[~] $ perl -Mre=debug -we'$_ = "abbaba"; /(.).*?\1/'
Freeing REx: `,'
Compiling REx `(.).*?\1'
size 11 first at 3
   1: OPEN1(3)
   3:   REG_ANY(4)
   4: CLOSE1(6)
   6: MINMOD(7)
   7: STAR(9)
   8:   REG_ANY(0)
   9: REF1(11)
  11: END(0)
minlen 1
Omitting $` $& $' support.

EXECUTING...

Matching REx `(.).*?\1' against `abbaba'
  Setting an EVAL scope, savestack=3
   0 <> <abbaba>          |  1:  OPEN1
   0 <> <abbaba>          |  3:  REG_ANY
   1 <a> <bbaba>          |  4:  CLOSE1
   1 <a> <bbaba>          |  6:  MINMOD
   1 <a> <bbaba>          |  7:  STAR
  Setting an EVAL scope, savestack=3
   1 <a> <bbaba>          |  9:    REF1
                              failed...
                           REG_ANY can match 1 times out of 1...
   2 <ab> <baba>          |  9:    REF1
                              failed...
                           REG_ANY can match 1 times out of 1...
   3 <abb> <aba>          |  9:    REF1
   4 <abba> <ba>          | 11:    END
Match successful!
Freeing REx: `(.).*?\1'