perl -Mre=debug -le 'print /(.)[^\1]\1/ ? "match" : "no match"'

Freeing REx: `","'
Compiling REx `(.)[^\1]\1'
size 19 Got 156 bytes for offset annotations.
first at 3
   1: OPEN1(3)
   3:   REG_ANY(4)
   4: CLOSE1(6)
   6: ANYOF[\0\2-\377{unicode_all}](17)
  17: REF1(19)
  19: END(0)

##</code><code>##

perl -le '$_ = "B\1B"; print /(.)[^\1]\1/ ? "match" : "no match"'