[boost] [Review] Xpressive

From: Fred (fred_at_[hidden])
Date: 2005-09-13 12:19:12

         * What is your evaluation of the design?
         * What is your evaluation of the implementation?
It's an extremely interesting library, and I really liked many of the design
decisions made. 
I tried converting a simple Spirit grammar into an xpressive regex. I'll
admit that this isn't a good idea, at least in my case, but it was an
entertaining test of the library. The library seemed to do very well at most
things I threw at it. I did run into one or two bugs, but Eric has already
fixed them. MSVC7.1 frequently generated internal compiler errors, but the
regex I was creating is hopefully significantly more complex than a regex
should be. (I didn't profile it, but it looked as though it would be
extremely slow as a regex.)
One thing that I'd like to see improved is the ability to organize complex
regexes. I think one example that I hope is easy to implement would be an
ability to name a set of characters. There are a number of literals (_d, _w,
etc.) that already do this. I would have found it useful to be able to name
my own character sets in static regexes. 
        * What is your evaluation of the documentation?
Overall, it was extremely helpful. I did have a few minor complaints that
I'll detail below:
The syntax cheat sheet should have a string literal added to it. I'd like to
see it be complete, and this is the only thing I could find that wasn't
there.
In addition, I'd like to be able to define character sets of my own that
could be used in multiple places in my static regex.
I'd like the documentation to better describe the _w identifier. I couldn't
find the definition of “word character”. (I can imagine only one definition
making sense, but that's not the same as having a definition.)
On the page grammars_and_nested_matches.html, there is a discussion of how
the library nicely handles all sorts different lifetime scenarios, but it
does not discuss the lifetime of the regex refered to by a regex object that
is refered to only through a “by_ref” clause in another regex. Does by_ref
copy the object and manage the lifetime or not?
One big advantage of static xpressive regexes is the potential of making a
complex regex relatively readable. I'd like to see some discussion of this
in the docs. (Something along the lines of
http://www.boost.org/libs/spirit/doc/style_guide.html) 
The documentation of ~as_xpr( character_constant ) should be included. This
is a useful construct.
        * What is your evaluation of the potential usefulness of the
library?
This will be an extremely useful library. I can see three reasons why this
would be valuable to a user:
It gives users another set of options to look at when trying to improve the
performance of a regex.
It makes it possible to create some fairly complex regexes that would be
impossible or unreadable with the current boost regex library.
The ability to refer to subexpressions in a static regex allows you to make
a regex significantly easier to read and understand. Static regexes may be
easier to read and understand than traditional string based regex languages,
but I don't have enough experience to be confident of that statement.
        * Did you try to use the library?  With what compiler? Did you have
any problems?
I played with it a bit. (See above.) I used MSVC7.1 and had slight troubles.
(I also broke my personal record for long error messages. Even Spirit has
never generated a 10MB error message for me before!)
        * How much effort did you put into your evaluation? A glance? A
quick reading? In-depth study?
I read the documentation reasonably carefully and spent an afternoon writing
some code with it. I did not test the code carefully, but I did get it to
compile. 
        * Are you knowledgeable about the problem domain?
I've used boost::regex a fair amount. I've also used Spirit occasionally. 
        * Do you think the library should be accepted as a Boost library? Be
sure to say this explicitly so that your other comments don't obscure your
overall opinion.
Yes.