If your end string is really something static like that, you may not want to use a regex at all. Here's a case insensitive replacement using rindex and substr: 

use Benchmark qw(cmpthese);


fbard@devo1:~/spot_camp_layout$ cat rindex 
use Benchmark qw(cmpthese);

my $x = ("x" x 100_000)."</body></html>"; 
my $lc_x = lc($x);
my $end_html = qr{(</body></html>)};

cmpthese(100_000,{
    rindex_substr => sub{
     my $str = $x;
     substr($str,rindex($lc_x,"</body>"),0,"yyyy"); 
    },
    regex => sub{
     my $str = $x;
     $str =~ s|$end_html$|yyyy$1|i;
    }
});

                 Rate         regex rindex_substr
regex         18416/s            --          -75%
rindex_substr 73529/s          299%            --

[download]

In reply to Re: Optimizing a regex that replaces near the end of a string by thunders
in thread Optimizing a regex that replaces near the end of a string by perl5ever

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.