Re: [Boost-users] xpressive: set vs standard alternation compile problems

From: Zachary (unceldrot_at_[hidden])
Date: 2008-02-19 19:01:11

Thank you very much, the two points below cleared a lot up for me, 
though I should have realized the construction issue on my own. The 
construction and negation sets from characters does raise one other 
inconvenient point, which is maybe misleading in the documentation:

All of the examples which refer to matching "tags" whether parentheses 
or html, have the following limitations, if I am understanding 
correctly. They either:

a) are limited to single character "tags"

or

b) the examples will not work as desired in the case of nested tags

Am I correct about this? And if so, can you suggest any purely regex 
magic to deal with multi-character nested tags?

Thanks,
Zak
> I see how my docs led you astray. Although they match the same strings, 
> there actually is a difference between (set='a','b','c') and 
> (as_xpr('a')|'b'|'c'). The first is a set and can have its complement 
> taken (e.g., operator~). The second is just a bunch of regexes in 
> alternate, and cannot have its complement taken.
>
> Consider this a doc bug, which I'll fix.
>   
>>  Additionally, combining the original lines into one construction, like so:
>>
>>     sregex parentheses = '(' >> *(keep(+~(boost::xpressive::set='(',')'))|by_ref(parentheses)) >> ')';
>>
>> compiles but running the code fails with:
>>     
> <snip>
>
> This is an invalid use. It's no different than:
>
>    int i = i + 1; // oops!
>
> You may be surprised to find this compiles, but invokes undefined 
> behavior. It's a sad fact in C++ that objects are in scope before they 
> are initialized, leading to these sorts of nasty situations.
>
> If you would like to put this all in one line, use xpressive::self, as:
>
>    sregex parentheses = '(' >> *(keep(+~(set='(',')'))|self) >> ')';
>