import re


def extract_adif_column(adif_file, column_name):
    """
    Extract data column from ADIF file (e.g. 'OPERATOR' column).

    Parameters
    ----------
    adif_file: file object
        ADIF file opened using open().
    column_name: str
        Name of column (e.g. OPERATOR).
    Returns
    -------
    matches: list of str
        List of values extracted from the ADIF file.
    """

    pattern = re.compile('^.*<' + column_name + ':\d+>([^<]*)<.*$', re.IGNORECASE)
    matches = [re.match(pattern, line)
            for line in adif_file]
    matches = [line[1].strip() for line in matches if line is not None]
    adif_file.seek(0)

    if len(matches) > 0:
        return matches
    else:
        return None