in reply to Re^2: Performance optimization question
in thread Performance optimization question

Are you sure the regex engine isn't "cheating" by caching something? 
#!/usr/bin/perl
use strict;
use warnings;
use Time::HiRes qw(time);

my $num = 3_000_000;
my $str = 'x'x$num . 'reg exp' . 'x'x$num; 

my $start = time;

my $res = $str =~ m/reg exp/;

print time - $start, $/;

$start = time;

my $idx = index $str, 'reg exp';

print time - $start, $/;
__END__
0.0114099979400635
0.011760950088501

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Admittedly, index is still a bit slower, but the difference isn't that huge.

BTW on my machine (with perl 5.8.8) the difference isn't there at all: 

         Rate REGEX INDEX
REGEX 87061/s    --   -2%
INDEX 88494/s    2%    --

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The results only differ slightly for 5.10.0. Which perl did you use?

I thought that index and regexes use the same algorithm, but the regex goes through the pain of compiling the regex first

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Re^4: Performance optimization question
by BrowserUk (Patriarch) on Apr 03, 2008 at 08:09 UTC
    Which perl did you use?
    c:\test>perl -v

This is perl, v5.8.6 built for MSWin32-x86-multi-thread
(with 3 registered patches, see perl -V for more detail)

Copyright 1987-2004, Larry Wall

Binary build 811 provided by ActiveState Corp. http://www.ActiveState.
+com
ActiveState is a division of Sophos.
Built Dec 13 2004 09:52:01

    [download]

    Things do seem to have swung the other way again with 5.10: 

    for( 30, 300, 3000, 30000 ) { 
    $s = 'x'x$_ . 'reg exp' . 'x'x$_; 
    cmpthese -1, { 
        REGEX => q[ $x = $s =~ /reg exp/;], 
        INDEX => q[ $x = index $s, 'reg exp';] 
    } 
};;
           Rate REGEX INDEX
REGEX 2526899/s    --  -27%
INDEX 3444838/s   36%    --

           Rate REGEX INDEX
REGEX 1083665/s    --   -9%
INDEX 1193837/s   10%    --

          Rate REGEX INDEX
REGEX 159444/s    --   -3%
INDEX 163587/s    3%    --

         Rate REGEX INDEX
REGEX 14881/s    --   -3%
INDEX 15292/s    3%    --

for( 30, 300, 3000, 30000 ) { 
    $s = 'x'x$_ . 'reg exp' . 'x'x$_; 
    study $s; 
    cmpthese -1, { 
        REGEX => q[ $x = $s =~ /reg exp/;], 
        INDEX => q[ $x = index $s, 'reg exp';] 
    }
};;
           Rate REGEX INDEX
REGEX 2103160/s    --  -41%
INDEX 3552278/s   69%    --

           Rate REGEX INDEX
REGEX  806412/s    --  -32%
INDEX 1187789/s   47%    --

          Rate REGEX INDEX
REGEX 110166/s    --  -33%
INDEX 163886/s   49%    --

         Rate REGEX INDEX
REGEX 11587/s    --  -24%
INDEX 15292/s   32%    --

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    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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