import re


def islatitude(value):
    """
    Return whether or not given value is valid latitude.
    If the value is valid latitude, this function returns ``True``, otherwise ``False``.

    Examples::

        >>> islatitude('-90.000')
        True

        >>> islatitude('+99.9')
        False

    :param value: string to validate latitude
    """
    lat = re.compile(r'^[-+]?([1-8]?\d(\.\d+)?|90(\.0+)?)$')
    return bool(lat.match(value))