$ perl -Mre=debug -le " q/yabba dabba doo/ =~ /y(.)(.)\2\1/ "
Compiling REx "y(.)(.)\2\1"
Final program:
   1: EXACT <y> (3)
   3: OPEN1 (5)
   5:   REG_ANY (6)
   6: CLOSE1 (8)
   8: OPEN2 (10)
  10:   REG_ANY (11)
  11: CLOSE2 (13)
  13: REF2 (15)
  15: REF1 (17)
  17: END (0)
anchored "y" at 0 (checking anchored) minlen 3
Guessing start of match in sv for REx "y(.)(.)\2\1" against "yabba dabba doo"
Found anchored substr "y" at offset 0...
Guessed: match at offset 0
Matching REx "y(.)(.)\2\1" against "yabba dabba doo"
   0 <> <yabba dabb>         |  1:EXACT <y>(3)
   1 <y> <abba dabba>        |  3:OPEN1(5)
   1 <y> <abba dabba>        |  5:REG_ANY(6)
   2 <ya> <bba dabba >       |  6:CLOSE1(8)
   2 <ya> <bba dabba >       |  8:OPEN2(10)
   2 <ya> <bba dabba >       | 10:REG_ANY(11)
   3 <yab> <ba dabba d>      | 11:CLOSE2(13)
   3 <yab> <ba dabba d>      | 13:REF2(15)
   4 <yabb> <a dabba do>     | 15:REF1(17)
   5 <yabba> < dabba doo>    | 17:END(0)
Match successful!
Freeing REx: "y(.)(.)\2\1"