(?^:foo$)
foo\nbar\n =~ /(?^:foo$)/s      false
foo\nbar\n =~ /(?^:foo$)/m      false
fred\nfoo\nbar\n =~ /(?^:foo$)/s        false
fred\nfoo\nbar\n =~ /(?^:foo$)/m        false
foo =~ /(?^:foo$)/s     true
foo =~ /(?^:foo$)/m     true
\nfoo =~ /(?^:foo$)/s   true
\nfoo =~ /(?^:foo$)/m   true
foobar =~ /(?^:foo$)/s  false
foobar =~ /(?^:foo$)/m  false
bar\nfoo\n =~ /(?^:foo$)/s      true
bar\nfoo =~ /(?^:foo$)/m        true
bar\nfoo\n =~ /(?^:foo$)/       true
foo\nbar\n =~ /(?^:foo$)/       false
fred\nfoo\nbar\n =~ /(?^:foo$)/         false
fred\nfoo\nbar\n =~ /(?^:foo$)/         false
foo =~ /(?^:foo$)/      true
foobar =~ /(?^:foo$)/   false
\nfoo =~ /(?^:foo$)/    true