In the second edition of his book, Friedl drily announces that readers of the first edition need not worry any longer about the /i modifier, since the issue has been already fixed.

Here is a test, showing that the difference, if any, is rather small. Getting less than 0.20 sec difference with ten thousand iterations on a one-million-char string, I would choose the /i modifier any time.

It all depends on your version of Perl and your machine speed, but if you have a recent release of both, you can safely use the /i modifier without losing much sleep.

#!/usr/bin/perl -w
use strict;
use Benchmark qw(timethese);

for my $size (10_000, 100_000, 1_000_000) {
    my $string = 'a' x $size  . ' While ';
    print "string size $size\n", ;

    my $x;
    timethese(10_000, {
        i_modifier => sub {
            $x = 1 if $string =~  m/\bwhile\b/i;
        },
        char_class => sub {
            $x = 1 if $string =~ m/\b[Ww][Hh][Ii][Ll][Ee]\b/;
        }
    });
}

__END__
Perl 5.6.1
==========
string size 10000
Benchmark: timing 10000 iterations of char_class, i_modifier...
char_class:  1 wallclock secs ( 0.62 usr +  0.00 sys =  0.62 CPU)
i_modifier:  1 wallclock secs ( 0.62 usr +  0.00 sys =  0.62 CPU)

string size 100000
Benchmark: timing 10000 iterations of char_class, i_modifier...
char_class:  6 wallclock secs ( 6.05 usr +  0.01 sys =  6.06 CPU)
i_modifier:  6 wallclock secs ( 6.05 usr +  0.01 sys =  6.06 CPU)

string size 1000000
Benchmark: timing 10000 iterations of char_class, i_modifier...
char_class: 64 wallclock secs (62.17 usr +  0.31 sys = 62.48 CPU)
i_modifier: 63 wallclock secs (61.87 usr +  0.25 sys = 62.12 CPU)

ActiveState Perl 5.8.0 (optimized for Pentium architecture)
===========================================================
string size 10000
Benchmark: timing 10000 iterations of char_class, i_modifier...
char_class:  1 wallclock secs ( 0.51 usr +  0.00 sys =  0.51 CPU)
i_modifier:  1 wallclock secs ( 0.52 usr +  0.00 sys =  0.52 CPU)

string size 100000
Benchmark: timing 10000 iterations of char_class, i_modifier...
char_class:  5 wallclock secs ( 5.12 usr +  0.02 sys =  5.14 CPU)
i_modifier:  5 wallclock secs ( 5.23 usr +  0.02 sys =  5.25 CPU)

string size 1000000
Benchmark: timing 10000 iterations of char_class, i_modifier...
char_class: 51 wallclock secs (50.74 usr +  0.15 sys = 50.89 CPU)
i_modifier: 51 wallclock secs (50.91 usr +  0.15 sys = 51.06 CPU)
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