The short reason it doesn't accept a complex number is that I haven't decided what forms of complex number to accept. My first impression is that the format should be something like this regular expression:
(a)? [+-] ((b)*?i)?
where a and b are numbers in whatever notation style is acceptable.
It gets a bit tricky when you think about scientific notation for numbers, which introduces another multiplication, so the i could be on the end: 5*10^3*i, or in the middle: 5i*10^3. I suppose you could insist that it's on the end.
The other option is to treat it as a mathematical expression, and user pattern-matching to enforce it's of the right form, but allow for terms to be rearranged.