import re


def is_isin(isin: str) -> bool:
    """
    Checks whether a string is a valid ISIN or not.

    :param isin: the string to be checked
    :return: true if the given string is a valid ISIN, otherwise false
    """
    if re.match("^[A-Za-z]{2}[A-Za-z0-9]{10}", isin):
        return True
    else:
        return False