s,(?<!\w)($worda)(\W+)($wordb)(?!\w),<B>$1</B>$2<B>$3</B>,i


##</code><code>##

my $material = join "\0", @material;

foreach $phrase (@key_phrases) {
    my($worda, $wordb) = split / /, $phrase;

    $material =~ s{(?<!\w)($worda)([^\w\0]+)($wordb)(?!\w)}
                  {<B>$1</B>$2<B>$3</B>}i;
}

@material = split /\0/, $material;