import re

_PREFIX_PATTERN = re.compile(
    r"^(?:(?:re|fw|fwd)\s*(?:\[\d+\])?\s*:\s*)+",
    flags=re.IGNORECASE,
)


def normalize_subject(subject: str | None) -> str:
    value = (subject or "").strip()
    while True:
        next_value = _PREFIX_PATTERN.sub("", value, count=1).strip()
        if next_value == value:
            break
        value = next_value
    return re.sub(r"\s+", " ", value).strip().lower()