Based on the documentation for Raku's lookaround assertions, I read the regex / <?[abc]> <alpha> / as saying "starting from the left, match but do not not consume one character that is a, b, or c and, once you have found a match, match and consume one alphabetic character."
Thus, this output makes sense:
'abc' ~~ / <?[abc]> <alpha> / # OUTPUT: «｢a｣ alpha => ｢a｣»
Even though that regex has two one-character terms, one of them does not capture so our total capture is only one character long.
But next expression confuses me:
'abc' ~~ / <?[abc\s]> <alpha> / # OUTPUT: «｢ab｣ alpha => ｢b｣»
Now, our total capture is two characters long, and one of those isn't captured by <alpha>. So is the lookaround capturing something after all? Or am I misunderstanding something else about how the lookaround works?
<?[ ]> and <![ ]> does not seem to support some backslashed character classes. \n, \s, \d and \w show similar results.
<?[abc\s]> behaves the same as <[abc\s]> when \n, \s, \d or \w is added.
\t, \h, \v, \c[NAME] and \x61 seem to work as normal.