If the client keeps on changing the requirements every now and then, then what testing method should be followed?

I always perform regression testing as soon as the changes come up. The case is the client comes up with changes or additional requirement every now and then and that makes the case more messy. I test something and then the whole things get changed. Again I have to test the changed module and perform integration testing with other modules that is linked with it.

How to deal with such cases?

Need the answer urgent.

Thanks.