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

Question

Is it possible to extract the n^th match in a string of single-quoted words?

use strict;
use warnings;

my $string1 = \"\'I want to\' \'ex         


        

Answer 1

It may look ugly, but

my $quoted = qr/'[^']+'/;  # ' fix Stackoverflow highlighting
my %_extract_wanted_cache;
sub extract_wanted_memo { 
  my($string, $index) = @_;
  $string =~ ($_extract_wanted_cache{$index} ||=
                qr/^(?:.*?$quoted.*?){@{[$index-1]}}($quoted)/)
    ? $1
    : ();
}

benchmarking suggests it might be worthwhile:

sub extract_wanted { 
  my($string, $index) = @_;
  $string =~ /^(?:.*?$quoted.*?){@{[$index-1]}}($quoted)/
    ? $1
    : ();
}

sub extract_wanted_gindex {
  my($string, $index) = @_;
  ($string =~ /$quoted/g)[$index-1];
}

use Benchmark;
timethese -1 => {
  nocache => sub { extract_wanted        $string2, 3 },
  memoize => sub { extract_wanted_memo   $string2, 3 },
  index   => sub { extract_wanted_gindex $string2, 3 },

  nocache_fail => sub { extract_wanted        $string2, 100 },
  memoize_fail => sub { extract_wanted_memo   $string2, 100 },
  index_fail   => sub { extract_wanted_gindex $string2, 100 },
}

Results:

Benchmark: 
running
 index, index_fail, memoize, memoize_fail, nocache, nocache_fail
 for at least 1 CPU seconds
...

     index:   1 w/c secs (1.04 usr + 0.00 sys = 1.04 CPU) @183794.23/s (n=191146)
index_fail:   1 w/c secs (1.03 usr + 0.00 sys = 1.03 CPU) @185578.64/s (n=191146)
   memoize:   1 w/c secs (1.00 usr + 0.00 sys = 1.00 CPU) @264664.00/s (n=264664)
memoize_fail: 0 w/c secs (1.03 usr + 0.00 sys = 1.03 CPU) @835106.80/s (n=860160)
   nocache:   0 w/c secs (1.03 usr + 0.00 sys = 1.03 CPU) @196495.15/s (n=202390)
nocache_fail: 2 w/c secs (1.03 usr + 0.00 sys = 1.03 CPU) @445390.29/s (n=458752)