Well, since you resorted to benchmarks (updated)... 

            Rate    bt_c    mult      bt     opt     mod    chop
bt_c    180870/s      --     -1%     -5%    -28%    -62%    -74%
mult    181987/s      1%      --     -4%    -27%    -62%    -74%
bt      189426/s      5%      4%      --    -24%    -60%    -73%
opt     249612/s     38%     37%     32%      --    -48%    -64%
mod     476214/s    163%    162%    151%     91%      --    -31%
chop    692944/s    283%    281%    266%    178%     46%      --

[download]
So chop is over 45% faster than mod even though it had to make an extra copy of the string! 
use Benchmark 'cmpthese';

$x = int (1_000_000 * rand 1_000_000);

cmpthese( -3, {
  mult => sub { $x =~ /(\d)*(\d)/  },
  bt_c => sub { $x =~ /(\d*)(\d)/  },
  bt   => sub { $x =~ /\d*(\d)/    },
  opt  => sub { $x =~ /(\d)$/      },
  mod  => sub { $x % 10            },
  chop => sub { my $x= $x; chop $x },
});

[download]

Following are the original bogus results. Thanks to dkubb for mentioning my over local. I realized I'd made a mistake and came back but not quick enough. So it looks like local is quite a bit slower than my (which makes sense), so I'd be interested in how japhy's machine compares the new code. 

        Rate    mult   bt_c     bt    opt    mod     chop
mult 180759/s     --    -1%    -7%   -31%   -62%     -71%
bt_c 182581/s     1%     --    -6%   -31%   -62%     -70%
bt   193680/s     7%     6%     --   -26%   -60%     -69%
opt  263234/s    46%    44%    36%     --   -45%     -57%
mod  481067/s   166%   163%   148%    83%     --     -22%
chop 618559/s   242%   239%   219%   135%    29%       --

[download]
So chop is almost 30% faster than mod even though it had to make an extra copy of the string! 
use Benchmark 'cmpthese';

$x = int (1_000_000 * rand 1_000_000);

cmpthese( -3, {
  mult => sub { $x =~ /(\d)*(\d)/ },
  bt_c => sub { $x =~ /(\d*)(\d)/ },
  bt   => sub { $x =~ /\d*(\d)/   },
  opt  => sub { $x =~ /(\d)$/     },
  mod  => sub { $x % 10           },
  chop => sub { local $x; chop $x },
});

[download]

        - tye (but my friends call me "Tye")

In reply to (tye)Re: japhy regex analysis: case study (RE question...) by tye
in thread RE question...yup, another one ;) by snafu

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