(??{}) 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
+(WORDS|WORD)S WORDS y $1 WORD
+(X.|WORDS|X.|WORD)S WORDS y $1 WORD
+(WORDS|WORLD|WORD)S WORDS y $1 WORD
+(X.|WORDS|WORD|Y.)S WORDS y $1 WORD
+(foo|fool|x.|money|parted)$ fool y $1 fool
+(x.|foo|fool|x.|money|parted|y.)$ fool y $1 fool
+(foo|fool|money|parted)$ fool y $1 fool
+(foo|fool|x.|money|parted)$ fools n - -
+(x.|foo|fool|x.|money|parted|y.)$ fools n - -
+(foo|fool|money|parted)$ fools n - -
+(a|aa|aaa|aaaa|aaaaa|aaaaaa)(b|c) aaaaaaaaaaaaaaab y $1$2 aaaaaab
+(a|aa|aaa|aaaa|aaaaa|aaaaaa)(??{$1&&""})(b|c) aaaaaaaaaaaaaaab y $1$2 aaaaaab
+(a|aa|aaa|aaaa|aaaaa|aaaaaa)(??{$1&&"foo"})(b|c) aaaaaaaaaaaaaaab n - -
+^(a*?)(?!(aa|aaaa)*$) aaaaaaaaaaaaaaaaaaaa y $1 a # [perl #34195]
+^(a*?)(?!(aa|aaaa)*$)(?=a\z) aaaaaaaa y $1 aaaaaaa