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  1. or download this
     beri beri 
    ^,               then '(hana)' fails. Fail.
    
  2. or download this
     hana    bbs
    ^(hana)  (bbs),  then '\2' fails. Backtrack;
    ^(hana)\2,       then '\1' fails. Fail.
    
  3. or download this
    'beriberi' =~ m#^(beri)(bbs)?(?(2)\2\1|\1)$#;
    
  4. or download this
     beri        beri
    ^(beri)(bbs)?\1  $   Match.
    
  5. or download this
    'beribbsbbsberi' =~ m#^(beri)(bbs)?(?(2)\2\1|\1)$#;
    
  6. or download this
     beri  bbs   bbsberi
    ^(beri)(bbs)?\2 \1  $   Match.