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