def ordinal(s):
    """Gets a number from a standard ordinal.

    >>> ordinal("15th")
    15

    >>> ordinal("34st")
    None

    >>> ordinal("foobar")
    None
    """
    if not s[:-2].isdigit():
        return None
    value = int(s[:-2])
    v = int(s[-3])
    th = s[-2:]
    if v is 1 and th == 'st' or v is 2 and th == 'nd' or v is 3 and th == 'rd' or th == 'th':
        return value

    return None

print ordinal("213rd")