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).

def validate_usr(username):
    import re
    if len(username) < 4 or len(username) > 16:
        return False
    else:
        return bool(re.match("^[a-z0-9_]*$", username))