Very nice comparison. You might as well have a swing at index, which is probably fast. 
sub s_index{
 my $pos = 0;
 my $cnt = 0;
 while (index($str, $sep, $pos)){
  $pos++;  
  $cnt++;
 }
 $cnt{s_index}=$cnt;
}

[download]

Jeroen
"We are not alone"(FZ)

Update: Thanks to danger for pointing my typos out. I took his code, and benchmarked it: 

my $_s_index = sub {
  my $pos = 0;
  my $cnt = 0;
  local $[ = 1;  # ook!
  my $pos = -1;
  ++$cnt while $pos = index($dta,"\n\n",$pos+2);
  $cnt{_s_index}=$cnt;
};
 
my $_while_re = sub{
  my $cnt = 0;
  my $recsep = "\n\n";
  $cnt++ while $dta =~ /$recsep/g;
  $cnt{_while_re} = $cnt;
};
=>resulted in:
Benchmark: timing 300 iterations of m_array, m_array_M, m_for, m_for_M
+, m_while, m_while_M, s_index, tr, while_re...
   m_array: 11 wallclock secs ( 5.13 usr +  0.04 sys =  5.17 CPU) @ 58
+.03/s (n=300)
 m_array_M:  4 wallclock secs ( 1.57 usr +  0.02 sys =  1.59 CPU) @ 18
+8.68/s (n=300)
     m_for:  9 wallclock secs ( 4.19 usr +  0.00 sys =  4.19 CPU) @ 71
+.60/s (n=300)
   m_for_M:  3 wallclock secs ( 1.41 usr +  0.01 sys =  1.42 CPU) @ 21
+1.27/s (n=300)
   m_while:  5 wallclock secs ( 2.46 usr +  0.00 sys =  2.46 CPU) @ 12
+1.95/s (n=300)
 m_while_M:  2 wallclock secs ( 0.94 usr +  0.00 sys =  0.94 CPU) @ 31
+9.15/s (n=300)
   s_index:  2 wallclock secs ( 0.88 usr +  0.00 sys =  0.88 CPU) @ 34
+0.91/s (n=300)
        tr:  1 wallclock secs ( 0.36 usr +  0.00 sys =  0.36 CPU) @ 83
+3.33/s (n=300)
            (warning: too few iterations for a reliable count)
  while_re:  2 wallclock secs ( 0.90 usr +  0.00 sys =  0.90 CPU) @ 33
+3.33/s (n=300)
 
=== check counts ===
         _while_re  2000
          _m_for_M  2000
        _m_array_M  2000
               _tr  6000
        _m_while_M  2000
          _m_while  6000
          _s_index  2000
            _m_for  6000
          _m_array  6000

[download]
With tr still being the winner, and closely tied 2nd and 3rd place for index and danger's while, respectively.

Than the light showed me that the separators should be all the same. That gave some shuffle: 

Benchmark: timing 300 iterations of m_array, m_array_M, m_for, m_for_M
+, m_while, m_while_M, s_index, tr, while_re...
   m_array: 12 wallclock secs ( 5.04 usr +  0.04 sys =  5.08 CPU) @ 59
+.06/s (n=300)
 m_array_M: 11 wallclock secs ( 5.08 usr +  0.03 sys =  5.11 CPU) @ 58
+.71/s (n=300)
     m_for:  9 wallclock secs ( 4.19 usr +  0.00 sys =  4.19 CPU) @ 71
+.60/s (n=300)
   m_for_M:  8 wallclock secs ( 4.10 usr +  0.00 sys =  4.10 CPU) @ 73
+.17/s (n=300)
   m_while:  5 wallclock secs ( 2.26 usr +  0.02 sys =  2.28 CPU) @ 13
+1.58/s (n=300)
 m_while_M:  5 wallclock secs ( 2.28 usr +  0.02 sys =  2.30 CPU) @ 13
+0.43/s (n=300)
   s_index:  5 wallclock secs ( 2.13 usr +  0.00 sys =  2.13 CPU) @ 14
+0.85/s (n=300)
        tr:  0 wallclock secs ( 0.27 usr +  0.00 sys =  0.27 CPU) @ 11
+11.11/s (n=300)
            (warning: too few iterations for a reliable count)
  while_re:  5 wallclock secs ( 2.11 usr +  0.00 sys =  2.11 CPU) @ 14
+2.18/s (n=300)
 
=== check counts ===
         _while_re  6000
          _m_for_M  6000
        _m_array_M  6000
               _tr  6000
        _m_while_M  6000
          _m_while  6000
          _s_index  6000
            _m_for  6000
          _m_array  6000

[download]
Sadly enough the C-code doesn't compile on my machine. I don't have the time to get it working right now.

In reply to Re: quickly counting substrings by jeroenes
in thread quickly counting substrings by Crayman

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