[Manichean]

Huh. You sure? I just tested the expression in Python and it matches the test case you specified in your original post. Did you have a look at the source Calibre sees while applying the regex (use the magic wand symbol)?

Edit to add: Given your test case, I assumed you wanted to write a regex that catches a number followed by a dot, followed again by two groups of strings (including spaces) separated by one or more whitespaces. That's exactly what the regex I wrote matches.