Octal digit in ANSI C grammar (lex)

Question

I looked ANSI C grammar (lex).

And this is octal digit regex

0{D}+{IS}?      { count(); return(CONSTANT); }

My question is why do they accept something like 0898?

It's not an octal digit.

So i thought they would consider that, but they just have wrote like that.

Could you explain why is that? Thank you

Answer 1

You want reasonable, user-friendly error messages.

If your lexer accepts 0999, you can detect an illegal octal digit and output a reasonable message:

 int x = 0999;
          ^
 error: illegal octal digit, go back to school

If it doesn't, it will parse this as two separate tokens 0 and 999 and pass them to the parser. The resulting error messages could be quite confusing.

 int x = 0999;
          ^
 error: expected ‘,’ or ‘;’ before numeric constant

The invalid program is rejected either way, as it should, however the ostensibly incorrect lex grammar does a better job with error reporting.

This demonstrates that practical grammars built for tools such as lex or yacc do not have to correspond exactly to ideal grammars found in language definitions.