in reply to Seeking Audience with the Perl Regex Droids...

Firstly, you omitted the other contender: 

...
sub lookbehind {
    my $x = shift;
    split '(?:(?<=[^#]####)|(?<=[^#]##)|(?<!#))[@]', $x;
}

C:\test>junk8
Subroutine Benchmark::mytime redefined at c:/Perl/lib/Benchmark.pm lin
+e 459.
Benchmark:
timing 10000 iterations of
 lookbehind, rollahead, unroll
...
lookbehind: 2.34183 wallclock secs ( 2.19 usr +  0.00 sys =  2.19 CPU)
+ @ 4570.38/s (n=10000)
 rollahead: 3.46343 wallclock secs ( 3.23 usr +  0.00 sys =  3.23 CPU)
+ @ 3092.15/s (n=10000)
    unroll: 3.51775 wallclock secs ( 3.33 usr +  0.02 sys =  3.34 CPU)
+ @ 2990.43/s (n=10000)

             Rate     unroll  rollahead lookbehind
unroll     2990/s         --        -3%       -35%
rollahead  3092/s         3%         --       -32%
lookbehind 4570/s        53%        48%         --

[download]

Secondly, you didn't check your results. unroll and rollahead produce different results: 

          rollahead => @# #@## ##@  ##@## #@# # @# #@## ##@@# #@## ##@
+@# #@ ...
             unroll => @# #@## ##@  ##@## #@#  @# #@## ##@@# #@## ##@@
+# #@# ...

[download]

And both sets are wrong! Any output that has an even number of #s preceding a @ (as with the 3rd, 4th etc. outputs above) is wrong. If you look back at the op, you'll see that any @s that persist into the results set are preceded by an odd number of ##s.

The results set from the fastest algorithm: 

          lookbehind => #@## ###@#### #####@  #####@#### ###@## # #@##
+ ###@#### ...

[download]
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

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Re^2: Seeking Audience with the Perl Regex Droids...
by mobiusinversion (Beadle) on Mar 29, 2008 at 05:13 UTC
    Your solution is actually incorrect on parsing any string which has a consecutive stretch of pound signs (ie '#') that is a multiple of 4 and greater than or equal to 8.

    Try this: 
    my $x = '@########@';

    [download]
    it should parse as: 
        '',
    '####',
    '',

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    Your solution (as you've hard coded it for one example, and admittedly so in your post), returns: 
       '',
   '###@'

    [download]
    which would have been delimited as: 
    '@#######@'

    [download]
    That is, seven pound signs...

    I ask you to please consider the original post as I dont think you are making your assertions with an understanding of what was asked. For example, the escape character itself must be escaped when created a delimited line. Hence the list: 
    my @x = qw(## @ #);

    [download]
    would be delimited as: 
    '####@#@@##'

    [download]
    Finally, I would be glad to verify your second claim if you can provide the input string upon which rollahead and unroll return different results. I believe such a string does not exist.
[reply]
[d/l]
[select]
      Finally, I would be glad to verify your second claim if you can provide the input string upon which rollahead and unroll return different results. I believe such a string does not exist.

      Actually, it was your own benchmark: 

      #! perl -slw
use strict;
use Benchmark ':all', ':hireswallclock';

my $x = "#@##@###@####@#####@";
my $y = reverse $x;
my $z = "$x$x$x$y$y$x$x$y$y$y$y$y$x$x$x$x";

our %results;
cmpthese -3, {
    unroll    => sub {
       my @x = ([unroll($x)],[unroll($y)],[unroll($z)]);
       $results{ unroll } = \@x;
    },
    rollahead => sub {
       my @x = ([rollahead($x)],[rollahead($y)],[rollahead($z)]);
       $results{ rollahead } = \@x;
    },
    lookbehind => sub {
       my @x = ([lookbehind($x)],[lookbehind($y)],[lookbehind($z)]);
       $results{ lookbehind } = \@x;
    },
};

printf "%20s => %s\n", $_,  "@{[ map{ qq[@$_] } @{ $results{ $_ } } ]}
+" for keys %results;

sub unroll {
    my @x = $_[0] =~ /(?:^|@)((?:##|#@|[^#@])*)/g;
    for(@x){
        $_ =~ s/##/#/g;
        $_ =~ s/#@/@/g;
    } @x
}

sub rollahead {
    my $x = shift;
    $x = reverse $x;
    my @x = reverse(split/\@(?=(?:##)*(?!#))/,$x,-1);
    for(@x){
        $_ = reverse;
        $_ =~ s/##/#/g;
        $_ =~ s/#@/@/g;
    } @x
}

sub lookbehind {
    my $x = shift;
    split m[ (?: (?<=[^#]\#\#\#\#) | (?<=[^#]\#\#) | (?<!\#) ) [@] ]x,
+ $x;
}

      [download]

      Output (read'em and weep:):

      C:\test>junk8
             Rate     unroll  rollahead lookbehind
unroll     3096/s         --        -3%       -33%
rollahead  3175/s         3%         --       -32%
lookbehind 4639/s        50%        46%         --
           rollahead => @# #@## ##@  ##@## #@# # @# #@## ##@@# #@## ##
+@@# #@## ##@ ##@## #@# @##@## #@# # # #@## ##@@# #@## ##@ ##@## #@# @
+##@## #@# @##@## #@# @##@## #@# @##@## #@# # # #@## ##@@# #@## ##@@# 
+#@## ##@@# #@## ##@

              unroll => @# #@## ##@  ##@## #@#  @# #@## ##@@# #@## ##@
+@# #@## ##@ ##@## #@# @##@## #@# # # #@## ##@@# #@## ##@ ##@## #@# @#
+#@## #@# @##@## #@# @##@## #@# @##@## #@# # # #@## ##@@# #@## ##@@# #
+@## ##@@# #@## ##@

          lookbehind => #@## ###@#### #####@  #####@#### ###@## # #@##
+ ###@#### #####@#@## ###@#### #####@#@## ###@#### #####@ #####@#### #
+##@## #@#####@#### ###@## ## ## ###@#### #####@#@## ###@#### #####@ #
+####@#### ###@## #@#####@#### ###@## #@#####@#### ###@## #@#####@####
+ ###@## #@#####@#### ###@## ## ## ###@#### #####@#@## ###@#### #####@
+#@## ###@#### #####@#@## ###@#### #####@

      [download]
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
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In Section Seekers of Perl Wisdom