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main.py

import re
from typing import Optional

def parse_year(yearstring: str) -> Optional[int]:
    '''
    Something is wrong with this year parser! Can you guess what it is?
    '''
    year = int(yearstring) if re.match('[1-9][0-9][0-9][0-9]', yearstring) else None
    assert year is None or 1000 <= year <= 9999
    return year