[TxMt] Re: Use begin marker of a scope to end the previous one

[Édouard Gilbert]

Le 19 mars 09 à 19:00, Allan Odgaard a écrit :

> On 16 Mar 2009, at 11:01, Édouard Gilbert wrote:
>
>> [...]
>> Watch out for grammar bugs, however.  It’s easy to make TM crash if
>> you write things like:
>>
>> begin = '(?=a)'
>> end = '(?=b)'
>>
>> because neither expression consume a character. [...]
>
> For the records, you’d need to make it:
>
>   begin = '(?=a)'
>   end = '(?=a)'
>
> Here, if the ‘patterns’ active between begin/end do not consume
> anything, we have the infinite loop. In your example the parser would
> have advanced to “just before b”, so the same rule would not be
> applicable at the same position.


Actually, I ran into the problem with both look-ahead and look behind,  
as in

begin = '(?<=a)'
end = '(?=b)'

so I didn’t thought of that. It might help with some trouble I have on  
a grammar. Cheers.

While we’re discussing language grammars, I was wondering if we could  
hope some lexer-like capabilities in TM2.0 — I to have regexes  
everywhere in my grammars, they make anything barely readable.

And, on quite but not that much unrelated matters, what would be the  
better way to match something like:

a
 > I can apply set of rule A
b
 > I can apply set of rule B
c


My current is

begin = 'a'
end = '(?<=c)'
patterns= (
	{	begin = '(?<=a)'
		end = 'b'
		patterns = ( A )
		begin = '(?<=b)'
		end = 'c'
		patterns = ( B )
	}

but the restrictions on look-ahead bug me.

begin = '(?=a)'
end = 'c'
patterns= (
	{	begin = 'a'
		end = '(?=b)'
		patterns = ( A )
		begin = 'b'
		end = '(?=c)'
		patterns = ( B )
	}

might be a better solution. Or is there a efficient and elegant  
solution I didn't think of?

Édouard GILBERT
edouard.gilbert at gmail.com

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