I'm really surprised that \d under the Unicode semantics is just [0-9]. I would expect that if \w is \p{IsWord}, and \s is \p{IsSpace}, that then \d is \p{IsDigit}.

And I don't get /b at all. I would expect that if I use /b because I have a regexp that I want to behave exactly as in 5.8, I really want that. But /b isn't giving me that, as \d matches a lot more than [0-9] in 5.8. I appreciate changing the \d to [0-9], that gets my vote, but I don't get a broken "broken/legacy" switch.

Third, what's the deal with /a and \s? Currently, \s matches 5 ASCII code points, but under /a it's going to match four?