import re
from typing import Optional


def sub_clyde(username: Optional[str]) -> Optional[str]:
    """
    Replace "e"/"E" in any "clyde" in `username` with a Cyrillic "е"/"E" and return the new string.

    Discord disallows "clyde" anywhere in the username for webhooks. It will return a 400.
    Return None only if `username` is None.
    """
    def replace_e(match: re.Match) -> str:
        char = "е" if match[2] == "e" else "Е"
        return match[1] + char

    if username:
        return re.sub(r"(clyd)(e)", replace_e, username, flags=re.I)
    else:
        return username  # Empty string or None