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

From: Eric Niebler (eric_at_[hidden])
Date: 2006-04-07 13:00:25

Lynn Allan wrote:
> Whatever you could work out would be greatly appreciated.
> 
> ----- Original Message ----- 
> From: "John Maddock" <john_at_[hidden]>
>> It's backtracking that gets you: each time the "lowest sub-expression set"
>> gets updated you'd need to store a complete history of previous settings 
>> so
>> you can backtrack out again.  However, now I think about that some more, I
>> actually think it might be possible after all :-)

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,

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com