Re: [Pyparsing] [pyparsing] RecursiveGrammarException

[Haifeng <min...@gm...>]

Thanks a lot for the response. I was trying to apply part of C grammar in my
small parser. Your answer solved my problem.

Haifeng

On Mon, Aug 22, 2011 at 10:57 PM, Paul McGuire <pt...@au...> wrote:

> Haifeng -
>
> You have 2 problems going on here:
> - you are matching 'a' before trying to match 'a->b'; the leading 'a' will
> match, and the trailing '->b' will then raise an error
> - you have defined a left-recursive expression (to match 'a->b', you will
> try to match 'a->b', but first you will match 'a->b', etc.); the
> left-recursion is what validate() is complaining about
>
> Pyparsing is not like lex/yacc or LR parsers - it works purely
> left-to-right
> through the input text, according to the definitions in the grammar.  There
> is no lookahead to try to achieve longer matches, as is done in regular
> expressions.  In pyparsing expressions, the '|' is a short-circuiting
> alternative operator, creating MatchFirst expressions: the first
> alternative
> expression to match will be the "winner", even if a later alternative might
> match a longer part of the input string.  You can force all alternatives to
> be tested for matching (with the longest match chosen) using the '^'
> operator, which creates Or expressions.
>
> Pyparsing does not allow you to directly implement a BNF definition, since
> BNF follows different rules for matching, and has different constraints for
> notation. There is more description of the BNF->pyparsing process here
> (http://pyparsing.wikispaces.com/message/view/home/11262229 and
> http://pyparsing.wikispaces.com/message/view/home/41102255).
>
> You use some terms like "postfix" in your grammar, but I don't see a lot of
> postfixing going on.  You do describe an infix '->' operator, but this too
> is somewhat confusing; in your example, you just show "a->b", but from your
> implementation, it looks like you want to support unbounded repetition,
> presumably something like "a->b->c->d".
>
> To implement your grammar to parse your two examples, you would write:
>
> ARROW = Literal('->')
> identifier = Word(alphas+"_", alphanums+"_")
>
> primary_expression = identifier
> compound_expression = identifier + ARROW + identifier
>
> # must test for compound_expression before primary_expression
> expression = compound_expression | primary_expression
>
>
> To add repetition to compound_expression, you could simply change it to:
>
> compound_expression = identifier + OneOrMore(ARROW + identifier)
>
>
> -- Paul
>
>
>
> -----Original Message-----
> From: Haifeng [mailto:min...@gm...]
> Sent: Sunday, August 21, 2011 2:43 AM
> To: pyp...@li...
> Subject: [Pyparsing] [pyparsing] RecursiveGrammarException
>
> Hi
>
> I am new to pyparsing. I am trying to parse just the following C
> expressions:
> a
> a->b
>
> My code looks like:
>
> from pyparsing import *
>
> ARROW = Literal('->')
> identifier = Word(alphas+"_", alphanums+"_")
>
> primary_expression = identifier
>
> postfix_expression = Forward()
> postfix_expression << ( primary_expression | (postfix_expression + ARROW +
> identifier))
>
> But it failed on postfix_expression.validate(). What did I do wrongly here?
> Thanks
>
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