Hmm, if you're using a peg parser that doesn't directly support this you can emulate it by choosing between the rhs of the cut and a production that will always match

so

  expr : atom (op <cut> expr)*

Would be replaced with

  expr : atom (op (expr / cut))*
  cut  : "XXX"?

or perhaps simpler

  expr : atom (op expr?)*

Either way when you process the expr rule there will be an empty match where the rhs should be, and an appropriate error can be generated