$ perl -Mre=debug -le " @F = q/yabba yobbo yebbe yubbu/ =~ /y(.)(.)\2\1/g "
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 yobbo yebbe yubbu"
Found anchored substr "y" at offset 0...
Guessed: match at offset 0
Matching REx "y(.)(.)\2\1" against "yabba yobbo yebbe yubbu"
   0 <> <yabba yobb>         |  1:EXACT <y>(3)
   1 <y> <abba yobbo>        |  3:OPEN1(5)
   1 <y> <abba yobbo>        |  5:REG_ANY(6)
   2 <ya> <bba yobbo >       |  6:CLOSE1(8)
   2 <ya> <bba yobbo >       |  8:OPEN2(10)
   2 <ya> <bba yobbo >       | 10:REG_ANY(11)
   3 <yab> <ba yobbo y>      | 11:CLOSE2(13)
   3 <yab> <ba yobbo y>      | 13:REF2(15)
   4 <yabb> <a yobbo ye>     | 15:REF1(17)
   5 <yabba> < yobbo yeb>    | 17:END(0)
Match successful!
Guessing start of match in sv for REx "y(.)(.)\2\1" against " yobbo yebbe yubbu"
Found anchored substr "y" at offset 1...
Starting position does not contradict /^/m...
Guessed: match at offset 1
Matching REx "y(.)(.)\2\1" against "yobbo yebbe yubbu"
   6 <abba > <yobbo yebb>    |  1:EXACT <y>(3)
   7 <bba y> <obbo yebbe>    |  3:OPEN1(5)
   7 <bba y> <obbo yebbe>    |  5:REG_ANY(6)
   8 <ba yo> <bbo yebbe >    |  6:CLOSE1(8)
   8 <ba yo> <bbo yebbe >    |  8:OPEN2(10)
   8 <ba yo> <bbo yebbe >    | 10:REG_ANY(11)
   9 <a yob> <bo yebbe y>    | 11:CLOSE2(13)
   9 <a yob> <bo yebbe y>    | 13:REF2(15)
  10 < yobb> <o yebbe yu>    | 15:REF1(17)
  11 <yobbo> < yebbe yub>    | 17:END(0)
Match successful!
Guessing start of match in sv for REx "y(.)(.)\2\1" against " yebbe yubbu"
Found anchored substr "y" at offset 1...
Starting position does not contradict /^/m...
Guessed: match at offset 1
Matching REx "y(.)(.)\2\1" against "yebbe yubbu"
  12 <obbo > <yebbe yubb>    |  1:EXACT <y>(3)
  13 <bbo y> <ebbe yubbu>    |  3:OPEN1(5)
  13 <bbo y> <ebbe yubbu>    |  5:REG_ANY(6)
  14 <bo ye> <bbe yubbu>     |  6:CLOSE1(8)
  14 <bo ye> <bbe yubbu>     |  8:OPEN2(10)
  14 <bo ye> <bbe yubbu>     | 10:REG_ANY(11)
  15 <o yeb> <be yubbu>      | 11:CLOSE2(13)
  15 <o yeb> <be yubbu>      | 13:REF2(15)
  16 < yebb> <e yubbu>       | 15:REF1(17)
  17 < yebbe> < yubbu>       | 17:END(0)
Match successful!
Guessing start of match in sv for REx "y(.)(.)\2\1" against " yubbu"
Found anchored substr "y" at offset 1...
Starting position does not contradict /^/m...
Guessed: match at offset 1
Matching REx "y(.)(.)\2\1" against "yubbu"
  18 < yebbe > <yubbu>       |  1:EXACT <y>(3)
  19 < yebbe y> <ubbu>       |  3:OPEN1(5)
  19 < yebbe y> <ubbu>       |  5:REG_ANY(6)
  20 < yebbe yu> <bbu>       |  6:CLOSE1(8)
  20 < yebbe yu> <bbu>       |  8:OPEN2(10)
  20 < yebbe yu> <bbu>       | 10:REG_ANY(11)
  21 < yebbe yub> <bu>       | 11:CLOSE2(13)
  21 < yebbe yub> <bu>       | 13:REF2(15)
  22 < yebbe yubb> <u>       | 15:REF1(17)
  23 < yebbe yubbu> <>       | 17:END(0)
Match successful!
Freeing REx: "y(.)(.)\2\1"