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

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