$string =~ s/[^<B>]($swapString)[^<\/B>]/<B>$1<\/B>/gi;

##</code><code>##

$string =~ s/[^>]($swapString)[^<]/<B>$1<\/B>/gi;

##</code><code>##

#!/usr/bin/perl -w
use strict;

$_ = "<b>foo</b> foo <b>bar</b> bar <b>foo</b> foo <b>bar</b> bar\n";
print;

s/(?!<b>)(foo|bar)(?!<\/b>)/XXX/gi;
print;