^(<(?:[^<>]+|(?1))*>)$ <<><<<><>>>> y $1 <<><<<><>>>>
((?2)*)([fF]o+) fooFoFoo y $1-$2 fooFo-Foo
(<(?:[^<>]+|(?R))*>) <<><<<><>>>> y $1 <<><<<><>>>>
+(?<n>foo|bar|baz) snofooewa y $1 foo
+(?<n>foo|bar|baz) snofooewa y $+{n} foo
+(?<n>foo|bar|baz)(?<m>[ew]+) snofooewa y $+{n} foo
+(?<n>foo|bar|baz)(?<m>[ew]+) snofooewa y $+{m} ew
+(?<n>foo)|(?<n>bar)|(?<n>baz) snofooewa y $+{n} foo
+(?<n>foo)(??{ $+{n} }) snofooefoofoowaa y $+{n} foo
+/(?'n'foo|bar|baz)/ snofooewa y $1 foo
+/(?'n'foo|bar|baz)/ snofooewa y $+{n} foo
+/(?'n'foo|bar|baz)(?'m'[ew]+)/ snofooewa y $+{n} foo
+/(?'n'foo|bar|baz)(?'m'[ew]+)/ snofooewa y $+{m} ew
+/(?'n'foo)|(?'n'bar)|(?<n>baz)/ snobazewa y $+{n} baz
+/(?'n'foo)(??{ $+{n} })/ snofooefoofoowaa y $+{n} foo
+/(?'n'foo)\k<n>/ ..foofoo.. y $1 foo
+/(?'n'foo)\k<n>/ ..foofoo.. y $+{n} foo
+/(?<n>foo)\k'n'/ ..foofoo.. y $1 foo
+/(?<n>foo)\k'n'/ ..foofoo.. y $+{n} foo
+/(?:(?<n>foo)|(?<n>bar))\k<n>/ ..barbar.. y $+{n} bar
+/^(?'main'<(?:[^<>]+|(?&crap)|(?&main))*>)(?'empty')(?'crap'!>!>!>)$/ <<!>!>!>><>>!>!>!> y $+{main} <<!>!>!>><>>
+/^(?'main'<(?:[^<>]+|(?&main))*>)$/ <<><<<><>>>> y $1 <<><<<><>>>>
+/(?'first'(?&second)*)(?'second'[fF]o+)/ fooFoFoo y $+{first}-$+{second} fooFo-Foo
+(?<A>foo)?(?(<A>)bar|nada) foobar y $+{A} foo
+(?<A>foo)?(?(<A>)bar|nada) foo-barnada y $& nada
+(?<A>foo)?(?(1)bar|nada) foo-barnada y $& nada
+(?<A>foo(?(R)bar))?(?1) foofoobar y $1 foo
+(?<A>foo(?(R)bar))?(?1) foofoobar y $& foofoobar
+(x)(?<A>foo(?(R&A)bar))?(?&A) xfoofoobar y $2 foo
+(x)(?<A>foo(?(R&A)bar))?(?&A) xfoofoobar y $& xfoofoobar
+(x)(?<A>foo(?(R2)bar))?(?&A) xfoofoobar y $2 foo
+(x)(?<A>foo(?(R2)bar))?(?&A) xfoofoobar y $& xfoofoobar
+(?1)(?(DEFINE)(blah)) blah y $& blah
+/^(?<PAL>(?<CHAR>.)((?&PAL)|.?)\k<CHAR>)$/ madamimadam y $& madamimadam
+/^(?<PAL>(?<CHAR>.)((?&PAL)|.?)\k<CHAR>)$/ madamiamadam n - -
+/(a)?((?1))(fox)/ aafox y $1-$2-$3 a-a-fox
+/(a)*((?1))(fox)/ aafox y $1-$2-$3 a-a-fox
+/(a)+((?1))(fox)/ aafox y $1-$2-$3 a-a-fox
+/(a){1,100}((?1))(fox)/ aafox y $1-$2-$3 a-a-fox
+/(a){0,100}((?1))(fox)/ aafox y $1-$2-$3 a-a-fox
+/(ab)?((?1))(fox)/ ababfox y $1-$2-$3 ab-ab-fox
+/(ab)*((?1))(fox)/ ababfox y $1-$2-$3 ab-ab-fox
+/(ab)+((?1))(fox)/ ababfox y $1-$2-$3 ab-ab-fox
+/(ab){1,100}((?1))(fox)/ ababfox y $1-$2-$3 ab-ab-fox
+/(ab){0,100}((?1))(fox)/ ababfox y $1-$2-$3 ab-ab-fox