From: Clifford Heath <no@...>
Date: 2008-02-05T08:20:02+09:00
Subject: Re: Treetop problem

Marco Guiseppe wrote:
> It seems to appear when the first character of the string parsed is a
> valid match for both rules, and Treetop is unable to resolve the
> ambiguity.

Packrat parsers are greedy. When a rule succeeds, or when a repeated
element (*, +) repeats too many times, or when an optional rule is
taken, that decision will never be reversed - it's been memoized.
You must prevent it happening (perhaps by using lookahead) in the
first place.

In this case, you can do that by writing:

rule string
   [A-z] [A-z0-9]*
end

BTW, did you realize that there are non-alphabetic characters in the
range A-z? Did you mean to say [A-Za-z]?

Clifford Heath.