[talk] lex start conditions.

[Warner Losh]

What was the original OS this thing is from? 40 years is 1980, which
suggests V7. If so, I'd lookup how to boot V7 on a PDP-11 with simh and
compile it there to see what this translates to... If you have the source
code and can share off list, I can run it on one of mine...

Warner

On Wed, Oct 28, 2020 at 6:31 PM R. Clayton <factotum at rclayton.org> wrote:

> I'm working with a 40-year-old lex program that uses statements like
>
>   BEGIN PROGRAM + 1;
>
> I understand what
>
>   BEGIN PROGRAM;
>
> does, but what is the intention of the "+ 1"? Does it skip the first rule
> in
> the <PROGRAM> context? The generated code suggests not, but I haven't dug
> into
> it too deeply. Does it select the context listed after the <PROGRAM>
> context?
> That seems slightly more plausible, but a test lex program doesn't support
> that
> intention (it's likely the test program is ineffectively written). I'm
> using
> flex, not a 40-year-old version of lex.
>
> For completeness, here's the test program:
>
>   $ cat t.lex
>   %s A B C
>
>   %%
>                   BEGIN A;
>
>   <A>[BC]         if (yytext[0] == 'B') BEGIN B + 1; else BEGIN C;
>   <A>.            printf("A: '%c'\n", yytext[0]);
>
>   <B>[AC]         if (yytext[0] == 'A') BEGIN A; else BEGIN C;
>   <B>.            printf("B: '%c'\n", yytext[0]);
>
>   <C>[AB]         if (yytext[0] == 'A') BEGIN A; else BEGIN B;
>   <C>.            printf("C: '%c'\n", yytext[0]);
>
>   %%
>
>   main() { yylex(); }
>
>   $ echo abcBabc | ./a.out
>   A: 'a'
>   A: 'b'
>   A: 'c'
>   B: 'a'
>   B: 'b'
>   B: 'c'
>
>   $
>
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