Your regex was perfect. FWIW, I put it in my original prog (some bugs fixed in the prog), as the 2nd regex in the regex array. The reason I had them and the outputs in arrays was to compare several RE's. But I ended up with just the one as it passed the most cases. So lines for cases 3 and 4 (w/4+5 being the two that didn't pass in the regex I originally posted)

ResByLn:{ln=>3, wanted=>4, got=>[4, 4]},[" p  ", " p  "]
ResByLn:{ln=>4, wanted=>2, got=>[3, 2]},["FAIL:<4>", " p  "]
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The gots were count I got from the regex's, with your RE being in the 2nd position. The last brackets contained the p/f for each regex against that statement. So yours were 'p' straight down the 2nd column. Thanks. I had spaces in the earlier revisions of the re's, but I wasn't sure I had the 'x' flag applied to the sub-re's that needed them.

I guess each outer layer of the RE's flags get propagated to inner RE's.

I'm not sure if you were asking a question about your third group above where it you wrote: " 'what about these questionable cases?',"? I'm not sure what is questionable about them. In my use case, neither 'q{}' nor '?' have special meaning. Only the quotes and backslash were meta chars. So in the first line, I see 3 fields in both of the 1st 2 cases:

[ q{is this"really so"simple now?}, [ q{is}, q{this"really so"simple},
+ q{now?} ] ],
      ^                     ^               ^                         
+^
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Both of expressions had 2 breaks -- yielding 3 parts in each. Does that make sense?

One rule I forgot to list, though, at least your example handled it as expected, was what to do with overlapping quotes, and not making a quote of a different type have 'meta' properties. I.e.:

this "is a' test" of weird' stuff
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I may be wrong but I don't think most here would split that into 3 parts, as most of us are used to meta-properties of characters being disabled or modified within quotes, so the single quote above wouldn't start a quoted sub-expression overlapping with double quoted part. That would effectively make "is a' test" of weird' all 1 "word" as all the spaces are between quotes of some type. While that would be "a" way of interpreting overlapping quoted sections, I don't know how expected or useful it would be. Need to study your example and some others, but wanted to make some response. Just that about 3-4 other things cropped up and need attention just after I posted this...