The strange expression does seem wrong to me: the loop should terminate on end-of-line or ":", so continue while not end-of-line and not ":".

I think you're right that shifting negative integers is fine on most any platform you're likely to encounter, but until recently one could also assume that adding two signed integers would wrap in practice, despite being undefined in theory; due to gcc optimizing ever more aggressively, code assuming that signed integers wrap is now broken (see e.g. http://thiemonagel.de/2010/01/signed-integer-overflow/). So I'd be careful with shifts of negative integers in my own code.