It turns out that qr// is actually the worst way to build regexes, but adding /o mostly fixes the problem. 
use Benchmark qw( cmpthese );
print $], "\n"; 
open my $W, '<', '/usr/share/dict/words' or die;
my @words = <$W>;
close $W;
my $s = '(?<![cC])[eE][iI]';
my $re = qr/(?<![cC])[eE][iI]/;
cmpthese(-5, {
   re  => sub { grep /(?<![cC])[eE][iI]/, @words },
   qr  => sub { grep /$re/, @words },
   qro => sub { grep /$re/o, @words },
   s   => sub { grep /$s/, @words },
   so  => sub { grep /$s/o, @words },
});
__END__
5.026001
      Rate   qr    s  qro   so   re
qr  7.14/s   -- -57% -67% -68% -69%
s   16.8/s 135%   -- -22% -26% -26%
qro 21.4/s 200%  28%   --  -5%  -6%
so  22.5/s 215%  34%   5%   --  -1%
re  22.7/s 218%  36%   6%   1%   --

[download]

In reply to Re^2: Parser Performance Question by Anonymous Monk
in thread Parser Performance Question by songmaster

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.