Basic usage of Nick Gammon's Regexp library
You haven't defined any capture groups in your regexp.

The regexp [aeiou] will match any of those characters, but it won't capture them.  To capture you need to define a capture group which is a sequence of tokens surrounded by parenthesis.  For instance:

([aeiou])


In your test phrase "The quick brown fox jumps over the lazy wolf" that should match 11 times and capture each match, resulting in captures of e u i o o u o e e a o.

The reason why matching and capturing are two completely different things is so you can do things like [aeiou]([aeiou]) which would capture the second vowel in a two vowel sequence. In your example it would match the ui of quick and capture just the i.