How can I substitute the nth occurrence of a match in a Perl regex?

Question

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

Answer 1

See perldoc perlvar:

use strict; use warnings;

use Test::More tests => 5;

my %src = (
    q{'I want to' 'extract the word' 'PERL','from this string'}
    => q{'I want to' 'extract the word' 'Perl','from this string'},
    q{'What about', 'getting','PERL','from','here','?'}
    => q{'What about', 'getting','Perl','from','here','?'},
    q{'How can I','use' 'PERL','to process this' 'line'}
    => q{'How can I','use' 'Perl','to process this' 'line'},
    q{Invalid} => q{Invalid},
    q{'Another invalid string'} => q{'Another invalid string'}
);

while ( my ($src, $target) = each %src ) {
    ok($target eq subst_n($src, 3, 'Perl'), $src)
}

sub subst_n {
    my ($src, $index, $replacement) = @_;
    return $src unless $index > 0;
    while ( $src =~ /'.*?'/g ) {
        -- $index or return join(q{'},
            substr($src, 0, $-[0]),
            $replacement,
            substr($src, $+[0])
        );
    }
    return $src;
}

Output:

C:\Temp> pw
1..5
ok 1 - 'Another invalid string'
ok 2 - 'How can I','use' 'PERL','to process this' 'line'
ok 3 - Invalid
ok 4 - 'What about', 'getting','PERL','from','here','?'
ok 5 - 'I want to' 'extract the word' 'PERL','from this string'

Of course, you need to decide what happens if an invalid $index is passed or if the required match is not found. I just return the original string in the code above.