In fact, merlyn's code uses the approach Juerd showed you. It's just a bunch of negative look-aheads for the backreferences you've already matched. For instance, text matching "ABCADB" would be the regex /^(.)(?!\1)(.)(?!\1|\2)(.)\1(?!\1|\2|\3)(.)\2$/s.

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Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area)
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;