Re: s/// in a sub : passing expressions

Juerd explained how to do this with eval, but there is another way. I saw this illustrated in a node a couple of months back, which of course I can no longer find, so I'll try to reproduce the concept. Basically the idea is that we build an array of the $1 .. $n values then do a second substitution on that. The array is required because the substitution clobbers the variables:

sub dyn_replace {
  my ($replace) = @_;
  my @groups;
  {
    no strict 'refs';
    $groups[$_] = $$_ for 1 .. $#-;      # the size of @- tells us the
+ number of capturing groups
  }
  $replace =~ s/\$(\d+)/$groups[$1]/g;
  return $replace;
}
[download]

and we use it as follows

my $pat = qr/foo(\d+)bar(\d+)/;
my $rep = 'x$1y$2';
my $str = 'foo36bar47';
$str =~ s/$pat/dyn_replace($rep)/e;
print "$str\n";
[download]

and it prints "x36y47", just as desired.

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Re^2: s/// in a sub : passing expressions by Tanktalus (Canon) on Oct 30, 2005 at 01:29 UTC
I think you're referring to an old node of mine: Re: How to do regex backreferences within $variable replacement text?. When I used that solution at work, another perl programmer told me I was "too advanced." I don't think that this was very advanced - just a novel way of looking at the problem. Anyway, that entire thread may be relevant here.	[reply]
Re^3: s/// in a sub : passing expressions by Errto (Vicar) on Nov 01, 2005 at 01:41 UTC
Yes, that's the one. Your solution is neater than mine because it avoids the number variables, and possibly faster (though I can't be bothered to benchmark it), but mine has a small advantage in that you can use it with modifiers such as /i, /g, and /x.	[reply]