.Sprintf(":[^: %c]+:", zws))
	out := "???"

	out = spacematch.ReplaceAllString(args[1], "\n")
	out = charmatch.ReplaceAllString(out, fmt.Sprintf(":%s_$1:%c", args[0], zws))

	out = emojimatch.ReplaceAllStringFunc(out, func(s string) string {
		return strings.ToLower(s)
	})

	fmt.Println(out)
}