Using eval is perfectly fine if you aren't going to be doing lots of them, but you may run into performance problems if you are.

I did this the other day: I needed to relabel the digits in a string of [1-4]+ so that it would start "1234". I was calling this routine many times, but since there were only 24 possible transformations I could cache them to avoid repeated evals:

  my %tr;
  sub normalise { 
    my $s = shift;
    my $from = substr $s, 0, 4;
    my $tr = $tr{$from} ||= eval qq{ sub { tr{$from}{1234} } };
    &$tr for $s; 
    $s;
  }

[download]

If you are doing this many times and the character(s) you are counting may cover too many combinations to make memoization appropriate, the best general solution is probably Roy Johnson's approach:

  $count = () = ($s =~ /[$search]/g)

[download]

If you specifically know that a) the $search string is always a single character, and b) that the target string is not too long then BrowserUK's xor approach:

  $count = ($s ^ ($search x length $s)) =~ tr/\0/\0/

[download]
may be better.

Hugo

In reply to Re: Capturing occurrence counts via tr/// with variable interpolation by hv
in thread Capturing occurrence counts via tr/// with variable interpolation by KenW

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