Word Extraction

I want a dictionary of the words my students actually use. Text from their assignments, forum posts, and emails goes into a program that pulls the words out.

To make it easy to tell whether a word has been seen before, the text is cleaned up first. Each line of text is processed in this order.

1. Every upper case letter becomes lower case.
2. A letter is one of a to z and a digit is one of 0 to 9. Every other character except the space is punctuation.
3. If the character immediately before a punctuation mark and the character immediately after it are both letters, that mark is deleted and the two sides are joined. Otherwise the mark becomes a single space. The two neighbours are always read from the line before any processing, so a run of punctuation marks does not affect itself. A mark at the very start or the very end of a line has no character on one side, so it becomes a space.
4. The resulting string is split on spaces to give the words.
5. A word made only of digits is thrown away.
6. The remaining words are sorted into alphabetical order and duplicates are removed. Comparison uses ASCII code values, so digits come before letters.

For instance haven't has a letter on both sides of the apostrophe and becomes havent, and Hartley-Jones. becomes hartleyjones. In top-10 the character after the hyphen is a digit, so it splits into top and 10, and 10 is thrown away for being all digits.

Input consists of a number of lines, each line holding one piece of text to analyse. The input ends with a line containing the single character #, which is not text.

There is at least one line of text, and no line contains more than 250 characters. Every line is made of printable ASCII characters, that is codes 32 to 126.

Print the qualifying words of each piece of text, one per line, in alphabetical order. One line of text gives one set of words, and two neighbouring sets are separated by a single blank line.

A line that yields no words still counts as a set. That set prints nothing while the blank lines separating it stay, so blank lines can appear one after another.