I've come across something a bit strange when using substitution at a point determined by look-behind and -ahead assertions. I used a regex code block to print some diagnostic text whenever a match occurred but found to my surprise that the code block seemed to be called twice for each match. Here's a small script that demonstrates the behaviour. It is just inserting a plus sign into a string at any point between pseudo-tags with different sorts of bracket pairs.

use strict;
use warnings;

use re q{eval};

sub doSep { print q{-} x 40, qq{\n}; }

my $count     = 0;
my $string    = q{<x1>[x2]{x3}(x4)};
my $rxClose   = qr@[]>})]@;
my $rxOpen    = qr@[[<{(]@;
my $rxBetween = qr
   {(?x)
       (?<=($rxClose))
       (?=($rxOpen))
       (?{print qq{Match @{ [++ $count] }: left $1, right $2\n}})
   };

print qq{ Before: $string\n};
doSep();

$string =~ s{$rxBetween}{+}g;
doSep();

print qq{  After: $string\n};
[download]

Even though the code block seems to execute twice per match the substitution is only done once. Here's the output, six apparent matches, weird.

 Before: <x1>[x2]{x3}(x4)
----------------------------------------
Match 1: left >, right [
Match 2: left >, right [
Match 3: left ], right {
Match 4: left ], right {
Match 5: left }, right (
Match 6: left }, right (
----------------------------------------
  After: <x1>+[x2]+{x3}+(x4)
[download]

If I change the code so that look-arounds are not used the strange behaviour does not happen and the code block is only executed once per match.

use strict;
use warnings;

use re q{eval};

sub doSep { print q{-} x 40, qq{\n}; }

my $count     = 0;
my $string    = q{<x1>[x2]{x3}(x4)};
my $rxClose   = qr@[]>})]@;
my $rxOpen    = qr@[[<{(]@;
my $rxBetween = qr
   {(?x)
       ($rxClose)
       ($rxOpen)
       (?{print qq{Match @{ [++ $count] }: left $1, right $2\n}})
   };

print qq{ Before: $string\n};
doSep();

$string =~ s{$rxBetween}{$1+$2}g;
doSep();

print qq{  After: $string\n};
[download]

The output this time, just three matches as expected.

 Before: <x1>[x2]{x3}(x4)
----------------------------------------
Match 1: left >, right [
Match 2: left ], right {
Match 3: left }, right (
----------------------------------------
  After: <x1>+[x2]+{x3}+(x4)
[download]

I have done other testing to try and pin down what is happening. The code block also executes twice if I am just matching using look-arounds rather than substituting. I have also tried using just a single look-behind rather than both lokk-behind and -ahead but still get the double execution.

Can any Monk throw some light on what is happening here?

Cheers,

JohnGG

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