"""Decode a base64 string to its original bytes representation.

This function decodes a base64 encoded string into its original bytes. The input string is first validated to ensure it conforms to the base64 format. If the input is not a valid base64 string, the function returns None. The function handles both standard and URL-safe base64 strings.

Args:
    s (str): A base64 encoded string that can be used in text files.

Returns:
    Optional[bytes]: The original decoded data as bytes. Returns None if the input is not a valid base64 string.
"""
You should write code starting with:
import base64
import re

def base64decode(s: str) -> Optional[bytes]:
    # TODO: Implement this function
    pass

