Re: [Boost-Users] Re: Inconsistent regexp matching when using quantifiers?

From: Joshua B. Smith (josh_at_[hidden])
Date: 2003-04-25 16:52:17

On Fri, Apr 25, 2003 at 08:27:54PM -0000, Dean wrote:
> That's what I (eventually) guessed was happening.  Thanks for 
> confirming my suspicion.  However, it still seems possible that this 
> was not the original design intention.  I suppose only John Maddock 
> can answer that question...

Does he read this list?  

> It seems to me that when the code finds more than 1 "a", it should 
> either:
> 
> 1) skip past all subsequent "a"s before starting the scan again.  
> This would cause "a{1}b" to be found in "ab" but 
> not "aab", "aaab", "aaaab", etc.  This would be very "greedy". :-)
> 
> Or:
> 
> 2) restart the scan 1 character after where the previous scan 
> started.  This would cause "a{1}b" to be found 
> in "ab", "aab", "aaab", "aaaab", etc.
> FWIW, I'm told that the regex searcher in the .NET Framework exhibits 
> behavior #1.  I mention that only as a point of reference -- I 
> realize that different implementations can have somewhat different 
> correct behaviors.

Perl and Python both exhibit behavior #2.  I think emacs does too.
and it doesn't surprise me that .Net is the greediest. :P  

I think a lot of regex engines have been converging on a perlish 
implementation. In fact, I'd never taken seriously the thought of
another way, but most of my regex work is done in python/perl (until
recently at any rate, I like to boost regex lib a lot).  

> Anyway, it is either a bug or a "gotcha".  I've been using regexs 
> occasionaly for over 10 years and it "got" me. :-)

I wouldn't say it "got" you. Regex's are still 50% voodoo 50% trick and 2%
butterscotch ripple.

-jbs