^[^bcd]*(c+) aexycd y $1 c
(?{$a=2})a*aa(?{local$a=$a+1})k*c(?{$b=$a}) yaaxxaaaacd y $b 3
(?{$a=2})(a(?{local$a=$a+1}))*aak*c(?{$b=$a}) yaaxxaaaacd y $b 4
+# [perl #117917]
+^(a(?{ "x" =~ m{x}})b)(??{ $1 }) abab y $& abab
(>a+)ab aaab n - -
(?>a+)b aaab y - -
([[:]+) a:[b]: y $1 :[