:shrug:, seems like roughly the same amount of work to me. Unless by "work" you mean lines of code, in which case I was being rather more verbose than usual. My method could easily be written: 

my @keep;     # numbers to keep
while(chomp( my $line = <NUMBERS> ) ) { 
    my %count;
    ++$count{$_} for split '', $line;
    push @keep, $line if grep { $_ == 2 } values %count
}

[download]

I also wouldn't be surprised if the regex method was slower, but I'm too lazy to do a benchmark right now. :) (And for only 10,000 numbers, it probably does not matter much.)

In reply to Re^3: Find duplicate digits by friedo
in thread Find duplicate digits by Anonymous Monk

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