def split_string(string):
    """Given a string, return the core string and the number suffix.
    If there is no number suffix, the num_core will be None.
    """
    # Get the starting index for the number suffix:
    assert isinstance(string, str)
    i = 1
    for i in range(1, len(string) + 1):
        if string[-i] in [str(k) for k in range(10)]:
            continue
        else:
            break
    idx = len(string) - i + 1
    # Obtain string_core and num_core:
    string_core = string[:idx]
    if len(string[idx:]) > 0:
        num_core = eval(string[idx:])
    else:
        num_core = None

    return string_core, num_core