BCP 47 Language Tag

const re = new RegExp("^[a-zA-Z]{2,3}(?:-[A-Za-z0-9]{2,8})*$", "");
const input = "en";
const matches = [...input.matchAll(re)];
console.log(matches.map(m => m[0]));

import re

pattern = re.compile(r"^[a-zA-Z]{2,3}(?:-[A-Za-z0-9]{2,8})*$")
input_text = "en"
for m in pattern.finditer(input_text):
    print(m.group(0))

package main

import (
	"fmt"
	"regexp"
)

func main() {
	re := regexp.MustCompile(`^[a-zA-Z]{2,3}(?:-[A-Za-z0-9]{2,8})*$`)
	input := `en`
	for _, match := range re.FindAllString(input, -1) {
		fmt.Println(match)
	}
}