# How to define at least one occurrence of a string between two tokens in bottom up LALR(1) parser grammar

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

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if it's at least one, you must replace $\epsilon$ by the canonical element (here HOUR). –  didierc Feb 20 '13 at 17:59

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.

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very interesting! –  didierc Feb 21 '13 at 21:27