Grammar questions

[Anonymous]

Hello,

I am trying to implement JavaScript parser in LPG, but I have some problems how to express things.

How to write the following things:
1) but not
Example:
SingleStringCharacter ::= SourceCharacter but not (single-quote ' or backslash \ or LineTerminator)

2) How to write grouping?
SingleStringCharacter ::= SourceCharacter but not (single-quote ' or backslash \ or LineTerminator)

3) How to write optionality?
DoubleStringCharacters ::= DoubleStringCharacter DoubleStringCharactersopt

[Zoltán Ujhelyi]

Hi,

1, As far as I know, the parser generator does not support direct negation. Instead of that you have to manually invert the elements, e.g.
SingleStringCharacter ::= Alphabet | Numeric | '?' | '!' |  etc. etc.etc.

2, Sorry, but I don't see whats different from the first question.

3, You have to define a new nonterminal with options:
DoubleStringCharacters ::= DoubleStringCharacter DoubleStringCharactersOpt
DoubleStringCharactersOpt ::= DoubleStringCharacter | %empty

Or a similar solution

DoubleStringCharacters ::= DoubleStringCharacter | DoubleStringCharacter DoubleStringCharacters

I hope I have helped,
Zoltán Ujhelyi

[Anonymous]

Thanks, it helped.

And btw: how could I write the following?
EscapeSequence ::=
                   CharacterEscapeSequence
                    |  0 
                    | HexEscapeSequence
                    | UnicodeEscapeSequence

Thanks a lot
Jan