Write a simple regex to validate a username. Allowed characters are:
- lowercase letters,
- numbers,
- underscore
Length should be between 4 and 16 characters (both included).
from re import fullmatch

def validate_usr(username):
    return bool(fullmatch(r"[_0-9a-z]{4,16}", username))
from re import fullmatch

def validate_usr(username):
    return bool(fullmatch(r"[_0-9a-z]{4,16}", username))