s/^            # Beginning of string
   [^|]+       # Everything up to the next bar
   \|          # The bar
   (           # Capture to $1
     [^|]+     # Everything up to the next bar
   )           # End capture
   \|          # The bar
   [^|]+       # Everything up to the next bar
   \|          # The bar
   (?:         # Non-capturing parens
     internal  # the string "internal"
     |         # or
     partner   # the string "partner"
   )$          # End of string
   /$1/x       # Substitute $1 for string