You asked to match "xx", not "XX". Regexen (like most good programmers) are rather picky about clearly and correctly specifying what you want matched. Maybe you only wanted to match lower case letters. Maybe you wanted to match only upper case. Maybe you didn't care. The interpretation that it wouldn't match was perfectly valid given the inherent ambiguity in your original question.

I'd point you (yet again) at How (Not) To Ask A Question, but it's obvious you're not going to take the time to either read it nor take the care to make useful questions.

*plonk*