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

...
   6: ANYOF[\0\2-\377{unicode_all}](17)
  17: REF1(19)
  19: END(0)

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