(a.)..(..)		zzzabcdefzzz	y			basic match
(a.)..(..)		zzzabcdefzzz	<mob: abcdef @ 3>	basic $0
(a.)..(..)		zzzabcdefzzz	<mob[0]: ab @ 3>	basic $1
(a.)..(..)		zzzabcdefzzz	<mob[1]: ef @ 7>	basic $2
d()ef		zzzabcdefzzz	y			empty capture
()ef		zzzabcdefzzz	y			empty capture at start
(a(b(c))(d))		abcd		y			nested match
(a(b(c))(d))		abcd		<mob: abcd @ 0>		nested match
(a(b(c))(d))		abcd		<mob[0]: abcd @ 0>	nested match
(a(b(c))(d))		abcd		<mob[1]: bc @ 1>	nested match
(a(b(c))(d))		abcd		<mob[2]: c @ 2>	nested match
(a(b(c))(d))		abcd		<mob[3]: d @ 3>	nested match
(a)b(c)		abc		y			multiple matches
(a)b(c)		abc	<mob[0]: a @ 0>	basic $0
(a)b(c)		abc	<mob[1]: c @ 2>	basic $1
b(.)\1				bookkeeper	y			backreference
b(.)\1(.)\2				bookkeeper	y			backreference