use HTML::Parser;

my $keyword = quotemeta 'match';

sub highlight {
    my $text = shift;
    $text =~ s|\b($keyword)\b|<span style="background: #FFFFFF">$1</sp
+an>|g;
    return $text;
}

my $html = q{
    <p id="match"><b>this is html match this</b><u value="don't match 
+this">text html blah</u></p>
};

my $p = new HTML::Parser(api_version => 3, 
    handlers => {
        text=> [ sub { print highlight(shift) }, 'text'],
    default => [sub { print shift }, 'text'],
  });

$p->parse($html);
[download]

gav^

Completely un-optimal, completely un-tested.. but I think I can get away with something like that from your code?

$newline =~ s|(?<!\<)$keyword(?!\>)|<font style="BACKGROUND: #ffffff">
+$keyword</font>|img;
[download]

The (?<!\<) implies that this shouldn't match a < before the $keyword, and the (?!\>) shouldn't match a > after it... so it's not in a tag.(I guess)

There is nothing wrong with having a < before or a > after it.. so long as the < has a > after it before and vice versa for after.... so I don't think yours will quite work...
I came up with the following... which seems to work at least somewhat...

s|(\Q$text\E)(?![^<]*>)|<I>$1</I>|gi;
[download]

which matches the text so long as it is not followed by a > before it sees a <

Update \me laughs as joealba and I both post the code I gave him at the same time...

                - Ant
                - Some of my best work - (1 2 3)

Thanks, Chady!

suaveant gave me this little hunk of code that seems to be working great:

$text =~ s|($keyword)(?![^<]*[>\Z])|<B>$1</B>|img;
[download]

I've never used that explicit lookahead before. I love/hate it when I'm shown something so useful (when I should have known it before). I KNOW I read pp. 228-230 of Mastering Regular Expressions one day last year... :)