Which one of these sentences is more correct?

Question

If I'm explaining a specific rule to someone. I would like to say that the rule has exceptions. How to choose the correct proposition?

1. This rule is not valid in all cases.
2. This rule is not valid for all cases.

Are these sentences commonly used among native speakers? or Is there more powerful sentence?

Edit: I'm teaching Arabic to someone. I need to tell him if he sees a specific character at the end of a word. This word is categorized under "feminine words". But it is not a general rule. It has some exceptions. There are some irregular words.

Answer 1

Both of your sentences are equally correct

This rule is not valid in all cases
This rule is not valid for all cases

after which you might enumerate or explain the cases, or you can simply say

There are exceptions to this rule.

Answer 2

As Peter has said, both sentences are correct. For all cases has the approximate meaning of if applied to all cases, while in all cases has the approximate meaning of if made a part of all cases. As you can see, either meaning is equivalent in this context.

Answer 3

I agree that in a general sense there is not much difference, but based on the usage of "cases" meaning "words" in the Edited Question, I would use:

This rule is not valid for all words.

"This rule is not valid in all words." would not make sense.

So it depends on the context or the specific meaning of generalized words.