Re: VBScript grammar

Scott Nicol <>
22 May 2000 23:02:39 -0400

          From comp.compilers

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VBScript grammar (2000-05-20)
Re: VBScript grammar (Scott Nicol) (2000-05-22)
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From: Scott Nicol <>
Newsgroups: comp.compilers
Date: 22 May 2000 23:02:39 -0400
Organization: APK Net
References: 00-05-076
Keywords: Basic, parse, practice

Anton Belov -- Customer Engineering wrote:
> Does anyone have or know where to get the grammar definition (in any
> form) for VBScript ? I know it has been asked before - but i didn't
> see any answers ...

I've never seen a grammar published. You can't trust the reference
manual either, because it lies. The only way to write a grammar is to
play with an implementation of the language.

I wrote a parser for VB (sorry, not available, I don't own the code),
and since VBScript is based on VB, I'll assume you'll run into similar
problems. Here are a few:

- End requires an extra lookahead, because End is different from
    End If, End While, End Select, etc. Handle it in the scanner
    so that End <something> is returned to the parser as one token,
    not two.
- Line labels are a pain. I think I put some magic in the
    scanner for this.
- There are a zillion reserved words, but some depend on context.
    This can make tokenizing very difficult. For example, "In" is
    a reserved word but can also be a variable name. I think I
    fixed this by having YACC pass state info back to Lex, but
    you have to be careful about lookahead. Another possible fix
    is to allow a syntax error and try to recover in yyerror().
    Not easy...
- Scoping rules are quite complex, and are subtly different for
    variables and types.
- Parenthesis are significant at runtime, so don't throw them
    away during the parse if you ever want to build a runtime.
    In case you're wondering, parenthesis override references,
    so even if a function/subroutine is written to accept a
    parameter by reference, you can pass it by value by
    enclosing the argument in parenthesis.

Writing a runtime is even more exciting...

Scott Nicol

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