import re


def cs_classes(post):
    """
    Returns strings that look like a Berkeley CS class,
    starting with "CS", followed by a number, optionally ending with A, B, or C.
    Case insensitive.

    >>> cs_classes("Is it unreasonable to take CS61A, CS61B, CS70, and EE16A in the summer?")
    True
    >>> cs_classes("how do I become a TA for cs61a? that job sounds so fun")
    True
    >>> cs_classes("Can I take ECON101 as a CS major?")
    False
    >>> cs_classes("Should I do the lab lites or regular labs in EE16A?")
    False
    """
    return bool(re.search(r"[CS cs]\d+[A-C|a-c]", post))