If you are comparing in code, you should already know how to due this by converting both strings to lowercase.
In rules, you can do the samething with the XPath translate() function:
translate(my:field1, "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "abcdefghijklmnopqrstuvwxyz") = translate(my:field2, "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "abcdefghijklmnopqrstuvwxyz")