Exercise 3-3: Expand shorthand like a-z

Write a function expand(s1,s2) that expands shorthand notations like a-z in the string s1 into the equivalent complete list abc...xyz in s2. Allow for letters of either case and digits, and be prepared to handle cases like a-b-c and a-z0-9 and -a-z. Arrange that a leading or trailing - is taken literally.

Expand x-y only when x and y are both lower case letters, both upper case letters or both digits, and x comes no later than y; copy anything else unchanged. Each input line, at most 1000 characters, holds s1 without its newline. Print s2 on a line of its own.