Re: [boost] [regex] specify parenthesis

From: Lorenzo Bettini (bettini_at_[hidden])
Date: 2007-05-01 17:15:05

Eric Niebler wrote:
> John Maddock wrote:
>> Lorenzo Bettini wrote:
>>> do you happen to know whether there are any specifications for regular
>>> expressions for matching parenthesis (I mean in any regexp frameworks
>>> or libraries)?
>> Not really no, but I have another idea, assuming the x-modifier is on:
>>
>> (?x)
>> (?:
>>   (\()
>>   |(\[)
>>   |(\{)
>> )
>> foo
>> (?:
>>   (?(1)\)
>>   |(?:(?(2)\]
>>   |(?:\}
>> ))))
>>
>> The idea is to use conditional expressions to check which opening backet 
>> matched and then react accordingly.  You'll need to check I've got the ('s 
>> and )'s matching up 'cos I lost count while typing in :-(
> 
> 
> You can do something similar with xpressive (alternate regex engine 
> which will be in boost 1.34):
> 
>      cregex rx = cregex::compile(
>          "(\\(()|\\[()|\\{())foo(\\4\\}|\\3\\]|\\2\\))");
> 
>      if(regex_match("(foo)", rx))
>      {
>          std::cout << "match!\n";
>      }
>      if(!regex_match("(foo]", rx))
>      {
>          std::cout << "no match!\n";
>      }
> 
> For each alternate, you create an empty capture with "()". Then you 
> match that capture again in the balanced alternate on the other side. 
> Backreferences (even empty ones) only match if their capture 
> participated in the match.

mhh... this looks nice :-)

do you happen to know whether empty captures can be specified in 
boost::regex (I cannot try it at the moment, but I'll do it asap)

thanks
        Lorenzo

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