m{
    ([A-Za-z])  # find a letter (and capture it)
    \1          # followed by the same
    (?!\1)      # but not by the same again
  }x;

##</code><code>##

  m{
    (?:           # either
      ^           #   start of string
    |             # or
      (.) (?!\1)  #   any character that is not followed by a duplicate
    )
    # now proceed as before, keeping in mind this is now the second capture
    ([A-Za-z])
    \2
    (?!\2)
  }x;