[boost] Re: Announcing: xpressive

From: Eric Niebler (eric_at_[hidden])
Date: 2003-11-17 21:19:24

Joel de Guzman wrote:
> If yes, then how much? If none, why bother with ETs? 

(Lengthy rationale follows...)

Even if statically-bound is no faster than dynamically-bound, there are 
reasons to prefer ETs anyway.  Regular expressions are not *really* 
strings, and they're not *really* containers.  They're little programs 
written in a domain-specific language.  And in C++, the best way to 
represent domain-specific languages is to approximate their syntax in 
native C++ using expression templates, as Spirit does.  This brings a 
number of advantages:

- Invalid regular expressions can be detected at compile time instead of 
at runtime.

- ET's are executable C++ code, and can access other C++ code and data 
directly. A good example are Spirit's semantic actions. Boost.Regex and 
GRETA don't have anything quite like them -- the string-based 
representation places a wall between your patterns and the rest of your 
program.

- Ever try to write a regular expression to match balanced, nested tags? 
Since ET's are native code, they can refer to other ET's (and 
themselves!) in a natural way, so you can build recursive parsers out of 
them.  Doing the same thing with string-based regular expressions would 
be clumsy and fragile.

In short, expression templates make regular expressions first-class 
citizens of C++.

However, I'm not so naive as to think that people will never have 
occasion to use a string-based regex representation.  Hence the design 
of xpressive, which allows for both.

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