And to iterate over text, providing previous, next, and current values:

def curprevnext_iters (chars):
    """Returns an iterator which provides a
    (char, previous character, next character) tuple."""
    cur, prev, next = itertools.tee (chars, 3)
    prev = itertools.chain (iter (unichr (0)), prev)
    next = itertools.chain (itertools.islice (next, 1, None),
                                iter (unichr (0)))
    return itertools.izip (cur, prev, next)