So I'm working on a Perl version of grep for FB2 (XML based) ebooks, and just like grep it has a command line option that turns on case insensitive matching:

$re = $opts{i} ? qr/$pattern/i : qr/$pattern/;

Everything seems to be in place and working fine except for one major setback: matching a chunk of multibyte text against a pattern compiled with qr//i turns out to be 5-15 times slower than doing the same thing with m//i and just plain qr//, whereas one would expect the two approaches to be equally fast.

[ hmm, I'm a bit worried about the integrity of the UTF-8 (cyrillic) test string in this ]

#!/usr/bin/perl

use strict;
use utf8;
use Benchmark ':all';

# the pattern doesn't even need to contain anything fancy
# for the problem to manifest itself
my $pattern = 'dumbest pattern ever';

# it's all about whether or not the /i flag is embedded into the regex
my $re   = qr/$pattern/;
my $re_i = qr/$pattern/i;

# qr//i causes a noticeable slowdown even when dealing with 7-bit (US-
+ASCII)
# strings, but this being multibyte seems to make things _a lot_ worse
my $str = 'очень длинная строка ' x 10;


my $count = 300_000;

cmpthese($count, {
    'qr//i+m//' => sub { $str =~ /$re_i/ },
    'qr//+m//i' => sub { $str =~ /$re/i },
    'qr//+m//'  => sub { $str =~ /$re/ }
});

[download]
$ ./qr-utf8.pl
              Rate qr//i+m// qr//+m//i  qr//+m//
qr//i+m//  10881/s        --      -98%      -98%
qr//+m//i 505263/s     4543%        --       -7%
qr//+m//  540845/s     4870%        7%        --

[download]

One possible way around this would be to altogether abandon qr//i and instead eval() my matching subroutine with all the necessary /i flags textually inlined (there are two m//'s and one s///), but that's still quite ugly. Any suggestions?

In reply to qr//i versus m//i by hazylife

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