What do you make of this benchmark? :)

#!/usr/bin/perl

use Benchmark qw(cmpthese);

$substr = join('','a'..'j');
$str = $substr x 90 . 'hTmL'. $substr x 2000;
print "length of search string: ",length $str, "\n";
cmpthese(-3, {
    ' regex ' => sub {
        $str =~ /^\A.{0,996}?html/si
     or $str =~ /^\A.{0,996}?sgml/si;
     },
    ' substr' => sub {
        substr( $str, 0, 1000) =~ /html/i
     or substr( $str, 0, 1000) =~ /sgml/i;
     },
    'index'   => sub {
        1+index( lc substr( $str, 0, 1000 ), 'html' )
     or 1+index( lc substr( $str, 0, 1000 ), 'sgml' )
     } ,
});
print substr $str, 900, 10;;
__END__
C:\test>junk
length of search string: 20904
            Rate  regex   substr   index
 regex   93505/s      --    -54%    -65%
 substr 204367/s    119%      --    -23%
index   266809/s    185%     31%      --
hTmLabcdef
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