import re


def parse_chromosome(s):
    """
    Parse and normalize a chromosome string, which may be prefixed with 'chr'.
    """
    match = re.fullmatch(r'(?:chr)?([1-9]|1\d|2[0-2]|x|y|xy|mt?)', s, re.IGNORECASE)

    if not match:
        raise ValueError(f'Failed to match chromosome against {s}')

    return match.group(1).upper()