Prompted by the responses in Newbie Q:How do I compare items within a string?, in particular those of Zaxo and TedPride I had a go at expanding my attempt with some of their ideas. This lead to some confusion about the use of pos() as shown here. In order to explore this further I have written this script which attempts to find occurrences of one string within another and to annotate each occurrence with the offset (zero-based) within the string

use strict;
use warnings;

#                        1         2
#              012345678901234567890123456789
my $string = q(The cat scattered caterpillars\n);
#                  ^    ^        ^
#                  4    9        18

print "\n Match against string - $string\n";

print "Start matching\n\n";
my $match = 1;
while($string =~ /(cat)/g)
{
    print "Match @{[$match ++]}\n";
    print "                found - $1\n";
    print "       Value of \$-[1] - $-[1]\n";
    print "       Value of \$+[1] - $+[1]\n";
    print "       Value of \$+[0] - $+[0]\n";
    print "         Value of \$#+ - $#+\n";
    print "Value of pos(\$string) - @{[pos($string)]}\n";
}

print "\nStart annotation\n\n";

$string =~ s
   {
      (cat)(?{print "Value of pos(\$string) - @{[pos($string)]}\n"})
   }
   {
      $1 . "[@{[pos($string)]}]"
   }xeg;

print "\n     Annotated string - $string\n";
[download]

which when run produces

 Match against string - The cat scattered caterpillars

Start matching

Match 1
                found - cat
       Value of $-[1] - 4
       Value of $+[1] - 7
       Value of $+[0] - 7
         Value of $#+ - 1
Value of pos($string) - 7
Match 2
                found - cat
       Value of $-[1] - 9
       Value of $+[1] - 12
       Value of $+[0] - 12
         Value of $#+ - 1
Value of pos($string) - 12
Match 3
                found - cat
       Value of $-[1] - 18
       Value of $+[1] - 21
       Value of $+[0] - 21
         Value of $#+ - 1
Value of pos($string) - 21

Start annotation

Value of pos($string) - 7
Value of pos($string) - 12
Value of pos($string) - 21

     Annotated string - The cat[4] scat[9]tered cat[18]erpillars
[download]

As per the documentation, during the matching phase the value returned by pos() corresponds with the value of $+[1], pointing to just after the match. As Zaxo pointed out, $-[1] points to the start of the match.

In the annotation stage pos() again points to just after the match in the (?{ ... }) block, where the last / .../g match left off, I think the documentation says. However, when I use pos() as part of the substitution it seems to return values corresponding to $-[1] and not $+[1].

My question is, what could be causing this apparent change in behaviour?

Cheers,

JohnGG