my %trans = (zero  => 0,
             one   => 1,
             ones  => 11,
             two   => 2,
             three => 3);

my $words_re = join "|", map quotemeta, sort { length($b) <=> length($a) } keys %trans;
$string =~ s/\b($words_re)\b/$trans{lc $1}/ig;