import re


def isbase64(value):
    """
    Return whether or not given value is base64 encoded.
    If the value is base64 encoded, this function returns ``True``, otherwise ``False``.

    Examples::

        >>> isbase64('U3VzcGVuZGlzc2UgbGVjdHVzIGxlbw==')
        True

        >>> isbase64('Vml2YW11cyBmZXJtZtesting123')
        False

    :param value: string to validate base64 encoding
    """
    base64 = re.compile(r"^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{4})$")
    return bool(base64.match(value))