+^(a\1?)(a\1?)(a\2?)(a\3?)$ aaaaaa y $1,$2,$3,$4 a,aa,a,aa
+^(a\1?){4}$ aaaaaa y $1 aa
+^(0+)?(?:x(1))? x1 y - -
+^([0-9a-fA-F]+)(?:x([0-9a-fA-F]+)?)(?:x([0-9a-fA-F]+))? 012cxx0190 y - -
+^(b+?|a){1,2}c bbbac y $1 a
+^(b+?|a){1,2}c bbbbac y $1 a
+\((\w\. \w+)\) cd. (A. Tw) y -$1- -A. Tw-
+((?:aaaa|bbbb)cccc)? aaaacccc y - -
+((?:aaaa|bbbb)cccc)? bbbbcccc y - -
+(a)?(a)+ a y $1:$2 :a -
+(ab)?(ab)+ ab y $1:$2 :ab -
+(abc)?(abc)+ abc y $1:$2 :abc -
+'b\s^'m a\nb\n n - -
+\ba a y - -
+^(a(??{"(?!)"})|(a)(?{1}))b ab y $2 a # [ID 20010811.006]
+ab(?i)cd AbCd n - - # [ID 20010809.023]
+ab(?i)cd abCd y - -
+(A|B)*(?(1)(CD)|(CD)) CD y $2-$3 -CD
+(A|B)*(?(1)(CD)|(CD)) ABCD y $2-$3 CD-
+(A|B)*?(?(1)(CD)|(CD)) CD y $2-$3 -CD # [ID 20010803.016]
+(A|B)*?(?(1)(CD)|(CD)) ABCD y $2-$3 CD-
+'^(o)(?!.*\1)'i Oo n - -
+(.*)\d+\1 abc12bc y $1 bc
+(?m:(foo\s*$)) foo\n bar y $1 foo
+(.*)c abcd y $1 ab
+(.*)(?=c) abcd y $1 ab
+(.*)(?=c)c abcd yB $1 ab
+(.*)(?=b|c) abcd y $1 ab
+(.*)(?=b|c)c abcd y $1 ab
+(.*)(?=c|b) abcd y $1 ab
+(.*)(?=c|b)c abcd y $1 ab
+(.*)(?=[bc]) abcd y $1 ab
+(.*)(?=[bc])c abcd yB $1 ab
+(.*)(?<=b) abcd y $1 ab
+(.*)(?<=b)c abcd y $1 ab
+(.*)(?<=b|c) abcd y $1 abc
+(.*)(?<=b|c)c abcd y $1 ab
+(.*)(?<=c|b) abcd y $1 abc
+(.*)(?<=c|b)c abcd y $1 ab
+(.*)(?<=[bc]) abcd y $1 abc
+(.*)(?<=[bc])c abcd y $1 ab
+(.*?)c abcd y $1 ab
+(.*?)(?=c) abcd y $1 ab
+(.*?)(?=c)c abcd yB $1 ab
+(.*?)(?=b|c) abcd y $1 a
+(.*?)(?=b|c)c abcd y $1 ab
+(.*?)(?=c|b) abcd y $1 a
+(.*?)(?=c|b)c abcd y $1 ab
+(.*?)(?=[bc]) abcd y $1 a
+(.*?)(?=[bc])c abcd yB $1 ab
+(.*?)(?<=b) abcd y $1 ab
+(.*?)(?<=b)c abcd y $1 ab
+(.*?)(?<=b|c) abcd y $1 ab
+(.*?)(?<=b|c)c abcd y $1 ab
+(.*?)(?<=c|b) abcd y $1 ab
+(.*?)(?<=c|b)c abcd y $1 ab
+(.*?)(?<=[bc]) abcd y $1 ab
+(.*?)(?<=[bc])c abcd y $1 ab
+2(]*)?$\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
+(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 $1 aaaaaaa
+^(.)\s+.$(?(1)) A B y $1 A # [perl #37688]
+(?:r?)*?r|(.{2,4}) abcde y $1 abcd
+(?!)+?|(.{2,4}) abcde y $1 abcd
+^(a*?)(?!(a{6}|a{5})*$) aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa y $+[1] 12 # super-linear cache bug may return 18
+^((?>(?:aa)?b)?) aab y $1 aab
+^((?:aa)*)(?:X+((?:\d+|-)(?:X+(.+))?))?$ aaaaX5 y $1 aaaa