m/(            # capture whole match
   (.)?        # 0 or 1 middle char
   (           # capture matched pairs
    .          # 1 char
     {?\2[<]}  # ...left of \2
     {?[eq]}   # string equal
     {?\2[>]}  # ...right of \2
   )*          # zero or more times
  )            # close capture
 /x;