Re: [Boost-users] regex_search? Which subexpression smatch'ed?

From: John Maddock (john_at_[hidden])
Date: 2006-04-07 13:56:23

> Forgive me for jumping in late. This seems like a pretty silly
> optimization to me. Matching a regex, especially a complicated one
> like this, is going to take soooo much longer than a simple O(N)
> traversal of the resulting sub-matches that it just doesn't make
> sense to track this information during match time. In fact, it'll
> probably run *slower*. Don't do during match time what you can do
> before or after. You'll turn a simple O(N) operation into something
> far worse, and you'll make everybody pay for it.
>
> If you don't want to muddy up user code with a bunch of
> "if(results[n].matched)" checks, just write an iterator adaptor (using
> boost::filter_iterator<>) that iterates over only those sub-matches
> which matched, and also provides the index.
>
> My $.02,

Good point: even enumerating 100 sub-expressions to find out which was 
matched is going to be pretty quick compared to the alternatives.  The 
balance only really changes if you have thousands of marked sub-expressions, 
and I really hope we don't see regexes like that any time soon :-)

John.