2(]*)?$\1 2 y $& 2
(??{}) x y - -
a(b)?? abc y <$1> <> # undef [perl #16773]
+(\d{1,3}\.){3,} 128.134.142.8 y <$1> <142.> # [perl #18019]
+^.{3,4}(.+)\1\z foobarbar y $1 bar # 16 tests for [perl #23171]
+^(?:f|o|b){3,4}(.+)\1\z foobarbar y $1 bar
+^.{3,4}((?:b|a|r)+)\1\z foobarbar y $1 bar
+^(?:f|o|b){3,4}((?:b|a|r)+)\1\z foobarbar y $1 bar
+^.{3,4}(.+?)\1\z foobarbar y $1 bar
+^(?:f|o|b){3,4}(.+?)\1\z foobarbar y $1 bar
+^.{3,4}((?:b|a|r)+?)\1\z foobarbar y $1 bar
+^(?:f|o|b){3,4}((?:b|a|r)+?)\1\z foobarbar y $1 bar
+^.{2,3}?(.+)\1\z foobarbar y $1 bar
+^(?:f|o|b){2,3}?(.+)\1\z foobarbar y $1 bar
+^.{2,3}?((?:b|a|r)+)\1\z foobarbar y $1 bar
+^(?:f|o|b){2,3}?((?:b|a|r)+)\1\z foobarbar y $1 bar
+^.{2,3}?(.+?)\1\z foobarbar y $1 bar
+^(?:f|o|b){2,3}?(.+?)\1\z foobarbar y $1 bar
+^.{2,3}?((?:b|a|r)+?)\1\z foobarbar y $1 bar
+^(?:f|o|b){2,3}?((?:b|a|r)+?)\1\z foobarbar y $1 bar
+.*a(?!(b|cd)*e).*f ......abef n - - # [perl #23030]
+x(?# x c - Sequence (?#... not terminated
+:x(?#: x c - Sequence (?#... not terminated