How to Toggle / remove classes in wysihtml5

问题:

I need to be able remove the previous class from a span when adding a new one.

Below is the custom function which adds a span and a class that gets passed through the button.

wysihtml5.commands.custom_underline = {
  exec: function(composer, command, className) {
    return wysihtml5.commands.formatInline.exec(composer, command, "span", className, new RegExp(className, "g"));
  },
  state: function(composer, command, className) {
    return wysihtml5.commands.formatInline.state(composer, command, "span", className, new RegExp(className, "g"));
  }
};

Here are the buttons:

<a data-wysihtml5-command="custom_underline" data-wysihtml5-command-value="u grey">Grey</a>
<a data-wysihtml5-command="custom_underline" data-wysihtml5-command-value="u dark_grey">Dark Grey</a>
<a data-wysihtml5-command="custom_underline" data-wysihtml5-command-value="u black">Black</a>

If I click on both the grey and dark grey buttons it produces the following code:

<span class="u grey u dark_grey">Some text</span>

I need to find some way to remove the previous class before adding the new one.