import re


def is_iso_time(time_string):
    """
    Checks if string represents valid time in ISO format,
    with the default delimiter.
    Regex is somewhat convoluted, but general enough to last
    at least until the 9999 AD.

    :returns:
        True if string matches the pattern.
        False otherwise.
    """
    match = re.match(
        r'\d{4}-[01][0-9]-[0-3][0-9]T[0-3][0-9](:[0-5][0-9]){2}\.\d+Z',
        time_string)
    if match:
        return True
    else:
        return False