Re^3: How to use "less than" and "greater than" inside a regex for a $variable number

[AnomalousMonk]

Actually, I think the problem can be addressed without the need for exotic regex operators or constructs (although this uses the  \K operator introduced with 5.10). Unfortunately, this approach involves the replacement of a substring with the identical substring, an operation that I do not think the regex compiler can optimize away and that therefore may lead to a bit of inefficiency.

>perl -wMstrict -le
"my $book = qq{pg. 1 foo  pg. 2 bar baz  pg. 4 fee fie  pg. 5 foe \n}
          . qq{fum  pg. 6 hoo ha  pg. 9 deedle  pg. 10 \n}
          . qq{blah blah  pg. 14 noddle \n}
          ;
 print qq{[[$book]] \n};
 ;;
 my $pn = qr{ pg[.] \s+ }xms;
 $book =~
   s{ $pn (\d+) \K  (.*?)  (?= $pn (\d+))             }
    { my $m = missing($1, $3);  $m ? qq{$2$m  } : $2; }xmsge;
 print qq{(($book)) \n};
 ;;
 sub missing {
   my ($i, $j) = @_;
   ;;
   return if $j - $i < 2;
   ;;
   my ($ii, $jj) = ($i + 1, $j - 1);
   return $j - $i > 2
        ? qq{(pages $ii - $jj missing)}
        : qq{(page $ii missing)}
        ;
   }
"
[[pg. 1 foo  pg. 2 bar baz  pg. 4 fee fie  pg. 5 foe
fum  pg. 6 hoo ha  pg. 9 deedle  pg. 10
blah blah  pg. 14 noddle
]]

((pg. 1 foo  pg. 2 bar baz  (page 3 missing)  pg. 4 fee fie  pg. 5 foe
fum  pg. 6 hoo ha  (pages 7 - 8 missing)  pg. 9 deedle  pg. 10
blah blah  (pages 11 - 13 missing)  pg. 14 noddle
))