The PCRE (or any regex flavor supporting word boundaries, Unicode properties and lookaheads) \b(?!\p{Lu}+\b)(?=\p{L}\p{Ll})(?=\p{L}\p{Lu})(?!\p{Lu}\p{Ll}*\b)\p{L}+\b regex finds CaMeL words that:

• Have at least 1 lowercase letter
• Have at least 1 uppercase letter
• Do not follow the X, XXX or Xxxx patterns

Author's SO profile: http://stackoverflow.com/users/3832970/wiktor-stribi%C5%BCew