:`parse_string` for more information on parsing
        strings with embedded tabs.

        Example::

            source = "sldjf123lsdjjkf345sldkjf879lkjsfd987"
            print(source)
            for tokens, start, end in Word(alphas).scan_string(source):
                print(' '*start + '^'*(end-start))
                print(' '*start + tokens[0])

        prints::

            sldjf123lsdjjkf345sldkjf879lkjsfd987
            ^^^^^
            sldjf
                    ^^^^^^^
                    lsdjjkf
                              ^^^^^^
                              sldkjf
                                       ^^^^^^
                                       lkjsfd
        r