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Re: regular expressions

• To: xml-dev@l...
• Subject: Re: regular expressions
• From: David Tolpin <dvd@d...>
• Date: Fri, 30 Jan 2004 10:26:07 +0400 (AMT)
• In-reply-to: <4019A6ED.3000803@o...>

Bob Foster:
>  > Why isn't it done?
> 
> Good question. It should be. Of course, the notation suggests to most 
> compsci people that recursion will be permitted, i.e., that this is a 
> context-free instead of regular grammar, and that's probably not what 
> you intended.

The technique to detect recursions and stop context-grammars early is

 (resolve
   (lambda (piece)
     (cond
       ((string? piece) piece)
       ((symbol? piece)
	 (let ((entry (assv piece code)))
	   (if entry
	      (apply string-append (map resolve (cdr entry)))
	      (begin (error! piece " unresolved") ""))))))))

that is, just concatenate. If there is a recursion, the Scheme interpreter
will run out of space and the validator will immediately report an error
in the expression.

David

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