How to construct LEX for this ?

Question

I want to make the automata to accept following regular expression using lex and yacc. The regular expression is R = (ab + a)* .

Can anyone help me to construct this automata using lex and yacc.

Thanks.

Answer 1

First thing that comes to mind is something like this. Not complete programs, but something to get you started:

The scanner(lex):

%%
a     return TOKENA; /* for an a in the input */
b     return TOKENB; /* for a b in the input */
\n                      /* ignore end of line */;
[ \t]+                  /* ignore whitespace */;
%%

The parser(yacc):

commands: /* empty */
        | commands command
        { printf("found a (ab + a)* pattern"); }

command:
        ab
        |
        a
        ;

ab: TOKENA TOKENB
    ;
a: TOKENA
    ;

I'm not completely sure if the grammar works or has any reduction conflicts.