Return True if parameter contract is a custom contract, False otherwise.

def _get_custom_contract(param_contract):
    """Return True if parameter contract is a custom contract, False otherwise."""
    if not isinstance(param_contract, str):
        return None
    for custom_contract in _CUSTOM_CONTRACTS:
        if re.search(r"\b{0}\b".format(custom_contract), param_contract):
            return custom_contract
    return None