r, or an emphasis character
  | {{backticks}}                   # inline code
  | \[(?:                           # nested square brackets (one level deep)
        [^\[\]`]+(?=[\[\]`])        #  anything that isn't a square bracket or a backtick
        {{backticks}}?              #  balanced backticks
      )*\]                          #  closing square bracket
  )+                                # at least one character
)