PEG语法来解析可选内容

Question

任何人都可以告诉我如何在支持两者的Treetop上编写语法：

system u AAA1 car=5, motor=4

和

system u car=5, motor=4

Answer 1

您想要使用“可选表达式”（请参阅​​this page约60％）：

grammar Sokmesa
  rule line
    "system" wsp+ marker wsp+ (category wsp+)? attributelist
  end
  rule category
    "AAA1" # Or whatever
  end
  # Add rules for 'marker', 'wsp', and 'attributelist' here
end