a(?{}})b - c - /a(?{}})b/: Sequence (?{...}) not terminated or not {}-balanced
a(?{"{"})b - c - /a(?{"{"})b/: Sequence (?{...}) not terminated or not {}-balanced
a(?{"\{"})b cabd y $& ab
-a(?{"{"}})b - c - Unmatched right bracket
+a(?{"{"}})b - c - Unmatched right curly bracket
a(?{$bl="\{"}).b caxbd y $bl {
x(~~)*(?:(?:F)?)? x~~ y - -
^a(?#xxx){3}c aaac y $& aaac
a*abc?xyz+pqr{3}ab{2,}xy{4,5}pq{0,6}AB{0,}zz x n - -
a(?{$a=2;$b=3;($b)=$a})b yabz y $b 2
round\(((?>[^()]+))\) _I(round(xs * sz),1) y $1 xs * sz
+'((?x:.) )' x y $1- x -
+'((?-x:.) )'x x y $1- x-