One of the main benefits of alternation is that you can compile the RE. Essentially an alternation RE is very similar to the loop, but the loop code has been optimised to the task and is in C with alternation, but generalised and less efficient if you do it in perl. The difference is significant, at least that is what this Benchmark shows.....
use Benchmark 'cmpthese';
$iterations = 1000000;
%re = (
foo => 'foo1',
bar => 'bar1',
);
$re = join '|', keys %re;
$re = qr/($re)/;
$name1 = "RE";
$code1 = << 'END_CODE1';
$_ = 'foo bar';
s/$re/$re{$1}/g;
END_CODE1
$name2 = "Loop";
$code2 = << 'END_CODE2';
$_ = 'foo bar';
for my $sub( keys %re ) {
s/$sub/$re{$sub}/g;
}
END_CODE2
cmpthese( $iterations, {$name1 => $code1, $name2 => $code2} );
__END__
Benchmark: timing 1000000 iterations of Loop, RE...
Loop: 24 wallclock secs (24.77 usr + 0.00 sys = 24.77 CPU) @ 40
+377.94/s (n=1000000)
RE: 8 wallclock secs ( 7.27 usr + 0.00 sys = 7.27 CPU) @ 13
+7551.58/s (n=1000000)
Rate Loop RE
Loop 40378/s -- -71%
RE 137552/s 241% --
But if I change that to a more real world situation by making the string a 14Kb one (approximately a web page size)
$_ = 'foo bar' x 2000;
Benchmark: timing 10000 iterations of Loop, RE...
Loop: 20 wallclock secs (19.90 usr + 0.00 sys = 19.90 CPU) @ 50
+2.56/s (n=10000)
RE: 30 wallclock secs (29.44 usr + 0.00 sys = 29.44 CPU) @ 33
+9.64/s (n=10000)
Rate RE Loop
RE 340/s -- -32%
Loop 503/s 48% -
And now the loop is faster. In fact try this case:
$_ = 'fo ba' x 1000 . 'foo bar';
Benchmark: timing 10000 iterations of Loop, RE...
Loop: 1 wallclock secs ( 0.71 usr + 0.00 sys = 0.71 CPU) @ 14
+064.70/s (n=10000)
RE: 12 wallclock secs (11.10 usr + 0.00 sys = 11.10 CPU) @ 90
+1.23/s (n=10000)
Rate RE Loop
RE 901/s -- -94%
Loop 14065/s 1461% --
~This is a purpose designed worst case for alternation as it requires continouous back tracking. So I have shattered my own delusions! Perl loops are faster than RE alternation.
