Just a note since the other guys have answered your Q.

$string =~ s/[^<B>]($swapString)[^<\/B>]/<B>$1<\/B>/gi;
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Square brackets are for defining a list of characters to match any one of (or not with a ^) in a single character position. The [^<B>] and [^<\/B>] will match any character which is not inside of the square brackets. Not the string <B> or </B> as I believe you were trying to not match.

I expect I could explain this more clearly if it wasn't first thing in the morning! Basically with your $string the regex above would be as well written...

$string =~ s/[^>]($swapString)[^<]/<B>$1<\/B>/gi;
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and if you want to not match something that's more than one character then try...

#!/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;
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larryk