Escape square brackets when assigning a class name to an element

Does anyone know how to escape the square bracket character when setting a class name with jQuery?

Try the following:

$('#txtFirstname').addClass('test[someval]')

then

$('#txtFirstname').attr('class')

you'll see the class is there.

Now try

$('#txtFirstname').hasClass('test[someval]')

FAIL

The only reason I can think of is the square brackets.

And I need these for my jQuery validation you see.