re = ((a*)(b*)(c))*
$substring = aacbbbcac = test.pl
Group 0: <aacbbbcac>
$substring = ac = ac
Group 1: <ac>
$substring = a = a
Group 2: <a>
$substring =  =
Group 3: <>
$substring = c = c
Group 4: <c>
[
  "aacbbbcac",
  "ac",
  "a",
  undef,
  "c"
]