This isn't an answer to your question (since the other two respondents gave solutions), but I just wanted to point out that if there is a possibility of pseudonyms like AAA, BBB, etc., I would assume there is also the possibility of pseudonyms like QWERTY, ASDFGH, 12345, etc. You are going to need a more elaborate filter than that (which can not possibly get rid of all pseudonyms without also potentially getting rid of some real names).
... View more