You're right it will only work for one class. This is actually a perfect example of jQuery smoothing out the rough edges for you. I think the split method would work (provided you remember to split with \s instead of just space). It actually took me a while to come up with a regex replace that won't break in some corner cases ( I think)

    document.getElementById('foo').className = document.getElementById('foo').className.replace(/(\s|^)foo(\s|$)/, ' ');

\b is out because it matches - and would kill stuff like foo-bar.