Just out of interest, I thought I'd compare the gain from using constructs such as these compared with $`, $& and $'. I am no benchmark expert, but basically I added this, inside gmax's timethese() loop:
'vanilla' => sub {
if ($text =~ /gotcha/) {
$pre = $`;
$post = $';
$match = $&;
}
},
}
and this inside the if ($text =~ /gotcha/) {} loop:
print "vanilla\n";
print "prematch :", $`, "\n";
print "match :", $&, "\n";
print "postmatch :", $', "\n";
The (slightly edited) output, for 500,000 iterations, is clear:
Benchmark: timing 500000 iterations of substr, unpack, vanilla...
substr: 5 wallclock secs <snip> @ 106678.05/s
unpack: 8 wallclock secs <snip> @ 54241.70/s
vanilla: 0 wallclock secs <snip> @ 492125.98/s
So, on this basis alone, the much decried $`, $& and $' appear to be respectively almost five and nine times faster than the proposed 'substr' and 'unpack' subs.
Of course, the real problem, in a programme of any length, does not lie here, but in the fact that "Any occurrence (of $`, $& and $') in your program causes all matches to save the searched string for possible future reference". So I tried modifying each timethese() loop by adding a fairly large number of subsequent match attempts:
my ($i, $j, $k);
for ('a' .. 'z', 'A' .. 'Z') {
$i++ if $text2 =~ /$_/;
}
$j++ if $text3 =~ /quux/;
$k++ if $text4 =~ /H/;
(having, of course, approproately defined strings $text2, $text3 and $text4) and I was surprised to see the following output:
substr: 25 wallclock secs ...
unpack: 26 wallclock secs ...
vanilla: 26 wallclock secs ...
(Full code here:)
use strict;
use warnings;
use Benchmark qw(timethese);
open OUT, '>', 'temp.txt' or die "can't open $!";
select OUT;
my $repeat = 10;
my $text = ('abc' x $repeat) . 'gotcha' . ('xyz' x $repeat);
my $text2 = 'Fork over rice before serving';
my $text3 = 'foo bar baz quux';
my $text4 = 'Just Another Perl hacker';
my ($pre,$match,$post);
print "OS: $^O - Perl: $]\n";
timethese( 100000, {
'unpack' => sub {
if ($text =~ /gotcha/) {
$pre = prematch($text);
$post = postmatch($text);
$match = match($text);
}
my ($i, $j, $k);
for ('a' .. 'z', 'A' .. 'Z') {
$i++ if $text2 =~ /$_/;
}
$j++ if $text3 =~ /quux/;
$k++ if $text4 =~ /H/;
},
'substr' => sub {
if ($text =~ /gotcha/) {
$pre = substr_prematch($text);
$post = substr_postmatch($text);
$match = substr_match($text);
}
my ($i, $j, $k);
for ('a' .. 'z', 'A' .. 'Z') {
$i++ if $text2 =~ /$_/;
}
$j++ if $text3 =~ /quux/;
$k++ if $text4 =~ /H/;
},
'vanilla' => sub {
if ($text =~ /gotcha/) {
$pre = $`;
$post = $';
$match = $&;
}
my ($i, $j, $k);
for ('a' .. 'z', 'A' .. 'Z') {
$i++ if $text2 =~ /$_/;
}
$j++ if $text3 =~ /quux/;
$k++ if $text4 =~ /H/;
},
}
);
if ($text =~ /gotcha/) {
print "unpack\n";
print "prematch :", prematch($text), "\n";
print "match :", match($text), "\n";
print "postmatch :", postmatch($text), "\n";
print "substring\n";
print "prematch :", substr_prematch($text), "\n";
print "match :", substr_match($text), "\n";
print "postmatch :", substr_postmatch($text), "\n";
print "vanilla\n";
print "prematch :", $`, "\n";
print "match :", $&, "\n";
print "postmatch :", $', "\n";
}
sub prematch {
return unpack "a$-[0]", $_[0];
}
sub postmatch {
return unpack "x$+[0] a*", $_[0];
}
sub match {
my $len = $+[0] - $-[0];
unpack "x$-[0] a$len", $_[0];
}
sub substr_match {
substr( $_[0], $-[0], $+[0] - $-[0] )
}
sub substr_prematch {
substr( $_[0], 0, $-[0] )
}
sub substr_postmatch {
substr( $_[0], $+[0] )
}
Does this mean:
1 The much decried $`, $& and $' need to be rehabilitated?
2 There's a benchmarking problem?
3 I've missed something (Most likely reply :-)?
thanks
dave
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