[boost] Regex ease-of-use ideas

From: David Abrahams (dave_at_[hidden])
Date: 2004-04-05 14:22:10

I was just writing up a simple tutorial example; finding the subject
in a set of email headers.  Here's what I got:

    std::string line;
    boost::regex pat("^Subject: (Re: )?(.*)");
    boost::smatch matches;

    while (std::cin)
    {
        std::getline(std::cin, line);
        if (boost::regex_match(line,matches, pat))
            std::cout << matches[2];
    }

1. There's no way to search a stream for a match because a regex
   requires bidirectional iterators, so I have to do this totally
   frustrating line-by-line search.  I think Spirit has some kind of
   iterator that turns an input iterator into something forward by
   holding a cache of the data starting with the earliest copy of the
   original iterator.  Could something like that be added?

2. Seems to me that if match objects could be converted to bool, we
   might be able to:

    std::string line;
    boost::regex pat("^Subject: (Re: )?(.*)");

    while (std::cin)
    {
        std::getline(std::cin, line);
        if (boost::smatch m = boost::regex_match(line, pat))
            std::cout << m[2];
    }

   which would be much smoother to the touch.  Are match objects
   expensive to construct?

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com