HTMLElement extension to add, remove, toggle, detect Classes.

DOMClass.js

HTMLElement = typeof(HTMLElement) != 'undefiend' ? HTMLElement : Element;

HTMLElement.prototype.addClass = function(string) {
  if (!(string instanceof Array)) {
    string = string.split(' ');
  }
  for(var i = 0, len = string.length; i < len; ++i) {
    if (string[i] && !new RegExp('(\\s+|^)' + string[i] + '(\\s+|$)').test(this.className)) {
      this.className = this.className.trim() + ' ' + string[i];
    }
  }
}
 
HTMLElement.prototype.removeClass = function(string) {
  if (!(string instanceof Array)) {
    string = string.split(' ');
  }
  for(var i = 0, len = string.length; i < len; ++i) {
    this.className = this.className.replace(new RegExp('(\\s+|^)' + string[i] + '(\\s+|$)'), ' ').trim();
  }
}

HTMLElement.prototype.toggleClass = function(string) {
  if (string) {
    if (new RegExp('(\\s+|^)' + string + '(\\s+|$)').test(this.className)) {
      this.className = this.className.replace(new RegExp('(\\s+|^)' + string + '(\\s+|$)'), ' ').trim();
    } else {
      this.className = this.className.trim() + ' ' + string;
    }
  }
}

HTMLElement.prototype.hasClass = function(string) {
  return string && new RegExp('(\\s+|^)' + string + '(\\s+|$)').test(this.className);
}

I can confirm that the RegEx for removeClass works. Only minor thing is that it doesn't remove all instances of a class name, if the class name should appear in the class list several times (it might happen if a function simply adds the classname without testing if its already there). Simply adding a "g" flag to the RegEx is not enough to deal with that.
There exists an alternative RegEx, which does not have that minor problem, and which is just as small:

// @author Bjørn Rosell (derived from Peter Boughton) 
function removeClass​(el, className​) {
   el​.className=el​.className​.replace​(new RegExp​("​(\\s|^​)"+className+"​(?!\\S​)","g"​),""​)​.trim​(​)
}

BTW:
I have done an extensive test of candidates for removing a class. You can find it here