import re


def normalize_mentions(text):
    """
    Replace Twitter username handles with '@USER'.
    """
    # ignores = set(["@CFKArgentina", "@mauriciomacri", "@alferdez", "@MiguelPichetto"])
    ignores = set(["@CFKArgentina", "@mauriciomacri"])
    
    def _replace(matched):
        if matched.group(0) in ignores:
            return "@USER"
        else:
            return matched.group(0)

    # pattern = re.compile(r"(^|(?<=[^\w.-]))@[A-Za-z_]+\w+")
    # return pattern.sub(_replace, text)

    pattern = re.compile(r"(^|(?<=[^\w.-]))@[A-Za-z_]+\w+")
    return pattern.sub('@USER', text)