Following up from an earlier question on extracting the n'th regex match, I now need to substitute the match, if found.

I thought that I could define the extraction subroutine and call it in the substitution with the /e modifier. I was obviously wrong (admittedly, I had an XY problem).

use strict;
use warnings;

sub extract_quoted { # à la codaddict

        my ($string, $index) = @_;
        while($string =~ /'(.*?)'/g) {
                $index--;
                return $1 if(! $index);
        }
        return;
}

my $string = "'How can I','use' 'PERL','to process this' 'line'";

extract_quoted ( $string, 3 );
$string =~ s/&extract_quoted($string,2)/'Perl'/e;

print $string; # Prints 'How can I','use' 'PERL','to process this' 'line'

There are, of course, many other issues with this technique:

• What if there are identical matches at different positions?
• What if the match isn't found?

In light of this situation, I'm wondering in what ways this could be implemented.