@array = ( "foo bar", "bar foo" );     
@array2 = ( "foo", "bar" );     
$string = "there is a foo bar and a bar foo and also foo and bar.";

print "string before - $string\n";

$swapString = join ("|", @array);
print "swapString 1 - $swapString\n";
$string =~ s/($swapString)/<B>$1<\/B>/gi;

print "string after first - $string\n";

##</code><code>##

$swapString = join ("|", @array2);
print "swapString 2 - [$swapString]\n";
$string =~ s/[^<B>]($swapString)[^<\/B>]/<B>$1<\/B>/gi; 

print "string after first - $string\n";

##</code><code>##

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