Re: [Boost-users] why the following regexp doesn't meet my requirement?

From: llwaeva_at_[hidden]
Date: 2007-03-16 07:00:09

>> But, as I mentioned above, the whole matached string, namely #P{x},
>> will be replaced once the match is found. So, the character [^#] will 
>> also be replaced, that is not what I want. 

>Try a negative lookbehind: "(?<!#)#" will match any single "#" that is  *not* preceded by another #. 
> The previous character (whatever it is) >does not form part of the result. 

>John. 

Thank you so much. It's amazing! I see that regex is really a powerful tool! 

BTW,  in order to replace the  pattern #P and avoid the string start with double #, I find it is all
right to  specify the exact times in matching when using replace_all_regex, for example

replace_all_regex( source, boost::regex("([#]\{1\}P)|([#]\{1\}G)"), string("(?1 FIRST)(?2 SECOND)"), format_all );

of course, using the "negative lookbehind" approach also works in this problem

replace_all_regex( source, boost::regex("((?<!#)#P)|((?<!#)#G)"), string("(?1 FIRST)(?2 SECOND)"), format_all );

For my case, the program might be used to handle a LARGE text (perhaps several megabytes). So, I am wondering which
approach is faster!

Thanks again!