Here's a simple progression you might follow to develop a solution for sentence_re:

1. Match sentences of the form "a.", "aaaa.", "aa." -- just one word that's one more or "a"s followed by a period.

2. Match sentences of the form "a.", "a a.", "a a a a a." -- one or more words separated by one space but all words are "a".

3. Match "a.", "aaaa a aaa.", "a a a aaaaaaaaa."

4. Like previous but first letter must be capitalized, like "A.", "Aa a aaaa.", etc.

5. Like previous but also handle the endings: "A a!", "Aaaa a aa?!", etc.

6. Finally, allow words to composed of letters other than just 'a's.