^(a(?(1)\1)){4}$ aaaaaaaaaa y $1 aaaa
^(a(?(1)\1)){4}$ aaaaaaaaa n - -
^(a(?(1)\1)){4}$ aaaaaaaaaaa n - -
+((a{4})+) aaaaaaaaa y $1 aaaaaaaa
+(((aa){2})+) aaaaaaaaaa y $1 aaaaaaaa
+(((a{2}){2})+) aaaaaaaaaa y $1 aaaaaaaa
(?:(f)(o)(o)|(b)(a)(r))* foobar y $1:$2:$3:$4:$5:$6 f:o:o:b:a:r
(?<=a)b ab y $& b
(?<=a)b cb n - -
(^|x)(c) ca y $2 c
a*abc?xyz+pqr{3}ab{2,}xy{4,5}pq{0,6}AB{0,}zz x n - -
a(?{$a=2;$b=3;($b)=$a})b yabz y $b 2
+round\(((?>[^()]+))\) _I(round(xs * sz),1) y $1 xs * sz