BaseForm function implementation

I tried to implement [BaseForm](http://reference.wolfram.com/mathematica/ref/BaseForm.html) function from Mathematica which I kind of missed but run into trouble with returning the correct output.

This is the expected behavior:

    >> BaseForm[33, 2] 
     = 10001_2 

and here is what I actually got

    >> BaseForm[33, 2] 
     = 10001_2[] 

Where do you think could be the problem?

If we figure this out do you think it would be worth to be able to input numbers in format like `base^^num` so for instance `16^^1e`?