import re


def normalize_field_name(field_name: str) -> str:
    """If field name contains special characters, encloses it in square brackets
    
    Arguments:
        field_name {str} -- original field name 
    
    Returns:
        str -- normalized field name
    """
    if not re.match("^[A-Za-z0-9]+$", field_name):
        field_name = "[%s]" % field_name

    return field_name