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Re: Validate an XPath with regex

• To: <xml-dev@l...>
• Subject: Re: Validate an XPath with regex
• From: "Gary Stephenson" <garys@i...>
• Date: Tue, 9 Apr 2002 10:33:17 +1000
• References: <3B1C9D22-47A6-11D6-8473-000502CB905D@n...> <200204080834.JAA03071@p...>

> > Shouldn't it be possible to validate the syntax of an XPath string using
> > regular expressions.
>
> No. You at least need to be able to match () pairs, this requires
> counting, so can't be done with a regular expression.

I"m afraid I simply don't understand.  :-(  Are you sure about this?  Is
there perhaps a qualitative difference between "counting" and (recursively)
detecting balancing delimiters?

Since reading the above I have been unsuccessfully trying to trip my XPath
parser up, by feeding it expressions containing unbalanced parens and/or
square brackets.  Could you perhaps provide some examples that you would
expect to cause problems.

Hmmm..., BNF grammars are used ubiquitously to generate parsers which are in
turn quite capable of detecting unbalanced delimiters in their input.  So,
if what you say is correct, then BNF grammars must be inherently more
powerful than regular expressions - yes?  But aren't they both simply forms
of regular grammars cum finite automata?

<only-ever-opening-my-mouth-to-change-feet>'ly yrs,

gary

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    • From: Niels Peter Strandberg <nielspeter@n...>
  • Re: Validate an XPath with regex
    • From: David Carlisle <davidc@n...>

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