[boost] Re: Boost and regular expressions

From: Dave Abrahams (abrahams_at_[hidden])
Date: 1999-09-04 09:18:28

>>Came across an excellent reg expr package at
> http://ourworld.compuserve.com/homepages/John_Maddock/regexpp.htm
>
> Can be used at many levels - from raw API to posix compatible interface to
> a
> more C++ approach using locales etc.<
>
> I hope you don't mind me jumping in here, but I've been lurking here for a
> few days after a "heads up" from Paul.
>
> The boost project is certainly an interesting idea, and I've no doubt that
> group partisipation results in stronger code, so I've been wondering about
> offering regex++ for integration with boost.  I can forsee many problems
> (differing namespaces and header file extensions amoung the easier to
> solve) and there is currently some duplication of effort (file/directory
> iterators, timer classes), so the question is what do you guys think?  Is
> it even remotely workable/desirable, or too much work?
>
> I look forward to your comments :-)

I like the idea in principle, but:

I directed a friend of mine who is a regex guru at your package. He said it
was interesting, but, in his words "it uses POSIX syntax but unfortunately
doesn't have POSIX semantics. He says he saw the words 'leftmost longest' in
the POSIX documentation but didn't know what they meant so he implemented
what Perl does". Now maybe my friend was misinterpreting what you wrote, but
what he says POSIX does is to build a DFA for the expression which
recognizes ALL alternatives simultaneously. The Perl style of interpreting
regular expressions leads to backtracking and all sorts of other
inefficiencies, including failure to recognize strings which match certain
expressions or combinatoric explosion. I am strongly in favor of a DFA
implementation.

-Dave