Comment by eadmund

> splitting a line into words is a whole project on its own

Is it[1]? My version below accumulates alphabetical characters until it encounters a non-alphabetical one, then increments the count for the accumulated word and resets the accumulator.

    (let (c accumulator (counts (make-hash-table :test #'equal)))
      (handler-case
          (loop
            (setq c (read-char))
            (if (find c "ABCDEFGHIJKLMNOPQRSTUVWXYZ" :test #'char-equal)
                (push (char-downcase c) accumulator)
                (when accumulator
                  (incf (gethash (coerce (reverse accumulator) 'string) counts 0))
                  (setq accumulator nil))))
        (end-of-file ()
          (when accumulator
            (incf (gethash (coerce (reverse accumulator) 'string) counts 0))
            (setq accumulator nil))
          (maphash #'(lambda (word count)
                       (push (list count word) accumulator))
                   counts)
          (format t "~{~&~{~a ~a~}~%~}" (reverse (last (sort accumulator #'< :key #'car)
                                                       (let ((n (second sb-ext:*posix-argv*)))
                                                         (if n (parse-integer n) 100))))))))

It’s not exactly pretty or idiomatic, but its 19 lines appear to get the job done.

1: Well, technically it is, because there is SPLIT-SEQUENCE: https://github.com/sharplispers/split-sequence