PEG語法解析可選內容

Question

誰能告訴我怎麼寫語法上Treetop同時支持：

system u AAA1 car=5, motor=4 

和

system u car=5, motor=4 

Answer 1

你想用一個「可選的表達式」（見約在this page下降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