I am trying to define a non terminal symbol in a LALR(1) grammar (with CUP parser). It is requested that the <code> token must appear exactly twice, while the <hour> token must appear at least once.In the end I came up with this definition:section     ::= hour_l CODE SC hour_l CODE SC hour_l ;hour_l      ::= /* epsilon */             | hour_l HOUR SC ;where SC is a separator (semicolon) between tokens and hour_l is the non terminal symbol for hour's list.This solution has a problem: HOUR can be absent, because epsilon can be reduced from hour_l.Is there a clever solution other than specifying all possibilities or using the semantic capabilities of CUP (ie. putting a counter of how many times HOUR is present in section)? I'd prefer not to use semantics in order to achieve this; in fact, it seems to me this is syntax related.Thanks
How to define at least one occurrence of a string between two tokens in bottom up LALR(1) parser grammar
formal grammars;parsing
My solution, suggested by a friend, is to use a Finite State Machine. I drew a Deterministic Finite Automata, and $C$ is the final state accepted by this machine:I then transformed it into a right regular grammar:section     ::= c ;a           ::= CODE SC ;b           ::= a CODE SC ;c           ::= c HOUR SC | b HOUR SC | e CODE SC ;d           ::= HOUR SC | d HOUR SC ;e           ::= e HOUR SC | a HOUR SC | d CODE SC ;Hope it helps.