^(?: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