%{ =head1 SYNOPSIS This grammar deals with the famous ambiguous PL/I phrase: if then=if then if=then The (partial) solution uses C<YYExpect> in the lexical analyzer to predict the token that fulfills the parser expectatives. Compile it with: eyapp -C PL_I_conflict2.eyp Run it with: ./PL_I_conflict2.pm -t -i -c 'if if=then then then=if' for more detail: ./PL_I_conflict2.pm -deb -t -i -c 'if if=then then then=if' and also ./PL_I_conflict2.pm -t -i -c 'if then=if then if=then' Also try: ./PL_I_conflict2.pm -t -c 'if then=if then if a=b then c=d' =cut %} %strict %token then = %/(then)\b/ # contextual token. Returned only if expected by the syntax analyzer %token if = %/(if)(?!\s*=)/ %token ID = /([a-zA-Z_]\w*)/ %tree bypass %% stmt: ifstmt | assignstmt ; ifstmt: %name IF if expr then stmt ; assignstmt: %name ASSIGN id '=' expr ; expr: %name EQ id '=' id | id ; id: %name ID ID ; %%