It is certainly possible to write a function that accepts information about the values you want to set for the variables, that returns a regexp to set them up in that way.

As you supposed, the parens are counted at regexp compile time, so it is not possible to embed all the logic in a regexp without fixing the paren count in advance.

$^N will be the first match capture that reaches the maximum value of @+[1..], which can be emulated by constructing the regexp to nest the captures that end at this point.

You may need to allow for some captures being unset, as in "ac" =~ /(a)?(b)?(c)?/.

For the simple case where the nesting is natural, most efficient would be to forget the lookaheads and just construct a nesting of dots and parens, along with a simple negative lookahead for unset parens ((?!))?. I think something like the below would do it, but I have not tested it exhaustively:

my $s = "abcdefghi";
my @a = ([ 0, 3 ], [ 0, 1 ], [ 1, 2 ], [ 2, 1 ]);
my $qr = matcher(\@a);
match($s, $qr);

sub matcher {
  my $array = shift;
  my(%pos, @undef);
  my($start, $length) = @{ $array->[0] };
  for (1 .. $#$array) { 
    my($pos, $width) = ($array->[$_][0], $array->[$_][1]);
    if (defined $pos) {
      $pos{$pos} .= '(';
      $pos{$pos + $width} .= ')';
    } else {
      push @undef, $_;
    }
  } 
  my $qr = '.' x ($start + $length);
  for (sort { $b <=> $a } keys %pos) { 
    substr $qr, $_, 0, $pos{$_};
  }
  for (reverse @undef) {
    $qr =~ s/((.*?\(){$_})/$1((?!))?/;
  }
  $qr =~ s{(.{$start})}{(?<=^$1)} if $start;
  qr/$qr/;
}

[download]

Extending this to add lookaheads for captures that are not naturally nested is left as an exercise for the reader. :)

With respect to your title, note that it is possible to plug in an alternate regexp engine - this is how use re 'debug' is implemented - but I'm not aware that anyone has ever taken advantage of this, nor do I imagine there is any documentation of how you might do so.

Hugo

In reply to Re: Plug for an alternate regex engine by hv
in thread Plug for an alternate regex engine by bsb

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