Look at Backreferences

Also for your prolem can be solved easily without character class. 

$_ = 'abcab';
if(/^(.)(?!\1)(.)(?:(?!\1|\2).)\1\2$/){
    print "$&\n";
}

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artist

In reply to Re: Pattern matching: Why no \1 in [ ]? by artist
in thread Pattern matching: Why no \1 in [ ]? by Not_a_Number

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